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Monte Carlo Simulations for Magnetic Field Correction Factors of Plane-Parallel Ionization Chamber

J Lee

Jaegi Lee1*, Jimin Lee1, Dongmin Ryu1, Hochan Lee2, and Sung-Joon Ye1 , (1) Seoul National University Graduate School of Convergence Science & Technology, Seoul, Republic of Korea, (2) Ajou University, Suwon, Gyeonggi-do, Republic of Korea


SU-F-108-7 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 108

Purpose: The reference dosimetry with ionization chambers in the presence of a magnetic field needs an additional correction factor. The objective of this study was to investigate such effects on a plane-parallel ionization chamber through Monte Carlo simulations of high-energy photon beam.

Methods: The EGSnrc user code, egs_chamber, was used to simulate a plane-parallel (PP) ionization chamber model (NACP-02). A full-head geometry of linear accelerators was also generated in this simulation. The beam quality factors were calculated by the ratio of the ionization chamber response based on the absorbed dose to water for three therapeutic energies (6, 10, and 15 MV) compared to that of the Co-60 beam. The position of ionization chamber was at a depth of 10 cm in water, and the source-to-surface distance was 100 cm. The field size was 10×10 cm². The magnetic field correction factors were calculated from the change of ionization chamber response in 1.5 T.

Results: The percent depth doses were 66.7% for 6 MV, 75.4% for 10 MV, and 77.8% for 15 MV. The beam quality factors of 6, 10, and 15 MV beams were 0.992, 0.980, and 0.973, respectively. These values were within 0.25% compared to the previous report. The magnetic field correction factors of those energies for 1.5 T were 0.922, 0.972, and 0.983, respectively. All of the simulation uncertainties were within 0.2%.

Conclusion: This work indicates a plane-parallel ionization chamber could be used for the reference dosimetry in the presence of magnetic field with an additional correction factor.

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