Small-cavity chamber dose response in megavoltage photon beams coupled to magnetic fields.

Small-cavity chamber dose response in megavoltage photon beams coupled to magnetic fields.
Phys Med Biol. 2020 Jul 16;:
Authors: Cervantes Y, Billas I, Shipley DR, Duane S, Bouchard H
Abstract
In MRgRT, dosimetry measurements are performed in the presence of magnetic fields. For high-resolution measurements, small-cavity ionization chambers are required. While Monte Carlo simulations are essential to determine dosimetry correction factors, models of small-chambers require careful validation with experimental measurements. The aim of this study is to characterize small-cavity chamber response coupled to magnetic fields. Small-cavity chambers (PTW31010, PTW31016, PTW31021 and PTW3022) are irradiated by a 6 MV photon beam for nine magnetic field strengths between -1.5T and +1.5T. The chamber axis is orientated either parallel or perpendicular to the irradiation beam, with the magnetic field always perpendicular to the beam. MC simulations are performed in EGSnrc. The sensitive volume of the chambers is reduced to account for the inefficiency adjacent to the guard electrode (dead volume) based on COMSOL calculations of electric potentials. The magnetic field affects the chamber response by up to 4.1% and 4.5% in the parallel and perpendicular orientations, respectively, compared to no magnetic field. The maximal difference in dose response between experiments and simulations is up to 6.1% and 4.5% for parallel and perpendicular orientati...

Source: Physics in Medicine and Biology - July 15, 2020 Category: Physics Authors: Cervantes Y, Billas I, Shipley DR, Duane S, Bouchard H Tags: Phys Med Biol Source Type: research