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Monte Carlo Computed KQmsr Correction Factors for Reference Dosimetry of Tomotherapy Beams for 7 Ion Chambers

E Sterpin

E Sterpin1*, R Mackie2, S Vynckier3, (1) Center for molecular imaging, radiotherapy and oncology, Université Catholique de Louvain, Brussels, Belgium ,(2) University of Wisconsin, Madison, WI, (3) Departement of radiotherapy, Cliniques Univ St. Luc, Brussels, Belgium

SU-C-213AB-3 Sunday 1:30:00 PM - 2:15:00 PM Room: 213AB

Purpose: To compute kQmsr correction factors for various types of ion chamber (Exradin A1SL, A12; PTW 30006, 30010, 31014, MicroLion and NE-2571) using Monte Carlo (MC) simulations according to the new formalism of Alfonso et al (Med. Phys. 35 2008)

Methods: TomoTherapy units are not able to realize reference conditions specified in current codes of practice like IAEA/TRS-398 and AAPM/TG-51. To cope with this issue, Alfonso et al described a new formalism introducing machine-specific (msr) factors kQmsr for reference dosimetry, applicable to static TomoTherapy beams. In this study, those factors were computed directly using MC simulations for Q0 corresponding to a simplified 60Co beam in TRS-398 reference conditions (at 10 cm depth). Msr conditions were a 10x5 cm² 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 60Co 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%.

Results: All correction factors were close to one, from 0.991 for the PTW 31014 to 1.000 for Exradin A12 (+- 0.002). The factor obtained for Exradin A1SL (0.997 +-0.002) confirmed previous findings. This work provides msr correction factors for additional chambers used in reference dosimetry.

Conclusions: Global msr correction factors obtained in this study can be used directly for reference dosimetry of static tomotherapy beams. In the Alfonso et al formalism, intermediate factors are introduced to link the new formalism to existing protocols. Those intermediate factors are not needed here.

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