A Monte Carlo Dose Verification Model for the Small Fields with 6FFF and 10FFF Energy Beams of the Varian True Beam (EDGE) Radiosurgery System

The measured dosimetry uncertainties in our clinical cases treated with diminutive static MLC fields of 10MV FFF arc beams in Stereotactic Radiosurgery have suggested a possible cause of MLC geometric uncertainties. To further investigate this with various MLC displacements, we created a Monte Carlo (MC) dose model for both 6FFF and 10FFF beams of Varian EDGE Clinac system using the EGSnrc MC program. 10-30 million incident particles were simulated to validate the MC model w/o MLC by using the phase space files provided by the Varian systems. The input parameters varied during the MC simulations were the number of incident particles, directional Bremsstrahlung splitting, range rejection, particle splitting and voxel size to reduce the dose uncertainty and optimize the efficiency. Preliminary MC dose simulations resulted dose uncertainties < 5% for 10x10, 5x5 cm² open fields with 6FFF and 10FFF energy beams at 100cm SSD, d=d_max as well as 95cm SSD, d=5cm water phantom set ups. Dose uncertainty of <4% was obtained for the reference MLC field 5.0x5.0 cm² and the test smallest field 0.5x0.5cm² at 95 cm SSD, d=5cm water phantom set up. Comparison of the MC dose model against the Eclipse TPS as well as the measurements by SRS Mapcheck have shown a satisfactory initial validation score.