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.
Monte Carlo, Small Fields, Modeling
TH- External Beam- Photons: Small field computational dosimetry