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The Influence of Magnetic Fields On the 1D Lateral Dose Response Function of Photon-Beam Detectors

B Delfs1*, D Poppinga2 , A Ulrichs1 , B Poppe1 , H Looe1 (1) University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany (2) PTW Freiburg, Freiburg, Germany


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The aim of this study is to determine the one-dimensional lateral dose response functions K(x) of two air-filled ionization chambers (PTW Semiflex 31021 and PinPoint 31022) and two solid-state photon-beam detectors (PTW silicon diode 60017 and microDiamond 60019) under the influence of magnetic fields of up to 1.42 T.

Methods: The lateral dose response function K(x) is defined as the convolution kernel transforming the dose profile D(x) into the distorted signal profile M(x) measured with a detector. Measurements were carried out using a 6 MV photon slit beam combined with constant, homogeneous magnetic fields generated by an electromagnet. Signal profiles M(x) were measured for 0, 0.35 and 1.42 T, the dose profile D(x) using EBT3 film. The lateral dose response functions were then derived by an iterative deconvolution method.

Results: The dose response functions of the investigated detectors are shown to be distorted by the magnetic field. For the ionization chambers the FWHM of K(x) is decreasing with increasing magnetic field strength while K(x) gets more asymmetrical. The smaller dimension of the sensitive volume and the thinner central electrode of the PinPoint are causing less distortion by the magnetic field compared to the larger Semiflex. For the semiconductor detectors the FWHM values are not significantly changed in the magnetic fields, however the amplitudes of the negative values outside the detectors’ sensitive volume due to outward-directed secondary electrons are getting larger and more asymmetrical with increasing magnetic field. This effect is more prominent for the silicon diode than for the microDiamond.

Conclusion: The influence of magnetic fields on K(x) was characterized for four detectors. Hereby, the PinPoint and the microDiamond are influenced least by the magnetic field, making them suitable for clinical measurements in magnetic fields, when minimal distortions are permitted. Knowledge of K(x) allows for further correction strategies.


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