Purpose: The RefleXion™ X1 is a novel ring gantry system with a unique structural design made to accommodate both a linear accelerator (LINAC) and positron emission tomography (PET) detectors for biology-guided radiotherapy (BgRT). A dose monitor chamber (DMC) for measuring beam output from the LINAC is located between a fixed primary collimator and a movable secondary multi-leaf collimator (MLC). Backscattered radiation from the secondary collimator may contribute to the total charges collected in the DMC, may result in an artifact in the measurement. The present investigation utilized Monte Carlo (MC) simulation to characterize the impact of backscattered radiation to the DMC measurements.
Methods: The EGSnrc Monte Carlo package was used to model the X1 LINAC output of a 6 MV photon beam in flattening filter-free mode with 521 keV global electron cutoff energy and spin-on for reasonable backscattering calculation. The backscatter effect from downstream LINAC components was quantified by calculating the contribution of forward-directed and backscattered radiation to the energy deposited in the DMC. Scenarios involving 2, 4, 8, and 16 open MLC leaves with 1, 2, and 3 cm jaw settings were computed.
Results: A maximum backscatter contribution of 0.12%±0.1% was obtained for a simulated field of 10 cm (16 MLC leaves) with a 2 cm jaw opening. For a simulated field of 1.25 cm (2 MLC leaves), a maximum backscattering contribution of 0.27%±0.1% was recorded. On average, backscattering contribution decreased with an increase in the field size. Minimal variation in backscattering was observed for different jaw openings
Conclusion: For the X1 LINAC photon beam, the effect of backscattered radiation is minimal even with very small field sizes and is not correlated with jaw size. Therefore, correcting for field-size dependence of DMC output to account for backscatter is not necessary for the RefleXion X1 design.
Funding Support, Disclosures, and Conflict of Interest: Research supported by RefleXion Medical.