Small-cavity chamber dose response to megavoltage photon beams in the presence of magnetic fields
PH-0047
Abstract
Small-cavity chamber dose response to megavoltage photon beams in the presence of magnetic fields
Authors: CERVANTES|, Yunuen(1)*[y.cervantes.espinosa@umontreal.ca];Billas|, Ilias(2);Shipley|, David(2);Duane|, Simon(2);Bouchard|, Hugo(1);
(1)Université de Montréal, Département de physique, Montreal, Canada;(2)National Physical Laboratory, Chemical- Medical and Environmental Science Department, Teddington, United Kingdom;
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Purpose or Objective
With the advent of magnetic resonance guided radiation therapy (MRgRT), dosimetry measurements must be performed in the presence of magnetic fields (B-fields). Monte Carlo (MC) simulations play an essential role in determining dosimetry correction factors. In particular, high-resolution measurements require small cavity ionization chambers, MC models require careful validation with experiments. The objective of this study is to characterize small cavity ion chamber response in the presence of B-fields via experimental measurements and MC simulations.
Material and Methods
A small cavity ion chamber (models: PTW31010, PTW31021, PTW31016 and PTW3012) is irradiated in water by a conventional 6 MV Elekta linac beam for various B-field strengths at two polarities: 0 T, ±0.35 T, ±0.5 T, ±1 T, ±1.5 T. The B-field is always perpendicular to the irradiation beam and the chamber axis. Two orientations of the chamber axis are studied: parallel and perpendicular to the photon beam. The experimental setup is simulated using egs_chamber (EGSnrc) and the enhanced EMF macro. The sensitive volume is reduced to account for the inefficiency adjacent to the guard electrode (i.e. dead volume) based on COMSOL simulations of electric potentials.
Results
For small chambers, the dead volume represents a large volume (15%-23%) of the sensitive volume. The B-field affects the chamber response by up to 4.1% and 4.5% in the parallel and perpendicular orientations, respectively. In the parallel orientation, the maximal percentage difference in relative response was reduced from 4.37%, 6.06% and 2.81% to 0.60% for PTW31010, PTW31021, PTW31016 and PTW30122 and from 1.91 % to 1.57% for PTW31016 when the dead volume is removed. In the perpendicular orientation, for B > 0 T, the maximal difference was reduced from 2.10%, 3.57%. 2.24% and 1.73% to 0.31%, 0.67%, 2.15% and 0.70% for PTW31010, PTW31021, PTW31016 and PTW30122, respectively. Experimental and simulated relative response of PTW31022 considering the complete sensitive volume and removing the dead volume are shown in the attached figure.
Conclusion
Results suggest that B-fields highlights chamber geometry imperfections in simulations and in measurements, discrepancies between them increases with B-field strength. The reliability of design specifications plays an essential role in dose response characterization, including the dead volume considerations, especially in the presence of magnetic fields.