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The 1D lateral dose response functions of photon-dosimetry detectors in magnetic fields—measurement and Monte-Carlo simulation

The present study is concerned with clinical photon-beam dosimetry at radiotherapy units combined with magnetic resonance imaging devices. Due to the superposed constant magnetic field, the deflections of the secondary electron trajectories by the Lorentz force not only influence the 2D dose distribution, D(x,y), in water phantoms, but furthermore modify the secondary electron transport within the detectors and thereby the detectors’ signal profiles, M(x,y), across the photon beams. This second effect can be represented by the lateral dose response function, K(x,y), the convolution kernel transforming D(x,y) into M(x,y) via the convolution M(x,y)  =  D(x,y) * K(x,y). The 1D functions K(x) of a set of commercial gas-filled and solid-state photon-beam detectors were experimentally determined using a slit beam geometry together with a constant, homogeneous magnetic field of up to 1.42 T. As predicted by a recent Monte-Carlo study (Looe et al Phys. Med. Biol. 62 5131–48), the functions K(x) of these detectors are shown experimentally to be distorted in magnetic field. For the larger Semiflex 3D 31021 chamber, the FWHM value of K(x) decreases from 4.9 mm for the field free (0 T) case to 4.8 mm in 0.35 T and 4.1 mm in 1.42 T magnetic field, whereas the FWHM value of the smaller PinPoint 3D 31022 chamber decreases from 2.8 mm for the field free case to 2.6 mm in 1.42 T magnetic field. The FWHM values of the semiconductor detectors are not modified in magnetic fields. Additionally, the symmetry of K(x) is shown to be distorted in magnetic field. Using a 10 mm wide field as example, the signal profiles, M(x), predicted by the measured and simulated K(x) by convolution with D(x) (measured with EBT3 film) agree within 3% of the maximum value to the measured M(x) for all detectors, except for the silicon diode detector if the measured K(x) was used, where deviations of around 5% were observed at the field border.