Development of an EGSnrc multi‐leaf collimator component module and treatment head model for a low‐field MRI linear accelerator.

Background Purpose Methods Results Conclusions Monte Carlo (MC) modeling of MR‐guided radiotherapy (MRgRT) treatment machines enables the characterization of photon/electron interactions in the presence of a magnetic field. The EGSnrc MC code system is a well‐established system for radiation dose calculations. The multi‐leaf collimator (MLC) component modules presently available within the EGSnrc MC code system do not include a model of the double‐focused MLC available on a low‐field (0.35T) MRI linear accelerator (MR linac).Here we developed and validated a new component module (CM) for the low‐field MRgRT MLC using the EGSnrc/BEAMnrc/DOSXYZnrc code system. We performed detailed modeling of the treatment head and validated the model using measurements and calculations from the vendor‐specific treatment planning system (TPS).The detailed geometry of the low‐field MR linac MLC and other treatment head structures were modeled using BEAMnrc. Comparisons of DOSXYZnrc simulated dose against measurements and the low‐field MR linac TPS for a variety of AAPM TG‐53 task group report suggested square and shaped fields, as well as a step‐and‐shoot intensity‐modulated radiotherapy (IMRT) plan, are presented.Our model agrees with both measured and TPS calculated data on average within 2%/2 mm (dose/DTA) criterion for square field profiles. Output factors agreed within 1% for field sizes down to 2.49 × 2.49 cm2 and within 2% of TPS data for the smallest field size of 0.83 × 0.83 cm2. Shaped field and IMRT MC calculations agreed with measured and TPS data such that the gamma pass rates (3%/2 mm) were 99.5% and (3%/3 mm) 96.2%, respectively.We developed and validated an MLC CM (SYNCVRMLC) for the low‐field MR linac using the EGSnrc MC code systems. This new CM will facilitate MC computation of fluence and dose distributions using BEAMnrc/DOSXYZnrc for patients treated on the low‐field MR linac. [ABSTRACT FROM AUTHOR]