Room: AAPM ePoster Library
Purpose: develop and validate a Monte Carlo (MC) simulation framework for the imaging systems of the Linac TrueBeam STx, and study the scatter radiation in CBCT for the implementation and assessment of scatter correction methods.
Methods: performed the Linac head and x-ray tube simulations using the BEAMnrc code to get the phase spaces of the 2.5 MV and kV beams, respectively. The former is used in the EPID system and the second in CBCT images with the OBI system. The simulations were validated by comparing measured PDD curves, off-axis profiles and transmission curves with those calculated with the DOSXYZ code and a model of the detector response using the theoretical energy spectra. The transmission curves were obtained by imaging Al and Cu step-wedge phantoms with both the kV and MV systems. The egs_cbct code was used to simulate CBCT projections separating the scatter and primary contributions, taking care to optimize the code using variance reduction techniques (VRTs).
Results: transmission values obtained with the step wedge phantoms have excellent agreement with the theoretical detector signal curves. MC dosimetric data of both imaging systems have also been compared with previously reported data, obtaining good agreement. Optimization of the egs_cbct code using forced detection, delta transport, and fixed splitting allowed to speed-up the calculation of the scatter components by a factor 3.
Conclusion: MC models of the imaging systems of the TrueBeam STx developed in this work show very good agreement with experimental measurements and with previous results reported in the literature. The transmission measurements proved to be an excellent and fast method for the validation of the MC calculated photon energy spectra. The use of VRTs allowed a better estimation of the scatter contribution in considerably less time. This MC framework can be used as part of an iterative scatter correction method.
Funding Support, Disclosures, and Conflict of Interest: Acknowledgment for Scholarship by PNPC-CONACYT and funding support PAEP-PCF, UNAM