Impact of transverse magnetic fields on dose response of a radiophotoluminescent glass dosimeter in megavoltage photon beams.

PURPOSE

The purpose of this study was to investigate the impact of transverse magnetic fields on the dose response of a radiophotoluminescent glass dosimeter (RGD) in megavoltage photon beams.

METHODS

The RGD relative response (i.e., RGD dose per absorbed dose to water at the midpoint of the detector in the absence of the detector) was calculated using Monte Carlo (MC) simulations. Note that the Monte Carlo calculations do not account for changes of the signal production per unit dose to the RGD caused by the magnetic field strength. The relative energy response R , the relative magnetic response R , and the relative overall response R with the transverse magnetic fields of 0-3 T were analyzed as a function of depth, for a 10 cm × 10 cm field in a solid water phantom, for 4-18 MV photons. Although magnetic resonance (MR) linacs with flattening filter free beams are commercially available, flattening filter beams were used to investigate the RGD response in this study. R is the response in beam quality Q relative to that in the reference beam with quality 6 MV, R is the response in beam quality Q with the magnetic field relative to that in beam quality Q without the magnetic field, and the R is the response in beam quality Q with the magnetic field relative to that in the reference beam with quality 6 MV without the magnetic field. Two RGD orientations were considered: RGD long axis is parallel (direction A) and perpendicular (direction B) to the magnetic field. The reference irradiation conditions were at the depth of 10 cm for a 10 cm × 10 cm field for 6 MV, without the magnetic field. In addition, the influence of a small air-gap between the holder inner wall and the RGD on the dose response in the magnetic field, R , was analyzed in detail. R is the response in beam quality Q without/with the air-gap.

RESULTS

R decreased by up to 2.7% as the energy increased in the range of 4-18 MV, except in the buildup region. In direction A, the variation of R owing to the magnetic field strength was below 1.0%, regardless of the photon energy. In contrast, in direction B, R decreased with increasing magnetic field strength and decreased up to 4.0% at 3 T for 10 MV. The R for 0.03 and 0.05 cm air-gap models in direction A decreased up to 2.3% and up to 4.0%, respectively.

CONCLUSIONS

The variation of R changed with the direction of the RGD relative to the magnetic field. For dose measurements, RGDs should be positioned with the long axis parallel to the magnetic field, without air-gaps.