Masters Degrees (Medical Physics)

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Browsing Masters Degrees (Medical Physics) by Author "Lotter, M. G."

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    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.
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    Using Monte Carlo techniques to evaluate the dose distributions from a radiotherapy treatment planning system
    (University of the Free State, 2000-06) Awusi, Kavuma; Lotter, M. G.; Willemse, C. A.
    English: In this study we used Monte Carlo techniques to simulate the SL25 linear accelerator treatment head using the BEAM Code. The main purpose of study was to evaluate the dose distributions obtained by the CADPLAN treatment planning system (TPS) for 8 MV photon beams of a SL25 linear accelerator in realistic patient models. Simulation of the treatment head involves modeling of the main components of the treatment head that have influence on the absorption and scattering of radiation. Simulation of the accelerator was done in two parts to minimize the simulation time. Analysis of the data generated by the BEAM code was carried out using BEAMDP, another subsidiary of the BEAM code. We calculated the beam characteristics which are difficult to measure experimentally, such as angular distributions, spectral distributions, planar fluence and planar energy fluence at a plane located just above the jaws of the treatment head. The phase space files at the isocenter were used as source input for DOSXYZ, a MC code to calculate 3D dose distributions in water or CT based phantoms. The DOSXYZ code was used to calculate depth dose and cross plane profiles in a water phantom. The data obtained with Monte Carlo methods were compared with that obtained by ionization chamber measurements. Depth dose and cross plane profiles obtained by Monte Carlo methods and ionization chamber measurements generally agreed within 2%. We created patient models from CT data of real patients using the CTCREATE option of the DOSXYZ program. Dose distributions for a number of field sizes and different anatomical sites were calculated with the DOSXYZ code and compared with corresponding dose distributions calculated by the TPS. The modified BATHO and ETAR inhomogeneity correction methods used in the TPS were evaluated. Results show that Monte Carlo methods can accurately reproduce ion chamber measurements in a water phantom. Monte Carlo techniques are very useful for evaluating the accuracy of dose distributions generated by treatment planning systems in patient based models where measurements are impossible. The BATHO and ETAR methods showed comparable results to the Monte Carlo results. This could be due to the inefficiency of the method (visualization of the dose distributions) that we used for the comparison of the results. A more quantitative method like the use of the dose difference volume histogram could give a more comprehensive evaluation.