Development of a Monte Carlo simulation method for the evaluation of dose distribution calculations of radiotherapy treatment planning systems
Du Plessis, Frederik Carl Philippus
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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.