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Monte Carlo Simulations for Quality Assurance of Varian TrueBeam 6MV FFF RapidArc SBRT Treatments

T Teke

T Teke1*, C Duzenli2, A Mestrovic1, D Hyde1, M Milette1, I Popescu2, E Gete2, (1) BC Cancer Agency, Kelowna, BC, (2) BC Cancer Agency, Vancouver, BC,

TH-C-BRB-5 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

To establish feasibility of performing quality assurance for Flattening Filter Free (FFF) RapidArc stereotactic body radiotherapy treatments (SBRT) on a TrueBeam LINAC using Monte Carlo simulations.

Phase-space files for TrueBeam FFF photon beams were made available by Varian in IAEA-compliant format. Monte Carlo simulations were performed using BEAMnrc and DOSXYZnrc to validate the 6MV FFF phase space files for use in this study. The phase space data provided by Varian is in cylindrical geometry and required conversion into a format that was compatible with BEAMnrc prior to use. To establish validity of the phase space data, dose calculations in a water phantom for fields ranging from 3x3 cm^2 to 40 x 40 cm^2 were performed using DOSXYZnrc. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with measurements. Monte Carlo simulations of 6MV FFF SBRT RapidArc plans were performed using a 2mm^3 voxel size and compared with both ion chamber measurement and Eclipse Treatment Planning System (TPS) dose calculations. 3D gamma analysis (3%,3mm) comparing Monte Carlo and TPS results was performed.

Monte Carlo simulations and measured values agreed within 1% and 1.5% for PDDs and profiles respectively for all fields. The agreement between measured and calculated output factors was within 1% including for highly asymmetric fields. These results indicate that the 6MV FFF phase space data is sufficiently accurate for use in quality assurance in radiation therapy. For the 6 MV FFF RapidArc plans the agreement between MC and both measured and TPS dose calculations was within 2%. Over 95% of the points passed the 3D Gamma test

We have demonstrated the feasibility of performing patient specific quality assurance for 6 MV FFF SBRT RapidArc treatments using Monte Carlo simulations for a TrueBeam linac.

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