Medical Physics

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Open Access
Accuracy of iodine-131 activity quantification and dosimetry for three-dimensional patient-specific models
(University of the Free State, 2019-03) Ejeh, John Enyi; Van Staden, J. A.; Du Raan, H.
Iodine-131 (131I) therapy of thyroid related and other diseases is limited by critical organ toxicity. Therefore, accurate activity quantification and dose calculation are important to optimise dose to tumours while limiting dose to critical organs. The aim of this study was to evaluate the accuracy of 131I activity quantification and dosimetry for three-dimensional (3-D) patient-specific models. Retrospective patient Computed Tomography (CT) data were segmented to create clinically realistic patient 3-D voxel-based models. These were used to simulate Single Photon Emission Computed Tomography (SPECT) data with a Monte Carlo (MC) simulation software, which was validated against physical measurements. The simulated SPECT data were reconstructed using an ordered-subsets expectation maximization (OS-EM) algorithm which includes scatter correction, CT-based attenuation correction, and 3-D collimator-detector response compensation. Predetermined recovery coefficients were used to compensate for partial volume effects. Image counts were converted to activity by using a predetermined calibration factor. The patients’ reconstructed activity maps and density maps were used to perform 3-D dosimetry with the MC program, LundADose. LundADose calculated mean tumour and organ absorbed doses were compared with OLINDA/EXM calculated mean absorbed doses using statistical analysis. Validation of the simulation software resulted in a percentage difference of -6.50 % between the measured and simulated extrinsic energy resolution at the 131I peak energy of 364 keV and - 18.57 % error for the measured and simulated intrinsic energy resolution. The measured and simulated FWHM and FWTM of the camera for system spatial resolution had percentage differences of -7.41% and -7.38 % and an error of -1.50 % and -2.6 % for system sensitivity and collimator septal penetration fraction. SPECT activity quantification was evaluated by comparing the true tumour activities defined for the patient models with the quantified activities obtained from the models’ reconstructed SPECT images. The quantification error for the studied patient models was < 9.0 % and < 5.1 % for 3.0 and 6.0 cm spherical tumours situated in the lungs (mean values were 3.9 ± 3.3 % and -1.6 ± 1.9 %). The error for the two tumours in the liver was < 11.2 % (mean values of 7.7 ± 3.9 % and 8.4 ± 2.9 %). The mean percentage differences between the mean absorbed doses calculated by LundADose and OLINDA/EXM for the left lung, right lung, liver, 3.0 cm ‘tumour’ and 6.0 cm ‘tumour’ were comparable. These mean percentage differences were -2.23 ± 1.98 %, -3.06 ± 1.67 %, 1.31 ± 4.15 %, -28.44 ± 18.36 %, and -5.10 ± 2.87% for the listed organs and tumours when the 3.0 cm tumour was located in the lung and the 6.0 cm tumour in the liver. For the scenario where the 3.0 cm tumour was positioned in the liver and the 6.0 cm tumour in the lung, the corresponding results were -2.84 ± 3.42 %, -1.49 ± 2.68 %, 3.97 ± 4.12 %, -28.80 ± 5.05 %, - 8.21 ± 17.06 %. The SIMIND MC model of the gamma camera was accurately validated with good agreement between results calculated from the physical measurements and simulation. Good accuracy of 131I activity quantification and 3-D dosimetry was found for 3-D patient-specific models. Statistical analysis of the results of the comparison of LundADose and OLINDA/EXM showed that the two dosimetry programs were strongly correlated with R2 values ranging from 0.85 to 1.00 for the mean absorbed dose in the various organs and tumours. Furthermore, the two (MC and MIRD) methods were found to agree well using Bland-Altman analysis of the dosimetry results. For 131I, activity quantification and dosimetric accuracy better than 10 % were achieved using state-of-the-art hybrid equipment and sophisticated correction methods for image degrading factors.
Open Access
Accuracy of lutetium-177 SPECT activity quantification and patient-specific dosimetry: a Monte Carlo study
(University of the Free State, 2021) Ramonaheng, Keamogetswe; Van Staden, Johannes A.; Du Raan, Hanlie
𝑬𝒏𝒈𝒍𝒊𝒔𝒉 The goal of radiopharmaceutical therapy (RPT) is to deliver the maximum dose to cancerous tumours while sparing healthy tissue. Ideally, the radiopharmaceutical should accumulate in the tumorous tissue and the radiation entirely absorbed for tissue destruction; however, this is not the case. Dosimetry strives to balance the efficacy of delivering the maximum dose to the tumour cells with minimal toxicity to the healthy tissue. Patient-specific dosimetry offers the potential for RPT to reach its full potential as a powerful precision-based treatment. Efforts for patient-specific dosimetry remain a challenge due to the steps involved in the clinical dosimetry workflow. SPECT/CT imaging allows the estimation of the bio-kinetic distribution of the radiopharmaceuticals with good precision, which is required for accurate dosimetry. Different dosimetry software is available (commercial and non-commercial), but these should be benchmarked before being used. Optimising the imaging process for activity quantification and dosimetry in the NM discipline is essential, and Monte Carlo (MC) techniques have been used successfully in this endeavour. Furthermore, full MC dosimetry gained wide acceptance as the gold standard and the most accurate means for patient-specific dosimetry. MC simulations offer the advantage of having a gold standard against which the dosimetry can be benchmarked, and the dosimetry accuracy evaluated. Therefore this thesis aimed to assess the accuracy of 177Lu SPECT activity quantification and patient-specific dosimetry using MC simulations. The focus was on the bio-distribution of 177Lu- DOTATATE due to its clinical relevance in RPT of patients with metastasised neuroendocrine tumours. The kidneys are the dose-limiting organs for RPT with 177Lu-DOTATATE. Studies have shown kidney doses to vary significantly between patients. Given the relation between tumour absorbed dose and tumour reduction, for a complete efficacy evaluation, absorbed doses should be determined not only for the kidneys but, where possible, extended to the tumours. This study incorporated voxel-based phantoms generated from phantom and patient CT data to perform virtual image-based activity quantification and dosimetry using MC simulations. The voxel-based phantoms were modified to include spherical structures mimicking tumours. The first objective of this study was to validate a model of the Siemens Symbia T16 dual-head SPECT/CT gamma camera available in our clinic using the SIMIND MC program for 177Lu imaging. The validation was achieved by comparing experimental and simulated gamma camera performance planar and SPECT criteria tests. The results were in good agreement and provided adequate confidence that SIMIND could emulate the Symbia T16 successfully and be used for further investigations of 177Lu SPECT/CT image quantification. The second objective investigated the effect of sphere and cylinder calibration factor (CF) geometries and their corresponding recovery coefficients (RCs) on the quantification accuracy of 177Lu SPECT images using MC simulations. The investigations were performed using geometries of a cylindrical, an anthropomorphic torso, and patient-specific phantoms. The quantification accuracy was evaluated for tumours and the kidneys. The results demonstrated that 177Lu SPECT quantification accuracies compared favourably for sphere-based and cylinder-based CF and RC combinations when all SPECT corrections were applied. The absolute quantification accuracy of ≤ 3.5% compared well to literature findings and complied with the 5% requirements for accurate dosimetry. The third objective of the thesis aimed to compare the accuracy of the absorbed doses computed with the software LundADose and OLINDA/EXM 1.0 using three patient-specific voxel-based phantoms. The dosimetry accuracy was assessed by comparing the computed doses to the “true” activity images combined with full MC dosimetry to define the gold standard. The accuracy between LundADose (6.6%) and OLINDA/EXM 1.0 (8.1%) was comparable. The ≤ 10% dosimetry accuracy suggested that the software platforms approximated the true dose estimates and advocated for the dosimetry accuracy to be reliable. ___________________________________________________________________
Open Access
Accuracy of patient-specific dosimetry using hybrid planar-SPECT/CT imaging: a Monte Carlo study
(University of the Free State, 2021-07) Morphis, Michaella; Van Staden, J. A.; Du Raan, H.; Du Plessis, F. C. P.
Introduction: Theragnostics is a precision medical discipline aiming to individualise patient targeted treatment. It aims at treating cancer by the systemic administration of a therapeutic radiopharmaceutical, which targets specific cells based on the labelling molecule. With the renewed interest in radiopharmaceutical therapy, the importance of accurate image quantification using iodine-123 (I-123) and iodine-131 (I-131), for dosimetry purposes, has been re-emphasised. Monte Carlo (MC) modelling techniques have been used extensively in Nuclear Medicine (NM), playing an essential role in modelling gamma cameras for the assessment of activity quantification accuracy, which is vital for accurate dosimetry. This thesis aimed to assess the accuracy of patient-specific I-131 dosimetry using hybrid whole-body (WB) planar-SPECT/CT imaging. The study was based on MC simulations of voxel-based digital phantoms, using the SIMIND MC code emulating the Siemens SymbiaTM T16 gamma camera. To achieve the aim, the thesis was divided into four objectives, (i) validating the accuracy of an energy resolution (ER) model, (ii) verifying the SIMIND setup for simulation of static, WB planar and SPECT images for I-123 with a low energy high resolution (LEHR) and a medium energy (ME) collimator and for I-131 with a high energy (HE) collimator, (iii) evaluating SPECT quantification accuracy for the three radionuclide- collimator combinations and (iv) assessing the accuracy of I-131 absorbed dose calculations for tumours and organs at risk, based on hybrid WB planar-SPECT/CT imaging. Methodology: The proposed ER model was fit to measured ER values (between 27.0 and 637.0 keV) as a function of photon energy. Measured and simulated energy spectra (in-air, in-scatter and a voxel-based digital patient phantom) were compared. The SIMIND setup was validated by comparing measured and simulated static and WB planar (extrinsic energy spectra, system sensitivity and system spatial resolution in-air and in-scatter), as well as SPECT (simple geometry sensitivity) results. Quantification accuracy was assessed in voxel- based digital simple and patient phantoms, using optimised OS-EM iterative reconstruction updates, calibration factor and recovery curves. Finally, using the true and quantitative activity data from I-123 and I-131 voxel-based digital patient phantoms, full MC radiation transport was performed, to determine the accuracy of the absorbed dose for I-131-mIBG radiopharmaceutical therapy. Results: The fitted ER model better simulated the energy response of the gamma camera, especially for high energy photopeaks, (I-123: 528.9 keV and I-131: 636.9 keV). The measured and simulated system energy spectra (differences ≤ 4.6 keV), system sensitivity (differences ≤ 6.9%), system spatial resolution (differences ≤ 6.4%) and SPECT validation results (difference ≤ 3.6%) compared well. Quantification errors less than 6.0% were obtained when appropriate corrections were applied. I-123 LEHR and I-123 ME quantification accuracies compared well (when corrections for septal scatter and penetration are applied), which can be useful in departments that perform I-123 studies and may not have access to ME collimators. Average I-131 absorbed doses of 2.0 ± 0.4 mGy/MBq (liver), 20.1 ± 4.0 mGy/MBq (3.0 cm tumour) and 22.6 ± 4.2 mGy/MBq (5.0 cm tumour) were obtained in simulated patient studies. When using a novel method of replacing the reconstructed activity distribution with a uniform activity distribution, eliminating the Gibbs artefact, the dosimetry accuracy was within 10.5%. Conclusion: Using the proposed fitted ER model, SIMIND could be used to accurately simulate static and WB planar and SPECT projection images of the Siemens SymbiaTM T16 SPECT/CT for both I-123 and I-131 with their respective collimators. Accurate quantification resulted in absorbed dose accuracies within 10.5%. The hybrid WB planar-SPECT/CT dosimetry method proved effective for personalised treatment planning of I-131 radiopharmaceutical therapy, with either I-123 or I-131 diagnostic imaging.
Open Access
Accuracy of patient-specific I-131 dosimetry using hybrid whole-body planar-SPECT/CT I-123 and I-131 imaging
(SpringerOpen, 2024) Morphis, Michaella; van Staden, Johan A.; du Raan, Hanlie; Ljungberg, Michael; Sjögreen Gleisner, Katarina
𝗣𝘂𝗿𝗽𝗼𝘀𝗲: This study aimed to assess the accuracy of patient-specific absorbed dose calculations for tumours and organs at risk in radiopharmaceutical therapy planning, utilizing hybrid planar-SPECT/CT imaging. 𝗠𝗲𝘁𝗵𝗼𝗱𝘀: Three Monte Carlo (MC) simulated digital patient phantoms were created, with time-activity data for mIBG labelled to I-123 (LEHR and ME collimators) and I-131 (HE collimator). The study assessed the accuracy of the mean absorbed doses for I-131-mIBG therapy treatment planning. Multiple planar whole-body (WB) images were simulated (between 1 to 72 h post-injection (p.i)). The geometric-mean image of the anterior and posterior WB images was calculated, with scatter and attenuation corrections applied. Time-activity curves were created for regions of interest over the liver and two tumours (diameters: 3.0 cm and 5.0 cm) in the WB images. A corresponding SPECT study was simulated at 24 h p.i and reconstructed using the OS-EM algorithm, incorporating scatter, attenuation, collimator-detector response, septal scatter and penetration corrections. MC voxel-based absorbed dose rate calculations used two image sets, (i) the activity distribution represented by the SPECT images and (ii) the activity distribution from the SPECT images distributed uniformly within the volume of interest. Mean absorbed doses were calculated considering photon and charged particle emissions, and beta emissions only. True absorbed doses were calculated by MC voxel-based dosimetry of the known activity distributions for reference. 𝗥𝗲𝘀𝘂𝗹𝘁𝘀: Considering photon and charged particle emissions, mean absorbed dose accuracies across all three radionuclide-collimator combinations of 3.8 ± 5.5% and 0.1 ± 0.9% (liver), 5.2 ± 10.0% and 4.3 ± 1.7% (3.0 cm tumour) and 15.0 ± 5.8% and 2.6 ± 0.6% (5.0 cm tumour) were obtained for image set (i) and (ii) respectively. Considering charged particle emissions, accuracies of 2.7 ± 4.1% and 5.7 ± 0.7% (liver), 3.2 ± 10.2% and 9.1 ± 1.7% (3.0 cm tumour) and 13.6 ± 5.7% and 7.0 ± 0.6% (5.0 cm tumour) were obtained for image set (i) and (ii) respectively. 𝗖𝗼𝗻𝗰𝗹𝘂𝘀𝗶𝗼𝗻: The hybrid WB planar-SPECT/CT method proved accurate for I-131-mIBG dosimetry, suggesting its potential for personalized treatment planning
Open Access
Assessment of factors affecting accuracy of standardised uptake values in positron emission tomography
(University of the Free State, 2015-01) Du Toit, Petrus Daniel; Du Raan, H.; Rae, W. I. D.; Visvikis, D.
English: Positron emission tomography (PET) is an imaging method that uses tracers labelled with positron emitting isotopes for the monitoring and evaluation of in vivo molecular processes. Semi-quantitative determination of tracer uptake in a lesion is accomplished by calculating the standardised uptake value (SUV), an index that represents the amount of uptake in a given volume-of-interest (VoI) in relation to the average uptake throughout the body. The SUV is influenced by biological and physical factors that determine the uptake or detectability of the tracers which may result in false results. Changes in SUV of small lesions or lesions with low activity uptake cannot be determined with enough certainty and precision to be used for decision-making and it is therefore necessary to investigate the factors affecting the SUV. The aim of this study was to assess the relative importance of the physical factors that affect the accuracy of a single SUV measurement using Monte Carlo modelling. Phantom studies were performed to determine the influence of the partial volume effect due to spatial resolution using a PET scanner. Comparative Monte Carlo simulations were performed on a computer cluster using a voxelised version of the same phantom. The XCAT anthropomorphic phantom was used to assess the influences on SUV in a human-like configuration and was set-up to simulate movement in the thorax during breathing. SUVs were calculated using simulations of the phantom in 2D and 3D modes to assess the influence of the partial volume effect by variation of the size of the lesions, by variation of the contrast ratios and by placing the lesions in different areas in the lungs during. Influence of activity from outside the field-of-view (FoV) was also assessed as well as the impact the various coincidence types have. Statistical methods were used to compare the difference in data for statistical significance. It was found that the partial volume effect was present when evaluating the SUVs of the activity in the spheres of the phantom when scanned on a PET/CT scanner as well as when performing Monte Carlo simulations. Statistically there were no significant differences between the two scanning modes. The mean SUV increased as the voxel sizes became smaller. The choice of matrix influenced the amount of partial volume effect. The relative contributions of true-, scatter- and random coincidences demonstrated that the true coincidences were the major contributor when assessing the data from this phantom. The relative contribution of the trues-to-total coincidences decreased with a decrease in lesion size and contrast ratio whereas the relative contributions of the scattered- and random coincidences increased. The contributions of scatters and randoms increased during the 3D acquisition mode compared to 2D mode. The contribution of the trues-to-total coincidences decreased with an increase in VoI size and consequently caused a decrease in the mean SUV. The location of the lesion made a difference in SUV when the same size lesions are compared to each other. Apical lesions experienced the least amount of motion during breathing, were distorted less and had the least amount of variation in SUV. By moving the phantom partly outside the FoV, significant effects on the SUVs of objects still inside the FoV were found. An increase in the SUVs was observed when the true coincidences were used for the calculation. A decrease in true SUVs was found at the right basal lesion. In conclusion, partial volume effects play a significant role when determining the SUV of objects based on their size and contrast ratio; the location of pulmonary lesions affects SUV calculation during breathing; and activity outside the field-of-view of the scanner contributed to a change in SUV in particular to the central and basal regions of the lung.
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
Assessment of ventricular function using gated blood pool planar and - SPECT imaging: a phantom study
(University of the Free State, 2023) Pieters, Hané; van Staden, J. A.
In the field of Nuclear Medicine, Gated Blood Pool (GBP) investigations are essential in offering vital insights into cardiac function, specifically the Left Ventricular Ejection Fraction (LVEF). The evaluation of ventricle volume changes during the end-diastolic (ED) and end-systolic (ES) phases plays a critical role in detecting, diagnosing, and managing various cardiac diseases. While the longstanding preference for Gated Blood Pool Planar (GBP-P) methods lies in their validation, non-invasiveness, and straightforward application, challenges such as anatomical overlap and the need for background correction persist. The theoretical superiority of three-dimensional (3D) analogues, specifically Gated Blood Pool SPECT (GBP-S) studies, promises to overcome GBP-P challenges by offering true volumetric representation without the need for background correction. However, this transition introduces complexity in algorithms and processing software needed for GBP-S studies. Accuracy and precision in determining LVEF is paramount in both GBP-P and GBP-S methods to avoid misdiagnosis, improper treatment, or negligence. Rigorous testing and comparison to known or true values are imperative for validating GBP processing software programs to meet set standards. A key advancement in validating these software programs is the use of digital hybrid phantoms, notably the advanced 4D-XCAT model. The model, blending voxelised and mathematical elements, mimics human anatomy and physiology. Paired with the Monte Carlo (MC) code, SIMIND, these 4D-XCAT models can be used to simulate clinically realistic GBP images, generating a database for testing, and validating various GBP software packages. Importantly, this approach avoids radiation exposure to patients and researchers and enhances the reliability of outcomes by providing benchmark input parameters for software evaluation. The primary aim of this investigation was to assess ventricular function using MC-simulated GBP-P and GBP-S images of digital patient phantom studies based on 4D-XCAT models with varying cardiac volumes and functions. The aim was achieved by considering three objectives. Firstly, a Monte Carlo simulated cardiac phantom for planar and SPECT studies was validated. The modelled gamma camera was verified using routine quality control procedures outlined by the National Electrical Manufacturers Association (NEMA). Furthermore, 3D cardiac phantoms were printed and imaged according to GBP-P and GBP-S imaging guidelines. Simulated images of these phantoms were generated using the MC code SIMIND and compared to the gamma camera-acquired images. The successful verification of these simulated images led to the next step, namely verifying the use of the 4D-XCAT model in image simulations. By simulating GBP-P and GBP-S studies of a single 4D-XCAT model, the study demonstrates excellent agreement between known and calculated ventricular parameters, confirming the reliability of the 4D-XCAT model in simulating cardiac imaging. Building on the successful simulation of the 4D-XCAT model, the second objective involved the creation of a comprehensive database comprising 64 clinically realistic GBP patient models. GBP-P simulated images from these models were utilised to evaluate four commercially available GBP-P processing software programs. The study yielded a strong correlation between calculated LVEF values and known values, thereby affirming the reliability and accuracy of the GBP-P processing software. Lastly, the research was extended to include clinically realistic GBP-S studies as part of the third objective. The phase focused on assessing the commercially available GBP-S processing software, Quantitative Blood Pool SPECT (QBS), from Cedars-Sinai. GBP-S images of the 4D-XCAT GBP database were simulated according to established imaging guidelines. The study assessed the accuracy of QBS in terms of LV volumes and EF values for two different reconstruction techniques and found a strong correlation between the calculated and known LV volumes and EF values. This paved the way for future multi-centre studies to validate other GBP-S processing software packages.
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
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
Comparison between measured and simulated activity using Gafchromic™ film with radionuclides
(University of the Free State, 2020-08) Joubert, Maria Magdalena; Du Plessis, F. C. P.; Van Staden, J. A.
In this study, Gafchromic™ film XR-QA2 and RT-QA2 were used to characterise the film energy response against various radionuclides. The film response was investigated with respect to different backscatter materials. The sensitivity of the two types of films was compared, and a film stack method was tested to allow the user to obtain sequential, cumulative doses at different time points. Monte Carlo (MC) simulations were used to link optical density (OD) values from measurements to the absorbed dose in the film. This was achieved by using conversion factors obtained by BEAMDP, BEAMnrc and DOSXYZnrc simulations to get the absorbed dose in the film. A neutron depletion theoretical model was introduced that can describe film response as a function of cumulated activity and absorbed dose. Background: Gafchromic™ film has been used for quality assurance in various studies but not in nuclear medicine applications. Once the OD has been determined after film exposure to a radionuclide, it can be linked to the absorbed dose using the air kerma rate constant at distances that approximates point sources and the dose in water can be linked to the dose in film using MC simulations to get conversion factors. MC simulations are known as a gold standard to get the absorbed dose in materials. Materials and Methods: XR-QA2 and RT-QA2 Gafchromic™ film were irradiated with the following radionuclides: Am-241, Cs-137, Tc-99m and I-131. The OD was calculated, and a function describing the relationship between the OD and the time-activity was derived based on the neutron depletion model. Different backscatter materials such as Corrugated fibreboard carton (CFC) or air equivalent material, polystyrene, Polymethyl Methacrylate (PMMA or perspex) and lead were used to investigate the effect it has on film response. The sensitivity of each film was investigated and compared. BEAMDP, BEAMnrc and DOSXYZnrc simulations were used to link the film response, OD, to the absorbed dose. The MC simulations were done replicating the exact geometry as with the physical measurements to get the absorbed dose in the film. Results: The new neutron depletion model fitted the OD vs cumulative activity accurately as well as the OD vs absorbed dose. The XR-QA2 Gafchromic™ film has shown to be the most sensitive film when using air equivalent material with radionuclides, especially with low energy radionuclides such as Am-241. When using more than one layer, the OD sensitivity of the film can be increased as well. The film stack method investigated also showed to be less time consuming when relating stacked film data to single film data. The fluence obtained from BEAMDP confirmed that the radionuclide containers have an effect on the radionuclide spectra’s. Lead was also the backscatter material which showed higher OD change but lower absorbed dose values. Conclusions: The neutron depletion theoretical model is more accurate than higher-order polynomial fits because it contains less free parameters. The XR-QA2 Gafchromic™ is better to use in nuclear medicine because of its sensitivity at low energies and because the sensitivity can be increased by using multiple layers of film. Film stack methods can be used to decrease experiment times. BEAMnrc can be used to accurately model radionuclides within their containers to evaluate the container effects. Lead showed a higher induced OD with lower absorbed dose, and the air equivalent material showed the lower OD change but higher absorbed dose.
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
Development and evaluation of a spect attenuation correction method using an open transmission source and scatter correction
(University of the Free State, 2011-06) Van Staden, Johannes Abraham; Du Raan, H.; Van Aswegen, A.
Abstract not available
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
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
Development of a brachytherapy treatment planning module for cervix cancer utilising biological dose metrics
(University of the Free State, 2020-12) Van der Walt, Hester Catharina; Shaw, William
The contouring uncertainties associated with the use of computed tomography imaging for brachytherapy planning creates the need to investigate an alternative planning method for Image-Guided Adaptative Brachytherapy. This alternative method needs to be more robust against imaging and contouring uncertainties compared to the original GEC-ESTRO prescription regarding dose-volume histogram criteria. This study evaluates the utilisation of biological dose metrics (equivalent uniform dose (EUD)) during Image-Guided Adaptive Brachytherapy (IGABT) treatment planning and optimisation. A retrospective planning study was conducted. The eighteen patients that were included in the planning study received CT-based Image-Guided Brachytherapy (IGBT) in combination with external beam radiotherapy (EBRT) between 2014 and 2015. A novel biological optimisation model was developed and used to efficiently and automatically optimise brachytherapy (BT) plans by utilising either dose-volume, biological metrics/indexes or both for fast treatment plan generation. The module was refined to allow forward and inverse planning and optimisation of combined interstitial and intracavitary brachytherapy. Additionally, the utilisation of OAR total dose constraints during the planning process was applied and evaluated. The results of the study showed that the inverse optimisation tool could produce better target volume coverage compared to the manual optimisation tool. In some cases, the inverse optimisation tool led to higher OAR doses; however, the values recorded were still within set constraints. When comparing the conventional and biological planning methods, the biological planning produced superior CTV-T doses and dose distributions within the CTV-T. The inverse biological approach reported significantly higher average CTV-THR D98 % , D100 % values and CTV-TIR EUD, D90 % D98 % and D100 % values compared to inverse conventional planning approach. With this, the inverse biological approach also had the ability to record significant lower average bladder wall EUD and D0.1cm3D0.1cm3 values. Even though the inverse conventional planning approach reported significant lower average small bowel D2cm3 values compared to the IBG approach, both approaches were still well below the D2cm3 hard constraint of 75 Gy. Dose escalation was achieved in the CTV-T with a reduction in OAR dose with the combination of interstitial/intracavitary brachytherapy. It was concluded from the study that the incorporation and utilisation of biological metrics, which incorporates the entire dose distribution in the organ of interest, is the preferred approach when compared to the conventional physical dose-volume approach.
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
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
An equivalent uniform dose-based class solution for cervical cancer radiotherapy
(University of the Free State, 2014-06) Shaw, William; Rae, William Ian Duncombe; Alber, Markus Lothar
English: Cervix cancer radiotherapy treatment consists of external beam radiotherapy (EBRT) and brachytherapy (BT). Currently there exists no method to combine the dose of both modalities in a single dose value or dose distribution. This study derived a method to use the equivalent uniform dose (EUD) concept as a worst case dose estimate for both modalities, and the combination thereof. The EUD was used as dose evaluation tool in clinical brachytherapy planning of 10 patients that received conservative organ at risk (OAR) toxicity avoidance treatment plans. OAR EUD dose constraints were also derived for brachytherapy treatment planning so as to be equivalent to the Gyn GEC-ESTRO guidelines for cervix cancer brachytherapy based on a population of 20 patients receiving 5 high-dose-rate image guided brachytherapy treatments each. Furthermore, a method to escalate tumour dose without increasing OAR dose was investigated using the EUD as a safeguard against OAR over-dosage and exploiting the effects of fractionation radiobiologically and by organ geometry variations. The EUD was also used as an external beam IMRT evaluation tool to calculate suitable planning target volume (PTV) margin sizes for treatment plan optimization and as a quick cumulative dose computation to enable on-line and off-line image guided adaptive radiotherapy (IGART). This study utilizes the underlying mathematical properties of the EUD to act as a method for determining a worst case dose estimate for tumours and OARs. The method is accurate and reliable and easy to use. OAR dose constraints for brachytherapy treatment planning based on EUD prescription were derived and they compare well with existing Gyn GEC-ESTRO recommended methods and constraints. The safety of the EUD as a worst case dose estimate motivates the use thereof in fractionation compensation based treatment planning that strives to maximize OAR dose to a fixed constraint level and maximize tumour dose at no extra toxicity cost. The EUD derived external beam planning margins also corresponded well with the published margin recipes, but showed that margin recipes potentially overestimate the required margin size and that PTV dose levels could be reasonably lower in some cases compared to the CTV dose level and not lead to tumour under-dosage. The EUD is also an effective 4D dose evaluation and planning tool for IGART and can be used to ensure adequate total dose is delivered in a mobile and deforming tumour without overdosing the OARs. The quick and reliable application of this method is its biggest attribute. The mathematical properties of the EUD open the possibility to determine a worst case estimate of cumulative dose in different treatment modalities and when they are used in combination. The application of this estimate can be extended to safe tumour dose escalation in both image guided adaptive brachytherapy (IGABT) and IGART and their combination.
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.