Room: Stars at Night Ballroom 1
Purpose: Independent dose verification with Monte Carlo (MC) simulations is suggested as an important feature of proton therapy quality assurance. However, the MC dose engine must be accurately commissioned to ensure a reliable assessment of the plan optimized by the treatment planning system (TPS). This commissioning procedure is time consuming and relatively complex for non experts. We therefore propose a method to automatically generate the beam data library (BDL) for MCsquare, a fast, recently released, open source MC code.
Methods: The BDL requires energy-dependent parameters that describe 1) optical properties (beam divergence, spot size and their correlation), 2) the beam energy spectrum, and 3) the number of protons per MU for absolute dose computation. This BDL is optimized from measurements acquired during TPS commissioning.Optical parameters are retrieved by fitting beam divergence and spot sizes according to the Courant-Snyder theory, considering measured spot fluences as 2D Gaussians. The mean beam energy and its standard deviation are optimized in an iterative process using a Nelder-Mead method to reproduce measured depth dose profiles, assuming the spectrum is Gaussian. Protons per MU are deduced using GomÃ â€™s method.Finally, the resulting BDL is validated by comparing ranges and absolute doses obtained in MC simulations and measurements respectively, for various plans with different complexities.
Results: The method was applied to commissioning MCsquare for a proton therapy center. The optimization of the energy spectrum led to range errors in Bragg peaks below 0.03 mm. All validation tests led to range deviations of less than 1 mm. Absolute dose comparisons showed an accuracy better than 1% for 93% of the simulated plans, with a maximum error of 1.9%.
Conclusion: We propose an automatic tool to process measurements data in order to commission any proton PBS MC algorithm. The method reduces human intervention and provides good accuracy.