Angular dependence of detector responses in high-energy brachytherapy.

Objective:To characterize three small volume ionization chambers and a diamond detector concerning their angular dependent response under Ir-192 irradiation. Approach:Monte Carlo (MC) simulations and experimental measurements of the angular dependent response were performed at 10cm distance in air. Further simulations were performed at distances of 1-10cm in water. The detectors were placed in relation to their reference points specified by the manufacturer and rotated relative to the beam axis. Results for the different setups were compared to investigate the influence of water and air as well as a distance dependence of angular response. Detector specific properties such as ionization chambers dead volumes or asymmetries were taken into account. Main results:The MC simulations in air could be experimentally verified. The detectors showed an angular dependence <0.5% for rotations ≤20°, ≤35°, ≤40° and ≤50° at distances of 1-10cm in water, respectively. In air at 10cm distance, the angular dependence exceeded 0.5% for rotations of 20° for the microDiamond and for 50° for the Semiflex and PinPoint. Here, the angular dependence of the A26 was <0.5% at all angles. The slope of the angular dependence increased with rotation from frontal alignment for all detectors. Variations of detector angular dependent response caused by volume averaging were largest close to the source, i.e. up to 8% at 1cm distance. High-Z materials in the vicinity of the sensitive volumes were an additional source of angular response variations amounting up to 4% independent of the distance Significance:To the best of the authors' knowledge, this work is the first investigation of perturbation factors, also differential in energy, leading to angular dependent detector response in high-energy brachytherapy dosimetry. The results could be useful to estimate the angular dependent response when measuring complex dose rate distributions, e.g. multiple source positions or in a heterogeneous environment.