Medical Physics
Permanent URI for this community
Browse
Browsing Medical Physics by Subject "BEAMnrc"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
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 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.