N Anami*,

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  N Anami

SU-IePD-TRACK 6-3 (Sunday, 7/25/2021) 5:30 PM - 6:00 PM [Eastern Time (GMT-4)]

Purpose: To investigate the response of parallel-plate ionization chambers and a diode detector on the buildup dose measurement in inline magnetic fields using Monte Carlo (MC) simulation.

Methods: The ROOS, NACP-02 and MARKUS parallel-plate chambers and a PTW-60018 diode detector were modeled using the EGSnrc/cavity code. A 6 MV photon beam from a Varian Novalis Tx head was irradiated with a 10 × 10 cm² field at a source-to-surface distance of 100 cm. The magnetic field strengths of 0, 0.35 and 1.0 T were applied parallel to the beam axis. The dose to water, D₍w₎, and the dose in sensitive volume of detector, D₍det₎, were calculated at 0.1-1.5 cm depth in a water phantom. The overall perturbation correction factor, P₍Q,B₎, for the detectors was calculated with/without magnetic fields according to the Spencer–Attix cavity theory. The average restricted mass collision stopping-power ratio of water to air was calculated by the EGSnrc/SPRRZ code. In all dose calculations, photon and electron cutoff energies were set to 0.01 MeV and 0.521 MeV, respectively. The statistical uncertainty of the calculated dose was less than 0.1% in one standard deviation.

Results: The dose to water at a 0.1 cm depth in inline magnetic fields was higher up to 37.4% than that of B=0 T. The depth of the dose maximum in all inline magnetic fields was shifted 0.7 cm toward the surface compared to that of B=0 T. The ₍PQ,B₎ values for PTW-60018 were 0.814-0.889 at 1.0 T. Similarly, the P₍Q,B₎ values for ROOS, NACP-02, and MARKUS chambers they were 0.983-1.015, 0.960-1.022, 0.959-1.027 at 1.0 T, respectively, and larger than those of PTW 60018.

Conclusion: The response of parallel-plate ionization chambers and a diode detector is affected by focusing electrons in the buildup dose measurement under the inline magnetic fields.

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