Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers

Abstract

To determine k(Q(msr),Q(o) ) (f(msr),f(o) ) correction factors for machine-specific reference (msr) conditions by Monte Carlo (MC) simulations for reference dosimetry of TomoTherapy static beams for ion chambers Exradin A1SL, A12; PTW 30006, 31010 Semiflex, 31014 PinPoint, 31018 microLion; NE 2571. For the calibration of TomoTherapy units, reference conditions specified in current codes of practice like IAEA∕TRS-398 and AAPM∕TG-51 cannot be realized. To cope with this issue, Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] described a new formalism introducing msr factors k(Q(msr),Q(o) ) (f(msr),f(o) ) for reference dosimetry, applicable to static TomoTherapy beams. In this study, those factors were computed directly using MC simulations for Q(0) corresponding to a simplified (60)Co beam in TRS-398 reference conditions (at 10 cm depth). The msr conditions were a 10 × 5 cm(2) TomoTherapy beam, source-surface distance of 85 cm and 10 cm depth. The chambers were modeled according to technical drawings using the egs++ package and the MC simulations were run with the egs_chamber user code. Phase-space files used as the source input were produced using PENELOPE after simulation of a simplified (60)Co beam and the TomoTherapy treatment head modeled according to technical drawings. Correlated sampling, intermediate phase-space storage, and photon cross-section enhancement variance reduction techniques were used. The simulations were stopped when the combined standard uncertainty was below 0.2%. Computed k(Q(msr),Q(o) ) (f(msr),f(o) ) values were all close to one, in a range from 0.991 for the PinPoint chamber to 1.000 for the Exradin A12 with a statistical uncertainty below 0.2%. Considering a beam quality Q defined as the TPR(20,10) for a 6 MV Elekta photon beam (0.661), the additional correction k(Q(msr,)Q) (f(msr,)f(ref) ) to k(Q,Q(o) ) defined in Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] formalism was in a range from 0.997 to 1.004. The MC computed factors in this study are in agreement with measured factors for chamber types already studied in literature. This work provides msr correction factors for additional chambers used in reference dosimetry. All of them were close to one (within 1%).