Using Monte Carlo techniques to evaluate the dose distributions from a radiotherapy treatment planning system

[摘要] English: In this study we used Monte Carlo techniques to simulate the SL25 linear acceleratortreatment head using the BEAM Code. The main purpose of study was to evaluate thedose distributions obtained by the CADPLAN treatment planning system (TPS) for 8 MVphoton beams of a SL25 linear accelerator in realistic patient models. Simulation of thetreatment head involves modeling of the main components of the treatment head thathave influence on the absorption and scattering of radiation. Simulation of the acceleratorwas 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 aredifficult to measure experimentally, such as angular distributions, spectral distributions,planar fluence and planar energy fluence at a plane located just above the jaws of thetreatment head.The phase space files at the isocenter were used as source input for DOSXYZ, a MC codeto calculate 3D dose distributions in water or CT based phantoms. The DOSXYZ codewas used to calculate depth dose and cross plane profiles in a water phantom. The dataobtained with Monte Carlo methods were compared with that obtained by ionizationchamber measurements. Depth dose and cross plane profiles obtained by Monte Carlomethods and ionization chamber measurements generally agreed within 2%.We created patient models from CT data of real patients using the CTCREATE option ofthe DOSXYZ program. Dose distributions for a number of field sizes and differentanatomical sites were calculated with the DOSXYZ code and compared withcorresponding dose distributions calculated by the TPS. The modified BATHO andETAR inhomogeneity correction methods used in the TPS were evaluated.Results show that Monte Carlo methods can accurately reproduce ion chambermeasurements in a water phantom. Monte Carlo techniques are very useful for evaluatingthe accuracy of dose distributions generated by treatment planning systems in patientbased models where measurements are impossible. The BATHO and ETAR methodsshowed comparable results to the Monte Carlo results. This could be due to theinefficiency of the method (visualization of the dose distributions) that we used for thecomparison of the results. A more quantitative method like the use of the dose differencevolume histogram could give a more comprehensive evaluation.

[发布机构] University of the Free State