Absolute dose calibration and output factor simulation for flattened and flattening filter-free Monte Carlo model of 6 MV photon beams

Abstract

Objective To explore the Monte Carlo calculation methods for the absolute dose calibration and output factor of 6 MV flattening-filter (FF) and flattening-filter free (FFF) photon beams based on TrueBeam accelerator. Methods The BEAMnrc code was used to model the LINAC head of FF and FFF modes. The BEAM_up covers the components from the target to the monitor chamber, and BEAM_down includes the structures beneath the chamber, the dose deposit to the monitor chamber contributed by the incidence electrons and scattered particles from the secondary collimators were calculated respectively. The incidence electron-induced dose at certain depths on the central axis were simulated by means of the DOSXYZnrc code. By means of dose calibration equation, the calibration factor for the standard field (10 cm×10 cm) and the output factors for various fields (1 cm×1 cm-40 cm×40 cm) were computed respectively. Results For the 6 MV FF and FFF beams under the standard 10 cm×10 cm field, 1 MU equals to 7.747×1013±3.099×1011 and 3.248×1013±1.624×1011 electrons to the target, respectively, which deposited 21.53 and 35.01 cGy to the monitor chamber of the virtual accelerator respectively. The difference between the simulated and calculated output factors were 0.72%±1.4% and 0.56%±0.78% for FF and FFF, respectively. Conclusions The model generated and measured output factors agree well, indicating the good accuracy of the dose calculation by the model, which would provides basis for further clinical dosimetric studies. Key words: Flattened model; Flattening-filter free model; Monte carlo; Output factor; Absolute dose calibration