Masters Degrees (Medical Physics)

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Open Access
The assessment of potential radiation hazards from gold mines in the Free State Goldfields to members of the public
(University of the Free State, 1998-11) Ellis, Jozua Francois; Botha, J. C.; Van Aswegen, A.
The gold mines In the Free State Goldfields extract and process ore from underground, which contains naturally radioactive uranium and its associated decay products. This assessment aimed to cost effectively determine the major potential radiation hazards to the public from the gold mines in the area. The potential exposure sources from the mines are radon gas, radioactive dust, contaminated water and external gamma radiation. The assessment focussed mainly on the public's potential exposure to radon gas emanating from tailings dams, waste rock dumps and upcast shafts from underground workings. The rate of radon emanation from the dams was measured using several different techniques, and the potential dispersion of the radon was modelled using internationally accepted modelling codes and local weather data for the Free State Goldfields. A maximum potential contribution to the natural background radon levels of 6 Bq m-3 was calculated. This is a small increment to the background levels in the order of 25 to 35 Bq m-3". Environmental measurements of outdoor radon concentrations confirmed the modelling results to the extent that no significantly high radon concentration could be detected in the environment. Background radon levels in towns outside the Free State Goldfields are in the same order as those measured around the mines. Similar environmental measurements of airborne dust and water sources around the mines indicated relatively low levels of radiation. A conservative estimate of the total potential exposure of the public in the Free State Goldfields is in the order of 130 to 250 µSv/a. This can be interpreted as well within the internationally accepted public dose limit of 1000 µSv/a.
Open Access
Development of a Monte Carlo simulation method for the evaluation of dose distribution calculations of radiotherapy treatment planning systems
(University of the Free State, 1999-11) Du Plessis, Frederik Carl Philippus; Willemse, C. A.; Lotter, M. G.
In this study a method is described whereby the dose distributions calculated by any treatment planning system (TPS) could be evaluated using dose distributions calculated with Monte Carlo simulations. The Monte Carlo dose simulations can be regarded as the golden standard. The method developed in this study involved the Monte Carlo simulation of a Philips SL75/14 based generic accelerator with the BEAM code. This was done to obtain beam information stored in phase space files that were characteristic of the generic accelerator. This beam data were then used for the simulation of dose distributions in a mathematical water phantom using the Monte Carlo code, DOSXYZ. The same beam data were used to generate the data base for the TPS that uses it for dose calculations in CT based patient models. The BATHO and ETAR inhomogeneity correction algorithms implemented on a CADPLAN TPS were evaluated. The CT slices that make up the patient model, on the TPS, were transformed to material data. Each of these materials (57 in total) covered a discrete CT interval in a total CT number range of 3000 CT numbers. Each of the 57 materials was represented in the preprocessor code (pEGS4) to allow dose simulations in realistic patient models with the DOSXYZ code. Dose distributions were calculated in a maxillary sinus (head), lung and prostate patient for photon beams with size 2x2, 5x5 and 10xlO cm2 . These dose distributions were calculated on the TPS using the BATHO and ET AR methods. The DOSXYZ dose distributions were scaled to the TPS calculated dose distributions by normalization to the dose in water at 2 cm depth on the beam central axis. Dose difference volume histograms, percentage depth dose curves and 2D dose distributions were obtained to evaluate these dose distributions. The BATHO and ET AR methods cannot model lateral and longitudinal electron transport through complex media. These effects were apparent in large inhomogeneities such as in the lung model where the Monte Carlo dose simulation gave a wider beam penumbra for the large field, and in the deviation of the TPS dose distributions in these regions for the small field size. The method developed in this study could also be applied to any IPS that uses more sophisticated models. Manufacturers of IPS's in particular could use the methods described in this study to evaluate their dose calculation algorithms. Key words: Monte Carlo, CT based patient model, DOSXYZ, BEAM, Treatment planning, dose distributions, lung, maxillary sinus, water phantom, lateral electron transport, TPS, inhomogeneity.
Open Access
Kwantifisering van radionukliedverspreiding deur planare beelding met 'n sintillasiekamera
(University of the Free State, 2000-06) Marais, Johan; Van Aswegen, A.
English: In this study attenuation and scatter correction techniques in planar imaging were investigated as well as the practical implementation of these techniques in clinical studies. The attenuation correction technique that was investigated was the geometrical mean method where an iterative method was used to determine the attenuation coefficient. This adaptation made the attenuation correction method independent of depth. In this study the depth dependence of the geometrical mean method was solved which is an important contribution. The three-energy window scatter correction technique was investigated with Monte Carlo methods for planar imaging. These simulations showed that the scatter correction method under corrects by approximately 10%. The scatter correction technique cannot be performed on it's own but must be used in conjunction with the attenuation correction method. The overall effect of the quantitation (attenuation and scatter correction) was determined in two ways. First a known amount of activity was imaged. Then the correction was applied to the images. Subsequently the corrected activity obtained from the latter images was compared to the known value. The calculated activity was 98,6 % of the known activity value. Secondly clinical studies were performed where the bladder activity was quantitated in vivo and correlated to the in vitro measured value. In this case the correlation coefficient was 0,996. The quantitation technique was applied to the clinical determination of the glomerular filtration rate. The quantitation technique was compared to the routine method that requires handling of urine. The correlation coefficient was 0.96 and the linear regression line had a slope ofO.91 and a y-axis intercept of4.8l. The quantitation of the bladder activity leads to a simplification of the clinical study procedure since the physical handling of urine can be eliminated. The accuracy of this method depends on the conversion factor that compensates for the difference between the sensitivity of the camera and the well counter. This factor must be checked on a regular basis to ensure the accuracy of this method. The bladder as source organ was used in this study. The bladder is a relative isolated organ and therefor the influence of radioactive uptake in other organs is relatively small. If these quantitations are applied to organs that are not isolated, i.e. the heart where the liver can influence it, this problem has to be address. In this project a quantitative planar imaging technique was investigated that corrects for attenuation and scatter and thereby facilitates depth independent corrections.
Open Access
Using Monte Carlo techniques to evaluate the dose distributions from a radiotherapy treatment planning system
(University of the Free State, 2000-06) Awusi, Kavuma; Lotter, M. G.; Willemse, C. A.
English: In this study we used Monte Carlo techniques to simulate the SL25 linear accelerator treatment head using the BEAM Code. The main purpose of study was to evaluate the dose distributions obtained by the CADPLAN treatment planning system (TPS) for 8 MV photon beams of a SL25 linear accelerator in realistic patient models. Simulation of the treatment head involves modeling of the main components of the treatment head that have influence on the absorption and scattering of radiation. Simulation of the accelerator was done in two parts to minimize the simulation time. Analysis of the data generated by the BEAM code was carried out using BEAMDP, another subsidiary of the BEAM code. We calculated the beam characteristics which are difficult to measure experimentally, such as angular distributions, spectral distributions, planar fluence and planar energy fluence at a plane located just above the jaws of the treatment head. The phase space files at the isocenter were used as source input for DOSXYZ, a MC code to calculate 3D dose distributions in water or CT based phantoms. The DOSXYZ code was used to calculate depth dose and cross plane profiles in a water phantom. The data obtained with Monte Carlo methods were compared with that obtained by ionization chamber measurements. Depth dose and cross plane profiles obtained by Monte Carlo methods and ionization chamber measurements generally agreed within 2%. We created patient models from CT data of real patients using the CTCREATE option of the DOSXYZ program. Dose distributions for a number of field sizes and different anatomical sites were calculated with the DOSXYZ code and compared with corresponding dose distributions calculated by the TPS. The modified BATHO and ETAR inhomogeneity correction methods used in the TPS were evaluated. Results show that Monte Carlo methods can accurately reproduce ion chamber measurements in a water phantom. Monte Carlo techniques are very useful for evaluating the accuracy of dose distributions generated by treatment planning systems in patient based models where measurements are impossible. The BATHO and ETAR methods showed comparable results to the Monte Carlo results. This could be due to the inefficiency of the method (visualization of the dose distributions) that we used for the comparison of the results. A more quantitative method like the use of the dose difference volume histogram could give a more comprehensive evaluation.
Open Access
An evaluation of the effect of scatter and attenuation correction of gamma photons on the reconstructed radionuclide distribution in the myocardial wall during spect imaging
(University of the Free State, 2000-11) Mdletshe, Nhlakanipho; Van Aswegen, A.; Du Raan, H.
The purpose of this study was firstly to evaluate the selection of reconstruction parameters (i.e. the number of subsets and the number of iterations) based on phantom studies. The second aim was to evaluate the effect of the non-uniform attenuation and scatter correction on myocardial perfusion studies performed on healthy volunteers as well as patients with proven inferior wall perfusion defects. The quality of the images from the phantom studies showed that 16 subsets with 2 iterations gave the best results if considering image noise and image resolution. These number of subsets and iterations were therefore used as reconstruction parameters in the patient studies The application of an attenuation correction to the emission data required that attenuation coefficient maps of the subjects were obtained from transmission images. I39Ce was chosen as the transmission source and used in conjunction with 99mTcas the emission source. The emission data were corrected for scatter according to the triple energy window method. In the healthy male and female volunteers, the attenuation and scatter corrected myocardial SPECT images showed an improvement in the homogeneity of the counts distribution compared to the uncorrected images. The counts distribution in the inferior region improved after the attenuation correction was applied, however it exceeded the counts in the anterior region. After applying a scatter as well as an attenuation correction to the emission data, the counts in the inferior region of the myocardium were slightly reduced. This was a result of the scatter correction eliminating scattered counts in the inferior region originating mainly from the liver. The apparent lower counts in the anterior region could be a result of too little compensation for scatter in the inferior wall, and needs to be investigated further. The defects in the three unhealthy patients, were not obscured after applying the scatter and attenuation correction to the emission data. The correction technique did not introduce false negative results in these patients. The application of scatter and attenuation correction techniques shows promising results for the interpretation of myocardial perfusion studies. These correction algorithms however need to be investigated thoroughly before being used in the routine clinical practice to avoid the introduction of artefacts.
Open Access
Commissioning and optimization of a total skin electron therapy technique using a high rate electron facility
(University of the Free State, 2007-06) Yousif, Yousif Abd Alla Mohammed; Willemse, C. A.
English: Total skin electron therapy (TSET) is the treatment of choice for several malignant diseases of the skin (Kaposi sarcoma, mycosis fungoides). Several different techniques have been developed in various centers, in order to achieve homogeneous dose distribution over a large irradiation field (200 x 80 ern"). However, to implement a TSET technique one has to account for a variety of parameters, from geometric (room design, space constrains) to physical (number, angle and energy of the beams). To obtain the most acceptable dose distribution an extensive set of measurements and a large number of calculations have to be performed. Therefore Monte Carlo simulation of TSET can facilitate optimization of this technique. In this study we implemented and optimized a TSET technique using 4 and 6 MeVelectron beams. The dosimetrie procedure intended to obtain adequate dose uniformity over the entire surface of the patient, and to reduce the patient treatment time using a high dose rate facility on the Elekta Precise accelerator. The EGS4/BEAM code package running on a Windows based platform was used for the MC simulation. Percentage depth-dose curves and beam profiles were calculated and measured experimentally for the 40x40 cm2 nominal field at both 100 cm SSD and at the patient surface at the treatment plane (SSD 350 cm) for a single beam. The accuracy of the simulated beam was validated by the good correspondence (within less than 2%) between measured beam characteristic parameters (Rso, dmax, Rp) and Monte Carlo calculated results. To obtain a uniform profile vertically, two vertical angles of incidence were used. The angle between the two beams that gave best uniformity was considered the optimum angle. The patient is to be placed on a rotating platform perpendicular to the beam and rotated through 60 degree increments to obtain six horizontal directions of beam incidence. The doses expected in the patient were measured with Kodak EDR2 films positioned at different levels between slices of a Rando phantom. TLDs were placed on the surface to relate the film measurements to dose. The delivered doses in the treatment plane were compared to simulated data that was obtained from the MC simulation. The penetration depth of the dose distribution varied over various scanning directions between 2-3 mm and 3- 4 mm for 4 and 6 MeV respectively. This information is useful when treatment of lesions of different thickness are being considered. The composite percentage depth dose of all six dual fields for both 4 and 6 MeV yielded an 80 % dose at - 7 mm and - 9 mm depth, respectively. Good dose uniformity was achieved for both energies and it was about ± 5% for 4 MeVand about ± 3% for 6 MeVover a range of - 100 to +100 cm. The bremsstrahlung contamination was 0.9 and 1.3 %. Generally there was good agreement between the dose distribution calculated with MC and measured with films, thus validating our MC calculations. The dose distributions in phantom were found to comply with the guidelines described in the AAPM TG-23 protocol, showing the suitability of this technique for treatments of the skin diseases. The HDRE is a useful operational mode providing reasonable output, field size, and Xray contamination. Use of a dual field technique produces reasonable beam uniformity over an area large enough to allow total skin electron therapy in a conventional treatment room. Monte Carlo techniques provided a guiding principle to assist the verification of the beam characterization of a TSET technique. The absolute calibration of dose to the patient required the measurement of the ratio "skin dose to calibration point dose"; thiswas achieved by measurements with a parallel plate ionization chamber and TLDs.
Open Access
Manual quantification for right and left ventricular function using cardiac magnetic resonance imaging
(University of the Free State, 2012-07) Willemse, Nanette; Rae, W. I. D.; Herbst, C. P.
Abstract not available
Open Access
Development of a particle source model for a synergy linear accelerator to be used in Monte Carlo radiation dose calculations for cancer therapy
(University of the Free State, 2014-05) Van Eeden, Dete; Du Plessis, F. C. P.
English: In oncology patients are treated for cancer with various methods such as surgery, chemo therapy and radiation therapy. Accurate radiation treatment planning and dose delivery to the tumour is necessary for the successful outcome of cancer treatment. In order to achieve this goal accurate radiation dose calculation codes must be utilized. EGSnrc based Monte Carlo (MC) codes such as BEAMnrc and DOSXYZnrc have been developed for just this purpose. The problem that arises in using these MC codes is that they lack suitable x-ray beam source models. These models must be accurate in order to replicate the true clinical x-ray beam emanating from the linear accelerator. One such machine for which radiation source data must be derived is currently being used at the Oncology department in Universitas Hospital Annex. It is desirable to model this linear accelerator in order to perform MC based dose calculations for radiation treatment. The use of MC based dose calculations is certainly not new in the radiation physics environment. Various authors have studied the replication of radiation beam characteristics using source models to simulate the phase-space parameters of particles produced by the linear accelerator. These parameters include the charge, energy, direction, and position of each particle as it crosses a certain reference plane below the linear accelerator. An accurate source model should be able to re-generate particles with the exact set of above-mentioned parameters as would be produced by the real linear accelerator. Sources can be very simple such as a single point from which the particles are radiating with a single invariant energy spectrum. Studies have shown that these beam models can yield accurate beam data over relatively small field sizes and is not general enough to use over a whole range of clinically useful field sizes. A graphical user interface (GUI) was developed that can assist in the construction of the source model. The source model can describe energy and fluence distributions for photons and electrons as separate point sources each with their own SSD. The accuracy of the model was validated by comparing simulated profiles with measured data for an Elekta Synergy linear accelerator. The modified Schiff formula was used to derive the bremsstrahlung spectra emanating from the target. The x-ray fluence Gaussian distribution consisted of the primary fluence from the target, which was modified by the primary collimator, secondary collimators as well as the multileaf collimators. The truncation and beam scatter caused by the face of the collimators were modelled with error functions. Exponential functions were used to model off-axis collimator transmission. Profiles and percentage depth dose curves were obtained with the source for square field sizes of 1 × 1 cm2 up to a 40 × 40 cm2. Offset fields for 10 × 10 cm2, 15 × 15 cm2 and 20 × 20 cm2, rectangular fields as well as wedged fields were included. Irregular field shapes were simulated to evaluate the source model‘s capability of reproducing complex treatment fields. Film dose verification was done in an anthropomorphic Rando® phantom and compared with the MC source model for 6 MV x-ray beams. A criterion of 2% / 2 mm was used to compare MC data and measured data. This study demonstrated that a diversity of field sizes and percentage depth dose curves can be modelled within 2% / 2 mm. The model can replicate irregular field sizes used for complex treatments. Minor discrepancies were found for the relative dose comparisons between the MC and film data for the anthropomorphic phantom.
Open Access
The effect of tumour geometry on the quantification accuracy of 99mTc and 123I in planar phantom images
(University of the Free State, 2014-08) Ramonaheng, Keamogetswe; Van Staden, J. A.; Du Raan, H.
English: Accurate activity quantification is important for its application in radiation dosimetry. Planar image quantification plays an important role in the quantification of whole body images which provide a full assessment of bio-distribution from radionuclide administrations. In the Department of Nuclear Medicine at Universitas Hospital, 123I meta-iodobenzylguanidine [123I]-MIBG quantification of neuroendocrine tumours is performed prior to therapeutic radionuclide treatment. The bio-distribution of activity in these studies is mostly in the abdominal region. Factors influencing quantification include scatter, attenuation, background activity and close proximity of organs with radioactivity uptake. The aim of this study was to evaluate the effect of tumour geometry on the quantification accuracy of 99mTc and 123I in planar phantom images, by applying scatter and attenuation corrections, with the focus on neuroendocrine tumours. The tumour geometry investigated included: various tumour sizes, various tumour-liver distances and two tumour-background ratios (0.5 % and 1.0 %). The quantification technique was first developed with the readily available 99mTc and subsequently applied to the more costly 123I used for imaging neuroendocrine tumours. Adjustments were necessary due to the difference in physical properties between the two isotopes. An in-house manufactured abdominal phantom was developed to mimic the clinical geometries under investigation. The phantom was equipped with cylindrical inserts used to simulate tumours (diameters of the tumours were 63 mm, 45 mm, 34 mm and 23 mm) and a slider to vary the tumour-liver distance. The processing technique incorporated the use of the geometric mean method with corrections for scatter and attenuation performed on image counts. Scatter correction was performed using a modified triple energy window scatter correction technique for 99mTc and 123I, according to gamma camera manufacturer specifications. Attenuation correction was performed using transmission images obtained with an uncollimated 99mTc printed source. Scatter contribution from the abdominal phantom and transmission source combination was limited by setting the detector transmission source distance to 73 cm. A system calibration factor, processed in the same manner as the tumour quantified data was used to convert the image counts to units of radioactivity. Partial volume effect (PVE), was compensated for by the manner in which regions for tumour activity distribution were defined. The activity measured in the dose calibrators served as a reference for determining the accuracy of the quantification. The largest percentage deviation was obtained for the smallest tumours. The average activity underestimations were 29.2 ± 1.3 % and 34.6 ±1.2 % for 99mTc and 123I respectively. These large underestimations observed for the smallest tumours were attributed to PVE, which diminished with increasing tumour sizes. Better quantification accuracy was observed for the largest tumour with overestimations of 3.3 ± 2.6 % and 3.1 ± 3.0 % for 99mTc and 123I respectively. PVE compensation resulted in improved quantification accuracy for all tumour sizes yielding accuracies of better than 9.1 % and 12.4 % for 99mTc and 123I respectively. Scatter contribution to the tumours from the liver had minimal effect on the quantification accuracy at tumour-liver distances larger than 3 cm. An increased tumour-background ratio resulted in an increase in the quantification results of up to 16.6 % for calculations without PVE compensation. This contribution was increased to 26.3 % when PVE were compensated for, using larger regions. The literature often report accurate planar quantification results, however, this study shows that it is important to consider the specific tumour geometry for the study. It remains the responsibility of the user to evaluate the clinical available software and implement it in a responsible manner. When applying all relevant corrections for scatter, attenuation and PVE without significant background, quantification accuracy within 12 % was obtained. This study has demonstrated successful implementation of a practical technique to obtain planar quantitative information.
Open Access
Development and validation of an X-ray model for an Elekta Precise multileaf collimator to be used in Monte Carlo dose calculations
(University of the Free State, 2015-01) Smit, Jacobus Johannes Lodewikus; du Plessis, F. C. P.
English: Linear accelerators (Linacs) produce megavoltage (MV) energy photon and electron beams to irradiate tumour volumes in patients. More complex field shapes can be setup quickly with multileaf collimators (MLC’s), thus more advanced treatments like intensity-modulated radiation therapy (IMRT) are possible. This is one of the reasons why treatment planning models should be accurately commissioned and accurate dose calculation algorithms employed. Monte Carlo (MC) based dose calculations are very suitable to solve this issue. The aim of this project was to continue the development of an X-ray source model for MC dose calculations for an ElektaTM Precise MLC Linac. Methods & Materials An in-house developed graphical user interface (GUI) was used to calculate exit fluence based on a mathematical model and energy spectra derived from the Schiff formula. This produced an input source file for source number 4 in the DOSXYZnrc code. DOSXYZnrc was used to calculate X-ray dose distributions in water and RW3 solid water phantoms. These dose distributions were compared to actual measured film or water tank dose data. A gamma index was calculated to compare the MC and measured dose. The criteria used for the γ-index was 2 % dose / 2 mm distance-to-agreement. Dose distribution data for square, rectangular and off-set fields were compared. Results & Discussion Prior to source commissioning a GAFCHROMIC® EBT2 film dosimetry system, that entails using a film scanner, was setup. With the use of the EBT2 film, scanner properties like scanner uniformity, film orientation, film scanning side and repeatability were investigated. Film orientation produced the largest discrepancy of 3.5 % between portrait and landscape orientation. The remaining properties were within 1 % variation. A range of fields for 6, 8 and 15 MV beams were modelled, simulated and compared to corresponding measured water tank data. Parameters in the MC source model were adjusted until the gamma-index criteria were met for each comparison. These source parameter values were retained for further more complex field simulation and evaluation against measurements. Rectangular, small and medium sized off-set fields met the gamma-index criteria. For off-set fields greater than 15×15 cm2, the model failed the criteria at some dose points at the field edges. The jaws and MLC transmission parameters required adjustments for the irregular MLC field shapes comparisons. Conclusion The source model performed well and can be employed for dose verification ranging from simple regular fields to conformal treatments. In order to use the model for IMRT treatment verification, the model needs to be validated clinically. The only requirement is that Linac scatter factors must be measured separately to calculate the correct amount of monitor units necessary for patient treatment. Additional scatter sources can be implemented in the model to increase the accuracy at the field edges of the off-axis cases, which will require verification.
Open Access
Verification of a commercial treatment planning system based on Monte Carlo radiation dose calculations in intensity modulated radiation therapy
(University of the Free State, 2015-01) Strauss, Lourens Jochemus; Du Plessis, F. C. P.
English: Cancer treatment with external beam radiotherapy using the specialized technique of intensity modulation is a complex modality. The Treatment Planning System (TPS) is responsible for accurate calculation of dose to allow the radiotherapy team to make decisions on the patient treatment. The commercial TPS, XiO, utilizes a Multigrid Superposition algorithm as dose calculation engine, which is model based. Several approximations are inherent in this method. In-depth quality assurance (QA) of Intensity Modulated Radiation Therapy (IMRT) plans is necessary, and these tests are time-consuming and reduce the available clinical treatment time. Monte Carlo (MC) has been proven to be the most accurate method of radiation dose calculation. MC is a direct dose calculation method, and the EGSnrc codes are well suited for linear accelerator (linac) simulations. This study aims to be a first step towards full MC-based dose verification for IMRT dose distributions produced on XiO: developing the system and demonstrating the accuracy thereof. A generic virtual linac based on a typical Elekta linac was constructed using the EGSnrc MC software (BEAMnrc and DOSXYZnrc), for beam energies of 6 and 10 MV respectively. Simulations were either run on a watertank model or in air to produce beam data required for commissioning on XiO. Beam profiles, Percentage Depth Dose (PDD) curves and scatter factors for collimator and total scatter were extracted from the data. Software was developed to convert data to a format readable by the TPS. Modelling was done on XiO for all fields. A software graphical user interface (GUI) was developed to extract necessary information from dicom files required for MC calculations. This included CT data extracting and converting to EGSnrc format, reading all plan details, and creating scripts for automatic MC dose calculation execution. IMRT plans were created for 3 different treatment sites using the newly commissioned model on XiO. The modelling and simulation process was verified with MC dose calculations in scanned phantoms. After simulation, the IMRT plans were evaluated with isodose/profiles and 2D gamma analysis, as well as dose difference maps and Dose Volume Histogram (DVH) comparisons. The generic linac could successfully be created on BEAMnrc, and produced clinically acceptable beams. The data for commissioning was also generated successfully, and could be extracted and read into XiO after some de-noising filters were applied. Modelling on the TPS was done to an overall agreement level of 3%/3mm and 2%/2mm for small fields. Doses in the Prostate and Head-and-Neck IMRT plans compared well between XiO and MC for both energies. Gamma pass rates were above 90% for a criterion of 3%/2mm in a region of interest (ROI) covering the target and critical organs. Only slight overestimation of dose in bony regions was observed. The Esophagus IMRT plans however indicated some discrepancies in the dose calculation of XiO, especially in the low density regions, like lung. The 2D gamma pass rates were low, and DVH comparison indicated large overestimation of dose in the target volume, as well as in the Spine, as a direct consequence of errors in dose calculation of low density media. It is concluded that a dose verification system could successfully be developed for comparison of IMRT plans. Accurate modelling on the TPS was a vital step, and some possible issues were addressed. The system can be used routinely, and doses are calculated in a reasonable time with differences presented in a practical manner. The dose calculation of IMRT plans on XiO was compared to MC dose and found to be accurate for most treatment sites, independent of beam energy. However, caution is advised for cases where beams are directed through low density media, as clinically significant effects can possibly occur in patients.
Open Access
Evaluation 99mTc and 123I quantification using SPECT/CT
(University of the Free State, 2015-02) Mongane, Modisenyane Simon; Van Staden, J. A.; Du Raan, H.
English: A review of Single Photon Emission Tomography (SPECT) quantification shows that different protocols and phantoms are available to evaluate SPECT quantification accuracy. This study was necessitated by the lack of standardized protocols and the widespread use of a variety of non-standard phantoms. The aim of this work was to evaluate the influence of the geometry of a radionuclide distribution on SPECT quantification accuracy for 99mTc and 123I isotopes in an abdominal phantom. In order to achieve the aim, the following steps were taken: The preparatory phase of the study was to design and construct an abdominal phantom, verify the accuracy of the attenuation coefficients obtained with the Computed Tomography (CT) scanner, determine the accuracy of the source calibrator used in this study and then obtain a calibration factor in order to convert image counts to activity. During the quantification phase SPECT data were acquired, the influence of not applying scatter correction explicitly was evaluated and the final quantification was performed using the proposed standard clinical reconstruction protocols. The influence of different tumour sizes and locations in the abdominal phantom relative to a high uptake organ on the quantification accuracy was evaluated. Finally, parameters in the Ordered Subset Expectation Maximization (OSEM) reconstruction protocol were altered in order to investigate the influence of number of subsets and iterations on the quantified data. The non-standard Density Phantom with five different compounds was used for the verification of the 99mTc and 123I attenuation coefficients. The percentage difference between the measured and theoretical attenuation coefficients values were < 3%, except for Polystyrene (85% and 65% respectively). The SPECT calibration factor was determined for both 99mTc (11.0 ± 1.3 cpm/kBq) and 123I (10.8 ± 0.3 cpm/kBq) using the Cylindrical Phantom. The in-house built Abdominal Phantom was used to evaluate the tumour activity quantification accuracy. The quantification accuracy of 99mTc and 123I was found to change significantly (p < 0.05) as a function of tumour size after corrections for “spill out” counts due to the partial volume effect, scatter and attenuation were applied. On the other hand, there was no significant difference in the quantification accuracy (p > 0.05) for each tumour at different tumour-liver distances when appropriate scatter and attenuation corrections were applied. The influence of OSEM parameters showed no dependence on the tumour-liver distance and no significant difference (p > 0.05) between quantification with background activity as compared to no background activity. In conclusion, the study showed that the quantification accuracy for 99mTc and 123I was comparable to other published studies. It was found that the tumour quantification accuracy is not influenced by proximity of high uptake organs when appropriate correction factors were applied. Tumour size influenced the accuracy of SPECT quantification for both radionuclides. The results of this study also showed that at least 128 Maximum Likelihood Expectation Maximization (MLEM) equivalent iterations were needed during iterative reconstruction to achieve convergence and consistent SPECT quantification accuracy. Finally, it is recommended that the evaluated quantification protocol may be used in our nuclear medicine clinic for 99mTc and 123I quantification.
Open Access
A rabbit phantom study to reduce neonatal radiation dose without compromising image quality.
(University of the Free State, 2015-02) Erasmus, Anita; Herbst, C. P.
English: Neonates who are ill at birth often require a large number of radiographs during the period of hospitalization. Radiography refers to the imaging of body parts using x-rays and plays an important role in the diagnosis and follow-up of patients suffering from various medical conditions. X-rays fall into the category of ionizing radiation which has the potential to cause cell damage and therefore are associated with a radiation risk. The radiation risk to these neonates is higher than in adults. Therefore the dose should be kept as low as reasonable achievable (ALARA) while still rendering images that comply with the diagnostic task. Large variations have been found with respect to the beam parameters used to acquire neonatal anterior posterior (AP) chest x-ray images, emphasizing the need for optimization. Changing the beam parameters will not only influence the radiation dose, but also the image quality. Although physical image quality metrics such as contrast, resolution and noise can be used to quantify image quality these metrics do not give an indication of the diagnostic image requirements. Subjective image evaluation using appropriate phantoms is therefore also required. Aim The aim of this phantom study was to reduce neonatal radiation dose while maintaining diagnostically adequate image quality for neonatal radiography. Methods Entrance surface doses (ESD) and exit doses were determined for the beam parameters (55 kV, 2 mAs no additional filtration) used to acquire neonatal AP chest x-rays at Universitas Academic Hospital (UAH). Beam parameter combinations, reducing the ESD by 50 % or more compared to the current practice, were then selected keeping the dose to the Computed Radiography imaging plate (detector) constant. The effect of using different beam parameters on the image quality was investigated, using a chicken corpse as a phantom to simulate the neonatal chest. The contrast as well as contrast to noise ratios were determined. Ten observers comprising radiographers, radiologists and a medical physicist were then tasked to evaluate the overall subjective image quality using a multiple rank order method. Based on the image quality ranking the beam parameters delivering acceptable image quality were then used to image five living (sedated) rabbits with a weight of approximately 2000g, simulating the neonatal chest with a heart and lungs. Eight radiology registrars gave their opinion on the diagnostic quality of the images using a five-point scale. These radiologists then evaluated the image quality subjectively using the multiple rank order method with the focus on three anatomical regions namely the lung patterns, the heart borders and the diaphragm. Results and Discussion The contrast and contrast to noise ratio stayed constant in the range 55 – 66 kV, indicating the influence of post processing on digital images. Large variation in observer opinion of image quality was seen for both the chicken and rabbit phantom images obtained with different beam parameters (different dose values). Even amongst a single observer large variation in their opinion was seen when evaluating images of different rabbits at the same beam parameters. All the images were regarded as being diagnostically acceptable. This indicates that a lower dose option could be selected without compromising the diagnostic value of the images. Conclusion The chicken phantom proved to be a good simulation of the neonatal chest; however when one needs to investigate clinical features the rabbit phantom would be recommended. This study shows a possible dose reduction of 64 % when using beam parameters 60 kV, 2.0 mAs and 2 mm Al additional filtration. Afrikaans: Inleiding Siek neonate benodig ‘n groot aantal x-straal beelde tydens hospitalisasie. X-straal beelde val in die kategorie van ioniserende straling en hou dus ‘n stralingsrisiko in. Hierdie stralingsrisiko is hoër in neonate as in volwassenes. Dit is om hierdie rede belangrik om te verseker dat die laagste moontlike stralings dosis gebruik word. Die variasie in bundel parameters (kV, mAs en filtrasie) wat gebruik word vir neonatale anterior posterior borskas x-straal beelding dui op die noodsaaklikheid van optimalisering. ‘n Verandering in die bundel parameters beinvloed nie die stralings dosis alleenlik nie, maar ook die beeld kwaliteit. Alhoewel beeldkwalitiet parameters soos kontras, resolusie en geruis gebruik word om die beeld kwaliteit te kwantifiseer gee die parameters egter nie ‘n aanduiding of die beeld aan die diagnostiese vereistes voldoen nie. Vir hierdie doel is subjektiewe evaluasie van beeldkwaliteit met gebruik van die geskikte fantome genoodsaak. Doel Die doel van die fantoom studie was om die neonatale stralings dosis te verminder sonder om die diagnostiese beeldkwaliteit te verlaag. Metodes Die ingangsdosis en uitgangsdosis was bepaal vir die bundel parameters (55 kV, 2 mAs en geen addisionele filtrasie) huidiglik in gebruik by die Universitas Akademiese Hospitaal (UAH). Bundel parameter kombinasies wat die ingangs dosis met 50 % of meer verminder het, terwyl die detektor dosis konstant gehou is, is gebruik. ‘n Hoender fantoom is gebruik om die neonatale borskas te simuleer in ‘n poging om die effek van die gekose bundel parameters op die beeldkwalitiet te ondersoek. Die kontras en kontras-tot-geruis vehoudings is bepaal. Tien waarnemers wat radiograwe, radioloë en ‘n mediese fisikus insluit, het die beelde geëvalueer op grond van algemene beeldkwaliteit mbv. ‘n veelvuldige rangorde metode. Die bundel parameters wat aanvaarbare beeldkwaliteit gelewer het, is gebruik om x-straal beelde van vyf lewendige (gesedeerde) konyne, met ‘n massa van ongeveer 2000g, te verkry. Die konyne is gebruik om die neonatale borskas met ‘n hart en longe te simuleer. Agt radiologie kliniese assistente het die konyn beelde geëvalueer m.b.v. ‘n vyf-punt skaal. Die beeldkwaliteit is ook subjektief geëvalueer mbv. die veelvuldige rangorde metode met die fokus op drie anatomiese dele naamlik die long patrone, hart rande en diafragmas. Resultate en Bespreking Die kontras en kontras-tot-geruis verhoudings het konstant gebly in die reeks 55 – 66 kV. Dit gee ‘n aanduiding van die invloed wat “post” verwerking op die digitale beeld het. Groot variasies in waarnemer opinie van beeldkwaliteit is waargeneem, in beide die hoender en konyn fantoom beelde, met die gebruik van verskillende beeld parameters. Die verskille was selfs waargeneem vir ‘n enkele waarnemer tydens die evaluasie van die vyf konyn beelde by ‘n bundel parameter kombinasie. Al die beelde was van aanvaarbare diagnostiese kwaliteit wat aandui dat ‘n bundel parameter kombinasie met ‘n laer stralings dosis gebruik kan word. Gevolgtrekking Die hoender fantoom het die neonatale borskas goed verteenwoordig. Die konyn fantoom word egter aanbeveel wanneer gedetaileerde kliniese strukture evalueer moet word. Die studie bewys dat ‘n dosis vermindering van 64 % moontlik is met die gebruik van bundel parameters 60 kV, 2.0 mAs en 2 mm Al addissionele filtrasie.
Open Access
Optimisation of delivery efficiency in prostate intensity modulated radiotherapy planning
(University of the Free State, 2016-02) Fourie, Nicola Sieglinde; Muhammed, Omer Abdul-Aziz Ali; Rae, William Ian Duncombe
English: Evidence that supports dose escalation for prostate cancer is growing and with Intensity Modulated Radiation Therapy (IMRT) higher conformal target doses can be delivered. With more segments and higher monitor units (MU’s), target conformity can be improved, however this results in longer delivery times, which makes it difficult to ensure accurate dose delivery, as intra-fractional as well as target movement plays an increasing role. Evidence from the literature indicates that secondary radiation-induced cancer risk is proportional to the beam-on time (thus the MU’s). Improvements in IMRT delivery efficiency while maintaining plan quality can be achieved by reducing the complexity of an IMRT plan. This can be done by changing the optimization parameters during the optimization process. Less “complex” prostate IMRT plans will require fewer MU’s by using less segments resulting in shorter delivery times and therefore reduced risk of secondary cancers. The goal of this study was to recommend a set of optimization parameter values that will improve the delivery efficiency of prostate IMRT treatment plan while maintaining plan quality. Fifteen clinical prostate IMRT plans (15 MV), already used for treatment, were re-optimized, using a XiO treatment planning system (TPS). Changes in total MU’s and segments were evaluated for changes in some of the optimization parameter values. Eleven optimization parameters (some of them used more than once with different values) were used to generate 15 new IMRT combination plans (ICP’s) for each patient for both 6 and 15 MV, resulting in 450 plans being assessed. One parameter was changed at a time while all other variables were kept constant. Plan quality was evaluated in terms of four variables: MU, number of segments, homogeneity index and conformity index while the delivery efficiency was evaluated in terms of delivery time. To our knowledge no time delivery model has been proposed for a Siemens® ARTISTETM Linear Accelerator (Linac). Using the principles given in the literature we derived such a time delivery model by adding the radio frequency wave component and Multi Leaf Collimator delay time. K-means clustering was then used to analyse the data in terms of the five variables and the top 10 ICP’s in 3 patients in terms of a faster more conformal, delivered plan were identified. To confirm the delivery efficiency and accuracy, the fluences of these top 10 ICP’s were measured on a Siemens® ARTISTETM Linac with the step and shoot method and compared to the treatment planning system’s fluences. The evaluation criteria chosen were 3% and 3 mm, distance to agreement. A 3 dimensional dose volume histogram program was used to determine the percentage pass rates on the planned target volumes and the organs at risk. The optimization parameters such as the minimum MU’s per segment, intensity level, minimum segment size and minimum segment area; demonstrated the greatest influence on the total number of segments, while the total MU’s was most greatly influenced by the filters and intensity level optimization parameter. Controversy exists regarding which energy should be used, 6 MV or 15 MV, when treating prostate cancer. Both energies were considered here during the optimization process and it was concluded that the optimization parameters are not greatly influenced by the beam energy. However, it was seen that beam arrangement has an influence on optimization parameter behaviour. A limitation of this study is that the beam angle distribution was not investigated. Thus recommendations could be made in terms of which ICP demonstrated the most improved delivery efficiency of a prostate IMRT treatment plan while maintaining plan quality. The optimisation parameter which was introduced to the optimization process was a General High filter. Gaining knowledge about the behaviour of the optimization parameters during optimization makes it easier to advise and assist treatment planners preparing complex IMRT plans.
Open Access
Development and validation of a molecular assay and evaluation of the GeneXpert® MTB/RIF assay for the rapid detection of genital tuberculosis
(University of the Free State, 2016-02) Sokhela, Maxwell Cebolenkosi; Goedhals, D.; Hoosen, A. A.
English: Tuberculosis (TB) is a communicable disease which is caused by the bacterium Mycobacterium tuberculosis (MTB). According to the World Health Organization, globally in 2015 there were 10.4 million new cases and 1.4 million deaths due to TB. TB is one of the leading causes of death in South Africa resulting in approximately 8.4% of deaths in 2015. The most common manifestation of TB involves the lungs, defined as pulmonary TB (PTB), while TB affecting other organs is defined as extrapulmonary TB (EPTB). EPTB accounts for only 20% of all TB cases in human immunodeficiency virus negative individuals. Approximately 1.8% of all TB cases have a genitourinary site, with the prevalence of genital TB (GTB) in South Africa reported to range from 6.2-21.0%. One of the leading symptoms of GTB in females is infertility, usually resulting from the involvement of the fallopian tubes and endometrium. Approximately 40-80% of women with GTB will become infertile. The detection of microorganisms through microscopy is the oldest technique for laboratory diagnosis. While microscopy is rapid and inexpensive, it requires a high bacterial load which is not present in paucibacillary EPTB samples. Culture of MTB is widely regarded as the gold standard for TB diagnosis. While culture has a long turnaround time, culture remains important since it is more sensitive than microscopy. In addition, growth is required for species identification, drug susceptibility testing and genotyping of cultured organisms may be useful for epidemiological studies. Little is known regarding which technique is best for the detection of GTB from clinical samples apart from culture. Molecular based techniques hold the promise of a more rapid and accurate diagnosis of EPTB. The aim of this project was the development and validation of an in-house nested PCR assay and the validation of the GeneXpert® MTB/RIF (GeneXpert) assay for the laboratory diagnosis of GTB. In total 54 samples were submitted for GTB screening from women being investigated for infertility at the Unit for Human Reproduction, Universitas Academic Hospital, Bloemfontein. This included 44 endometrial tissue samples and 10 menstrual fluid samples. All samples underwent testing with the GeneXpert, the in-house nested PCR and culture. The nested PCR was designed targeting the insertion sequence element 6110 (IS6110) found in members of the MTB complex. The analytical sensitivity/limit of detection (LOD) for the GeneXpert was determined to be 250pg while the LOD for the nested polymerase chain reaction (PCR) was 62.5fg. Both assays displayed excellent analytical specificity by discriminating TB deoxyribose nucleic acid (DNA) from other bacterial and nontuberculous mycobacterial DNA. The diagnostic sensitivity and specificity was determined using culture as the reference method. Culture was able to detect GTB in 2 of the 54 samples including one menstrual fluid and one endometrial tissue sample, thus indicating a GTB prevalence of 3.7%. The GeneXpert detected 1 of the 54 samples as positive indicating a sensitivity of 50% and a specificity of 100%. The nested PCR detected both positive samples resulting in a sensitivity and specificity of 100%. The GeneXpert obtained a positive predictive value (PPV) of 100% and a negative predictive value (NPV) of 98.1%, while the nested PCR obtained a PPV and NPV of 100%. The two GTB isolates underwent genotyping using spoligotyping and mycobacterial interspersed repetitive unit – variable number of tandem repeats (MIRU-VNTR). The menstrual fluid isolate was characterised as a Beijing strain and the endometrial tissue isolate as an X3 strain. The nested PCR showed a greater sensitivity than the GeneXpert as a result of the better LOD. Despite this, both techniques could be implemented for GTB screening in combination with culture. Screening of menstrual fluid samples using the GeneXpert assay would be well suited for GTB screening in resource limited areas.
Open Access
Monte Carlo study on megavolt x-ray therapy for development of suitable targets for the evaluation of nano particle dose enhancement
(University of the Free State, 2016-09) Mutsakanyi, Stalyn; Du Plessis, F. C. P.
English: INTRODUCTION: Radiation dose enhancement with nanoparticles is a treatment technique involving the irradiation of tumour seeded with high atomic number (high Z) material. This work describes the generation of x-ray beams using a 6 MeV Elekta Precise linac head using low-Z Bremsstrahlung target materials, water and carbon combined with tungsten. The aim of the study was to simulate photon energy spectra appropriate for high-Z nanoparticles dose enhancement in tumour using EGSnrc MC codes. MATERIALS AND METHOD: BEAMnrc Monte Carlo (MC) code successfully modelled the treatment head components of a flattening filter free 6 MV Elekta Precise linear accelerator. Simulations were run using suitable histories to generate high energy x-ray beams of differing quality from electron spectra obtained using 6 MeV electron beam. Water and carbon layers were the primary target which were inserted in the path of the 6 MeV electron pencil beam before it hits the tungsten Bremsstrahlung target to act as moderators that slow down electron before they hit a tungsten layer. The electron spectra obtained just after the primary target was used as the incident beam to the tungsten target which acts as the secondary target to generate x-ray photon beams. Therefore the x-ray beam source target was either water/tungsten or carbon/tungsten combination. Different photon spectra were obtained for investigation in nanoparticles (NPs) based photon therapy. An original linac using a normal tungsten target of 0.3 cm thickness was also simulated to benchmark the results. The photon spectra obtained below X,Y jaws were used as input sources in the DOSXYZnrc MC code to simulate dose distribution in water and a patient CT phantom. The simulations were carried out using source 2 in DOSXYZnrc. A 40 x 40 x 40 cm3 water phantom was simulated at 100 cm SSD using a range of field sizes to characterize the beams. The phantom had voxel size of 0.2 × 0.2 × 0.2 cm3. The photon beams were characterised in terms of percentage depth doses and beam profiles. These x-ray beams were then used to quantify the variation of tumour dose enhancement in a constructed patient CT phantom. The prostate tumour was used as the planning target volume (PTV). The PTV composition was either a tumour only or a tumour volume seeded with atoms of gold nanoparticles with concentration of 7mg/g of tumour. These tumour/NPs model was manually drawn on to the CT dataset from actual CT images of the patient using MCSHOW graphical user interface (GUI). The tumour composition was made part of the patient CT data set using a locally-developed Interactive Data Language (IDL) code that converts the density of the drawn volume into the desired tumour density. The 3DCRT was used as the treatment strategy and 4, 5 and 6 field beams were investigated. With this model, we were able to estimate more accurately the effect of altered beams on NPs radiation dose enhancement. For both simulations using BEAMnrc and DOSXYZnrc the electron cut-off energy (ECUT) and photon cut-off energy (PCUT) was 0.521 MeV and 0.01 MeV respectively. The number of histories was chosen so that the statistical uncertainty along the CAX had an average value of 1% at 0 – 30 cm depth. RESULTS AND CONCLUSION: The results showed that the use of electron moderators in generating x-ray beams for use in NPs seeded tumours can lead to a significant dose enhancement. Photon spectra obtained with water/tungsten or carbon/tungsten Bremsstrahlung targets combinations showed significant changes at various target thickness. There is a significant dependence of dose enhancement factors (DEF) on the mean energy of the x-ray beams as well as the target thickness. DEFs ranging from 0.05% to 7.5% were obtained at various Bremsstrahlung target combinations. Based on the results, carbon is more efficient at moderating the electron beam to generate photon beams for dose enhancement at lower thickness (approximately 1.4 cm) compared to water (approximately 2.5 cm), although water can just be as good at larger thickness. At these thicknesses the mean photon beam energy is approximately 0.4 MV. In summary, the results of this work indicate that the use of photon beams from low-Z Bremsstrahlung targets as electron slowing down medium could enable significant clinical dose enhancement during external beam radiotherapy for NPs seeded tumours. MC techniques showed to be valuable tools for dose calculations in both water and patient CT phantom.
Open Access
Monte Carlo evaluation of the dose perturbation effect of various hip prostheses during pelvic megavoltage photon radiotherapy
(University of the Free State, 2016-11) Mahuvava, Courage; Du Plessis, F. C. P.
English: Introduction: Hip prostheses (HPs) are routinely used in hip augmentation surgery to replace painful or dysfunctional hip joints, especially in the elderly population. A number of patients with HPs are undergoing pelvic radiotherapy (RT) for localised prostate cancer. However, radiographic discrepancies between high-density and high-atomic-number (Z) inserts and surrounding tissue may cause considerable dose perturbations within the target volume and in regions where tissues interface with the prosthetic device. Furthermore, conventional treatment planning systems (TPSs) do not accurately predict dose effects incurred around metallic implants. Therefore, concerns regarding dose inhomogeneities near the prosthesis always arise, especially in patients with bilateral hip prostheses (bHPs) who require teletherapy of prostate cancer, where the tumour typically lies between the prostheses. The aim of this study was to evaluate the dosimetric effect of various HPs during 3D conformal prostate RT using Monte Carlo (MC) simulations. Materials and methods: The MC radiation transport simulation user-code BEAMnrc was used to simulate an Elekta Precise linear accelerator (linac) head, based on the manufacturer’s specifications. The MC linac model was validated by comparing dosimetric features including depth dose and dose profile data simulated in a cubic water tank (WT) with measured values. DOSXYZnrc was used to calculate 3D absorbed dose distributions in a CT based phantom (patient model) with and without HPs. Simulations were performed for 6, 10, 15 and 20 MV conformal photon beams using different beam arrangements. Three treatment plans were generated by XiO TPS and incorporated into MC simulations: a four–field (4F) box plan, a five–field (5F) plan and a six–field (6F) plan. The planning target volume (PTV) was generated by a 1 cm expansion of the prostate alone. The HP materials used were stainless steel (SS316L), titanium (Ti6Al4V) and ultra-high-molecular-weight-polyethylene (UHMWPE). These prosthetic models were manually drawn into the CT dataset from actual CT images of the patient pelvis using MCSHOW graphical user interface (GUI). The prosthesis was made part of the patient using a locally-developed Interactive Data Language (IDL) code that converts the density of the drawn volume into the desired HP material density. Both unilateral and bilateral models were considered in the simulations and dose perturbation factors (DPFs) were calculated on the proximal and distal interfaces of the implant. The dose reduction in the PTV as well as the dose to critical organs was also evaluated. Results: Results indicated that the central axis depth dose within and beyond the inhomogeneity drops significantly due to beam attenuation. For patients with bHPs, the dose contribution from lateral ports at 6 MV was attenuated by up to 23% and 17% for SS316L and Ti6Al4V, respectively. For a unilateral HP (uHP), the respective dose attenuations were 19% and 12%. The dose perturbation was always < 1% for a patient fitted with UHMWPE. Up to 38% dose increase was found at the proximal bone–HP interface due to backscattered electrons from the metal implant. There was a weak dependence of dose distribution on beam energy at the target isocenter, with the maximum dose reduction ranging only from 22.8 to 16.9% from 6 to 20 MV in a patient with bilateral steel HPs. However, interface effects were more pronounced at higher beam energies. However, increasing the number of treatment beams improved the plan quality. The greatest PTV dose perturbation was observed in a 4F box and lowest in a 6F plan. Production of scatter radiation was found to be larger for backscatter compared to forward scatter in this study. Conclusions: The dose perturbation effect of metallic HPs is significant and must be taken into account during treatment planning. UHMWPE poses no significant dose perturbation in the shadow of the implant and on the interface with tissue or bone. The use of MC–based TPSs is recommended for treatments using beam portals passing through HPs. MCSHOW allows the addition of HP contours in the virtual phantom from CT dataset of a patient without a HP. This allows one to carry out MC calculations for several implant models without metal artefacts. Results also highlight the significant influence of the implant’s composition and the beam position relative to the HP as well as beam energy on the dose distribution. Increasing the beam energy may help overcome the attenuation effects of metallic HPs and to improve target coverage. Therefore this study recommends plans with a larger number of beams that would allow avoiding the hip inhomogeneity in order to effectively compensate for dose attenuated in fields passing through HPs. 1It is also evident from the results that the shadowing effect is density-dependent, and its maximum value is for the SS316L HP. A more sophisticated, non-coplanar beam orientation may be necessary to avoid the HPs whilst sparing organs at risk (OARs) and giving sufficient target coverage.
Open Access
Characterization of small megavoltage photon beams for radiography
(University of the Free State, 2017-07) Setilo, Itumeleng; Du Plessis, F. C. P.
English: Introduction The landscape of radiation treatment techniques is ever evolving in pursuit of improved target coverage. The latest techniques such as IMRT, SBRT, SRS and VMAT, provide improved target coverage by controlling the intensity of the given dose through the use of multiple small fields in contrast to large fields in conventional treatments. The advantage of using these large fields is that, their characteristics are fully understood. The introduction of small fields leads to improved coverage, but the physics of these fields are not fully understood. So, when used in patient treatment, it resulted in unaccounted radiation exposure due to inaccurate commissioning and inaccurate absolute dose calibration at these field sizes. The errors were due to incorrect detectors used for data collection, and incorrect application of factors when performing absolute dose calibration. This report investigated the characteristics of these small fields using different detectors whilst varying the SSD and the incident photon beam energy. The measurements included beam profiles, percentage depth dose (PDD) curves as well as the relative output factors (ROF). Materials and Methods The photon energies, 6 MV, 10 MV and 15 MV were delivered using the Synergy LINAC, which is equipped with Agility multileaf collimators (MLCs). The detectors that were investigated were the CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film and the EDR2 radiographic film. Measurements were carried out using water as a medium for the CC01 ion chamber, EFD-3G diode and the PTW60019. Films were placed in between water equivalent RW3 phantom slabs. These measurements were carried out at 90 cm, 95 cm, 100 cm and 110 cm source to surface distances (SSD). The field sizes that were investigated were 1×1 cm², 2×2 cm², 3×3 cm², 4×4 cm², 5×5 cm² and 10×10 cm², these fields sizes were set using Jaws and MLCs. The 10×10 cm² field size was included as a reference field. Results and Discussion The results showed that the beam profiles were insignificantly different at the various SSDs for the detectors. The EBT2 film showed the sharpest penumbra, with the EDR2 and the CC01 showing broad penumbrae, but the difference was negligible. The PDD measurements showed that the difference between the detectors after Depth of maximum dose (Dmax) were insignificant. The films differed significantly at shallower depths, and this can be attributed to setup, as well as the artefacts that showed up when the films were being analyzed. The PDD measurements indicated that the setup used for the films was not adequate for measuring the 1 cm square field sizes and below. Dmax was used to compare the detectors, though it did not vary greatly for the detectors, it was shown that there is a change in the manner in which this factor changes with field size. Below a certain field size, 2 cm for the 6 MV and 10 MV and 3 cm for the 15 MV, the Dmax would start shifting back to the surface instead of moving deeper as expected. The relative output factor (ROF) increased with energy, and this is true for all the fields which had lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. This showed that the density of the detector had an effect on the output factor measured. Conclusion The fields were characterized with the different detectors, barring the artefacts experienced with film measurements in some instances, these detectors can be used safely for the small fields. The ROFs can be measured at longer SSDs as they showed little variation due to increased SSDs.
Open Access
Computed tomography radiomic texture features dependence upon imaging parameters
(University of the Free State, 2019-11) Makosa, Frank; Rae, W. I. D.; Court, L. E.; Acho, S. N. N.
Introduction and Aim: Few studies have been carried out to determine the influence of Computed Tomography (CT) acquisition parameters (slice thickness, tube potential difference (kVp), and tube current time product (mAs)) on the quantitative image features in radiomics studies. There is little evidence in the published literature, of studies that use mathematics to establish radiomic texture features that are independent of the CT scan technique parameters. The stability of radiomic texture features may have a great impact on the diagnosis and treatment of cancers. Robust texture features can be used to track radiotherapy treatment response. In this study radiomic texture features were investigated to identify features that did not depend on the CT technique parameters. Methodology: The credence cartridge radiomics (CCR) phantom was imaged at four CT units at the Universitas Academic and the National District hospitals. The tube current-time product (mAs) was varied from 75 to 400 mAs in steps of 25mAs while the kilovoltage peak and slice thickness were kept set at 120kVp and 5 mm respectively. The CT tube potential was investigated at 80, 100, 120 and 135 kVp whilst mAs and slice thickness was kept set at 300 mAs and 5 mm respectively. The slice thickness was varied from 1 mm to 5 mm whilst the mAs and kVp was kept constant at 300 mAs and 120kVp respectively. The acquisition field of view (FOV) and pitch were kept constant. The images obtained were processed using PyRadiomics software platform of 3D Slicer and the Matlab 2017a package. PyRadiomics was used to segment and extract a total of 105 radiomics texture features for each region of interest (ROI) delineated on an image. The 105 radiomic features included 13 shape features, 18 first order statistics features, 23 grey-level co-occurrence matrix, 14 grey level difference matrix, 16 grey-level run length matrix, 16 grey level size zone matrix and 5 neighbourhood grey tone difference matrix features. For each 10 CCR phantom inserts, 16 ROI of 2cm diameter was segmented by aligning the centre of the ROI at the centre of the insert. The Matlab package was used to segment and extract image matrices that were used to perform hand GLCM calculations. A kV Cone Beam Computed Tomography (kV CBCT) acquired cervical cancer data-set was used to establish the robust radiomic texture features response to radiotherapy treatment. The kV CBCT images were acquired first day and weekly during the 25 treatment fractions. Results: Five first order statistic radiomic features and six grey level co-occurrence matrix features were identified in the experimental test and mathematical manual calculations tests to vary with coefficients of variance of less than or equal to 10 % when the slice thickness was varied. Most of the radiomic texture features were weak and unstable (coefficients of variance above 10%) at very small slice thickness (≥2.5 mm) and robust at medium (≥2.5 mm) to large slice thickness (3.75 mm and 5 mm) (coefficients of variance ≤ 10 %). The above was attributed to an averaging effect (image smoothening) on the images when the slice thickness of image acquisition is increased. The image noise was observed to be less in large slice thickness when compared to noise at small slice thickness. Radiomics features were independent and stable to the tube potential at greater than 100 kV. At high tube potential the radiation attenuated signal detected at the CT detector was higher cancelling the noise effects. The robustness of these radiomic features depended on the material comprising the insert analysed. The extent of mAs dependence observed for the dense cork and plaster resin materials inserts was low compared to the dependence on the solid acrylic material insert. All the other phantom inserts (rubber particles, natural cork and the 3 acrylonitrile butadiene styrene plastic) data plots showed smaller variations around the central axis (zero feature value) of the skewness, uniformity, entropy and kurtosis features graphs. Irrespective of the mAs changes, the radiomic texture feature values obtained from all of the ABS materials inserts, rubber particles and natural cork inserts were consistently smaller, closer to zero. A general decrease in image noise as the mAs of image acquisition was increased in images of uniform or relatively uniform material was also observed. The patient tumour analysis showed some radiomic texture features response to radiotherapy treatment. This was shown by the changes observed on the inverse difference, inverse difference moment, entropy and difference variance texture features. The texture features had their values decrease from start of treatment (first fraction) to the last treatment fraction. The decrease was not smooth along the treatment period, there were some anomalies on the trends. This decrease was ascribed to the change in the heterogeneity of the tissues within the treatment region of interest evaluated. Conclusion: Overall, using theoretical analysis and a practical approach, robust radiomic features that were independent of the CT scan parameters were observed. The experimental approach showed that the phantom insert materials had influence on radiomic texture feature values obtained in investigations. Radiomic texture features demonstrated that tumours had a variation of heterogeneity between them. The observation agrees with other clinical studies that showed that tumours exhibit some extensive genetic and phenotypic variations. Radiomic texture features can be utilised to depict tumour texture changes along the treatment timeline as shown in this study. A great challenge would be to associate the radiomic texture feature changes to the clinical biological changes. For future robust radiomic feature studies, the use of phantoms with tissue like materials was proposed.
Open Access
Estimation of the eye lens doses in a catheterization laboratory from available image parameters
(University of the Free State, 2020-07) Phutheho, Mokete; Acho, Sussan; Rae, William; Rose, Andre
Background and objective: New data on eye lens dosimetry supports the theory that the threshold of radiation-induced cataracts could be substantially lower than previously believed with some investigators arguing that cataracts could be classified as a stochastic rather than a deterministic effect. Based on these new data, the International Commission on Radiological Protection (ICRP) has reduced the occupational eye lens dose limit from 150 mSv to 20 mSv averaged over a defined period of 5 years, with no single year exceeding 50 mSv. The new reduction in the annual dose limit will have considerable implications particularly in high exposure environments such as interventional cardiology and radiology. It is therefore imperative that strategies for effective dose reduction, radiation protection, eye dose monitoring, and dosimetry be implemented in countries that have already adopted the new eye dose limit. The main aim of this study was to develop methods that can be applied to estimate eye dose equivalent from the available imaging parameters and whole-body equivalent measured over the lead apron at the chest level. The study also aimed to establish a method to estimate eye lens dose based on the workload of interventionalists. Material and methods: The study included four interventional cardiologists. A total of 127 procedures were performed in a period of three months. The procedures were categorised into diagnostic (CA) and therapeutic (CA+PCI) procedures. During these procedures, two different active dosimeters were used to measure scatter dose (one attached on the canthus of the protective eyewear to measure eye lens dose (ELD) and the other at the chest level to measure whole-body dose) to the cardiologists. The dose area product (DAP), air kerma (Ka,r), fluoroscopic time, total cine images were recorded after every procedure. The efficacy of the protective eyewear used at Universitas Hospital was evaluated in a separate study. Results: Average eye dose per CA and CA+PCI procedures were 195.1±112 and 391.8±202.9 µSv, respectively. The average dose per procedure obtained by combining all the monitored procedures was 250.9±168.3 µSv. The minimum workload necessary to exceed the annual eye lens dose limit calculated using an equation established in this study was 80 procedures. The dose reduction factor of the protective eyewear was ~2. Applying this factor increased the minimum procedures necessary for a doctor to exceed the limit to 160 procedures per year. Excellent correlation was found between ELD and DAP (R2 = 0.78). Excellent correlation was also found between ELD and Ka,r (R2 = 0.72). A poor but significant correlation was found between ELD and chest dose (R2 = 0.45). Three methods based on the ratios of ELD to DAP, Ka,r and chest dose were established. The calculation error using the methods based on DAP and Ka,r was ±20%. The respective calculation error was ±37% using the method based on chest dose. Conclusion: The accumulated eye dose of interventional cardiologists working at the Universitas Hospital can easily surpass the newly set annual eye lens dose limit after performing relatively low numbers of interventional procedures. The high average dose per procedure reported in this study highlights immediate need for implementation of radiation optimization strategies to mitigate the risk of radiation-induced cataracts. This is the first study in South Africa to establish methods that can be used to estimate eye lens doses at any time. More research is needed in the South African context to further investigate eye lens dose in interventional suits. This will allow for comparison of results obtained at different institutions and improvement in accuracy of estimation methods.