Evaluation of volumetric modulated arc therapy and quality assurance based on monte carlo simulation

Resumo

Complex intensity modulated fields delivered by means of rotational dynamic techniques, such as Volumetric Modulated Arc Therapy (VMAT), can be considered a step forward in comparison to conventional, static technique, providing demanding dose distributions in short irradiation times. However this dynamic implementation involves two main sources of uncertainty: one related to the dose calculation accuracy, and the other linked to the continuous delivery of a discrete calculation. Therefore, require new quality assurance (QA) protocols and detailed verification capable of predicting the actual delivered dose to the patient. This is especially critical when used with hypofractionated schemes and for stereotactic body radiotherapy (SBRT) treatments. In this scenario, Monte Carlo (MC) simulation presents an ideal tool to complete the linac commissioning required for VMAT, as well as the gold standard for dose distribution verification. The present thesis reflects the work carried out in order to implement a routine MC verification of VMAT treatments, and to develop a QA model able to control and potentially reduce the inherent uncertainties for a fair and reliable evaluation of current VMAT solutions, including further evaluation of VMAT QA systems. The developed model consists on a system composed by a specific phantom integrated with MC simulation of VMAT log files in a feedback procedure by implementing an optimization process able to adjust the Monitor Units and reconstruct the dose-volume histogram on the patient CT. Several clinical cases, previously planned with different treatment planning systems and verified with different commercial solutions were selected in order to test operational feasibility of the proposed model. The proper operation of the feedback procedure was proved through the achieved high agreement between reconstructed dose distributions and the film measurements. The proposed model showed to be valid for VMAT assessment, and also for linac commissioning and evaluation of other QA systems. Besides, the results also showed enough robustness and efficiency of the model to be considered as a pre-treatment VMAT verification system.