Room: AAPM ePoster Library
Purpose: investigate the response of parallel-plate ionization chambers on the buildup dose measurement in inline magnetic fields using Monte Carlo (MC) simulation.
Methods: ROOS (0.39 cm³) and NACP-02 (0.16 cm³) parallel-plate chambers 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, 1.0, 1.5, and 3.0 T were applied parallel to the beam axis. The dose to water, D?w?, and the dose in air cavity of chamber, D?chamber?, were calculated at 0.1-1.5 cm depth in a water phantom. The perturbation correction factor, P?Q?, for the parallel-plate chambers 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: 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 dose increment on magnetic field strengths of 0.35 - 3.0 T was almost the same. 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 P?Q? values for the ROOS and NACP-02 chambers in magnetic fields were close to unity compared to those of B=0 T and the P?Q? variation on the magnetic field strength was slightly.
Conclusion: response of parallel-plate ionization chambers is affected by focusing electrons in the buildup dose measurement under the inline magnetic fields.
Not Applicable / None Entered.