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Browsing Medical Physics by Author "Du Plessis, F. C. P."
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Item 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.Item 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.Item 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.Item 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.Item Open Access Feasibility of tissue differentiation with multi-energy computed tomography: a Monte Carlo breast phantom study(University of the Free State, 2017-03) Van Eeden, Déte; Du Plessis, F. C. P.English: Dedicated breast CT is a new innovative way of imaging breast tissue without the limitations of overlapping anatomical features. It has been shown that the dose received by the patient is comparable to that of conventional mammography techniques. Further developments have led to the idea of a photon-counting detector that can be utilised in conjunction with breast CT. This will produce images with higher CNRs and will improve the detection of malignant masses. Other applications of multi-energy CT include image-based energy weighting and the differentiation of different tissues. The aim of this study was to explore the feasibility for tissue differentiation in breast tissue through the Monte Carlo simulation of a virtual multi-energy CT unit. The EGSnrc Monte Carlo code was used to simulate a virtual CT unit, similar to the Toshiba Aquillion LB 16 CT. The radiation source modelling code, BEAMnrc, was used to model the different components of the virtual CT. These components include the X-ray tube, suitable filters and beam-defining components such as collimators. A phase-space file was obtained consisting of all the particles generated by the different components. The energy spectrum of the Toshiba Aquillion LB 16 CT was approximated by the virtual CT using HVL measurements. The RMI electron density CT phantom was used to benchmark the virtual CT against the Toshiba Aquillion LB 16 CT. The phantom consists of several inserts with known electron densities that produce different CT numbers. A similar phantom was modelled with an in-house developed IDL program and used for the simulations. The reconstructed images were then used for the benchmarking of the HUs. This benchmarking ensured that the method used in this study produces a realistic model of a CT unit. Breast simulator software was used to model three breast phantoms consisting of different glandularities. The composition of the different breast tissues was taken from literature. The three phantoms were simulated at 20 keV up to 65 keV in 5 keV increments. All of the image reconstructions in this study was done with a filtered backprojection algorithm by using the OSCar reconstruction software. The CNRs of the different images obtained at different energies were assessed. Image-based energy weighting was investigated to further enhance the CNRs of the images by multiplying each energy bin with a specific weighting factor. The weighting factors were determined by a random number generator in an in-house developed IDL code. Good results were obtained with a 1.2-1.3 fold increase in the CNR. Further improvements were made by applying constraints to the weighting factors of the different energy bins. A new method was proposed to differentiate between different breast tissues by using the mass attenuation information from multiple energies. This technique showed promising results and can detect malignant tissue by using a single egs_cbct simulation. In conclusion, it is feasible to differentiate between different breast tissue types when using a multiple-energy CT unit. Better CNRs are obtained when utilising the information of the entire energy spectrum. This will lead to better tumour detection, even in dense breasts consisting of 89% glandular tissue.Item 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.Item 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.Item Open Access Sensitivity analysis of the integral quality monitoring system® for radiotherapy verification using Monte Carlo simulation(University of the Free State, 2017-07) Oderinde, Oluwaseyi Michael; Du Plessis, F. C. P.Advanced radiotherapy (RT) techniques have improved the quality of radiation treatment. Notwithstanding, advanced RT techniques have generated complexities in their quality assurance (QA). Therefore, there is a huge interest to verify treatment plan data in real-time treatment. The Integral quality monitoring (IQM) system® (iRT Systems GmbH, Koblenz, Germany) is an independent real-time treatment verifying system which checks the integrity and validates the accuracy of the treatment plan data. The IQM also functions as a pre-treatment quality assurance tool for radiotherapy. The prototype system (IQM) is currently undergoing its beta testing, and contributions from researchers across the globe are pivotal to its integration into the clinical workflow. The IQM is a large wedge-shaped ionization chamber that is attached to the treatment head of the linear accelerator (linac) for signal measurement in real-time treatment. The aim of this innovative study was to determine how sensitive the IQM is for small alterations in the multileaf collimator (MLC) leaf positions using Monte Carlo (MC) simulation. The sensitivity of the IQM system is essential for its integration into clinical workflow. The MC simulation technique is an accurate dose calculation engine that could score dose in regions that seem complicated for physical measurement. A new component module (CM) called IQM was successfully developed using TCL/TK, and MORTRAN codes. The newly created CM was added-on to the BEAMnrc MC User code. Also, a linac source model of an Elekta Synergy linac equipped with an Agility 160-leaf MLC head was developed using the EGSnrc/BEAMnrc. Accurate MC calculations for percentage depth doses, lateral beam profiles, and relative output factors were benchmarked with physical measurements using the Gamma analysis criterion of 2%/2 mm. Characterised photon beams of 10 MV for 1 × 1 up to 30 × 30 cm2 fields using the BEAMnrc MC Code were simulated. Photon beam data stored in the phase space files after the source model simulations were calculated in a homogeneous water phantom using the DOSXYZnrc MC Code. For the square field sizes considered, MC dosimetry features (percentage depth doses and lateral beam profiles) passed the gamma (γ) index criterion of 2%/2 mm. MC calculations and physical measurements agreed to approximate local difference of 1.44% for relative output factors. This accurate source model is suitable for the sensitivity study. It also has the potential to be used for dose calculation in advanced radiotherapy treatment planning. The accurate source model with the IQM CM positioned with its central electrode plate fixed perpendicularly to the photon beam in subsequent simulations was used. The spatial integral dose in the air region of the IQM CM was calculated. The IQM MC dose was calculated for 1 × 1 up to 30 × 30 cm2 fields at 10 MV photon beams and then correlated with physical measurement of the prototype IQM system. Secondly, systematic positional errors of 1, 2 and 3 mm were subtracted and added to the whole MLC bank of 1 × 1, 3 × 3, 5 × 5 and 10 × 10 cm2 fields. Thirdly, the IQM signal response for 1, 2, 3, 4 and five leaves shifted out of a 5 × 5 cm2 field for positional error of 1, 2, 3, 5, and 10 mm was calculated. Fourthly, the signal response was calculated for segments along the gradient of the IQM CM for 3 × 3, 5 × 5 and 7 × 7 cm2 fields at 10 MV photon beams. Lastly, eleven segments (regular and irregular) were altered randomly within ±1, ±2 and ±3 mm regarding its individual leaf positions as defined at the isocentre. Sensitivity analyses of leaf positioning errors were studied by using the following techniques such as scatter plots, brute force, variance-based and standard regression coefficient. The normalised IQM signal increases with an increase in square field sizes for the MC calculation and the physical measurement. The IQM model is highly sensitive to alterations of 1 × 1 cm2 more than other fields considered. For the segments considered, the magnitude of the signal response decreased and increased when systematic positional errors were subtracted from and added to individual MLC leaves. An increase in numbers of leaves shifted out causes an increase in IQM signal response and an increase in the position of moving leaves causes a further increase in the IQM signal. The sensitivity of the IQM model increases along the gradient of the IQM up to a noticeable plateau. The sensitivity analysis techniques utilised in this study deduced that the IQM model is highly sensitive to leaf positions of small segments compared to large apertures. The newly developed IQM MC model can now serve as a basis for researchers that have an interest in dose monitoring and MLC calibration using the wedge-shaped ionization chamber. The IQM model shows a potential platform for further study on advanced radiotherapy quality control. Application of MC techniques to dose monitoring is authentic. It demonstrates that the MC radiation transport method is virtually unlimited when it comes to solving radiation transport and dose calculation challenges.Item 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.