Purpose: RefleXion is a novel radiotherapy machine that tracks tumor using real-time positron emission signals and delivers radiation through a binary multi-leaf collimation (MLC) system. A Monte Carlo (MC) model of the RefleXion Linac is validated against commissioning measurement in this study.
Methods: The RefleXion Linac consists of a 6 MV FFF photon beam, a 64-leaf MLC system, and two pairs of y-jaws locating above and below the MLCs. The leaf side focuses on the source and each leaf provides a 0.625 cm opening along the x direction at 85 cm SAD, giving a total of 40 cm opening when all MLCs are retracted. The y-jaw pairs move simultaneously to open 1 or 2 cm at 85 cm SAD. The beam commissioning measurements were performed in a Blue Helix Phantom using a diode and a W2 scintillation detector for scanning and output factor acquisition, respectively. The MC model provided by the vendor includes phase space files above MLCs and detailed structures of MLC leaves and lower y-jaws. Beam characteristics of field sizes from 1.25 x 1 cm² to 40 x 2 cm², including percentage depth dose (PDD), beam profiles, and output factors were simulated using MC codes, BEAMnrc and GATE, and compared with measurement. The relative simulation uncertainty is approximately 0.6%.
Results: The mean and maximum differences of %dd(10) between measurement and simulation are 0.31% and 0.82%, respectively. The inline FWHM shows a deviation of 0.47 mm on average and 0.58 mm at most. The crossline off-axis ratio of the central 80% of field size differs by 0.88% on average (std = 0.92%). The mean output factor disagreement is 0.93% (std = 0.72%).
Conclusion: A complete and accurate MC model of RefleXion Linac has been validated. This model will help substantially in the upcoming dosimetric, imaging and quality assurance studies.