Three-dimensional characterization of the active volumes of PTW microDiamond, microSilicon, and Diode E dosimetry detectors using a proton microbeam.

PURPOSE: The purpose of this work is the three-dimensional characterization of the active volumes of commercial solid-state dosimetry detectors. Detailed knowledge of the dimensions of the detector's active volume as well as the detector housing is of particular interest for small-field photon dosimetry. As shown in previous publications from different groups, the design of the detector housing influences the detector signal for small photon fields. Therefore, detailed knowledge of the active volume dimension and the surrounding materials form the basis for accurate Monte Carlo simulations of the detector. METHODS: A 10 MeV proton beam focused by the microbeam system of the Physikalisch-Technische Bundesanstalt was used to measure two-dimensional response maps of a synthetic diamond detector (microDiamond, type 60019, PTW Freiburg) and two silicon detectors (microSilicon, type 60023, PTW Freiburg and Diode E, type 60017, PTW Freiburg). In addition, the thickness of the active volume of the new microSilicon was measured using the method developed in a previous study. RESULTS: The analysis of the response maps leads to active area of 1.18 mm2 for the Diode E, 1.75 mm2 for the microSilicon, and 3.91 mm2 for the microDiamond detector. The thickness of the active volume of the microSilicon detector was determined to be (17.8 ± 2) µm. CONCLUSIONS: This study provides detailed geometrical data of the dosimetric active volume of three different solid-state detector types.

The role of radiation-induced charge imbalance on the dose-response of a commercial synthetic diamond detector in small field dosimetry.

PURPOSE: Discrepancy between experimental and Monte Carlo simulated dose-response of the microDiamond (mD) detector (type 60019, PTW Freiburg, Germany) at small field sizes has been reported. In this work, the radiation-induced charge imbalance in the structural components of the detector has been investigated as the possible cause of this discrepancy. MATERIALS AND METHODS: Output ratio (OR) measurements have been performed using standard and modified versions of the mD detector at nominal field sizes from 6 mm × 6 mm to 40 mm × 40 mm. In the first modified mD detector (mD_reversed), the type of charge carriers collected is reversed by connecting the opposite contact to the electrometer. In the second modified mD detector (mD_shortened), the detector's contacts have been shortened. The third modified mD detector (mD_noChip) is the same as the standard version but the diamond chip with the sensitive volume has been removed. Output correction factors were calculated from the measured OR and simulated using the EGSnrc package. An adapted Monte Carlo user-code has been used to study the underlying mechanisms of the field size-dependent charge imbalance and to identify the detector's structural components contributing to this effect. RESULTS: At the smallest field size investigated, the OR measured using the standard mD detector is >3% higher than the OR obtained using the modified mD detector with reversed contact (mD_reversed). Combining the results obtained with the different versions of the detector, the OR have been corrected for the effect of radiation imbalance. The OR obtained using the modified mD detector with shortened contacts (mD_shortened) agree with the corrected OR, all showing an over-response of less than 2% at the field sizes investigated. The discrepancy between the experimental and simulated output correction factors has been eliminated after accounting for the effect of charge imbalance. DISCUSSIONS AND CONCLUSIONS: The role of radiation-induced charge imbalance on the dose-response of mD detector in small field dosimetry has been studied and quantified. It has been demonstrated that the effect is significant at small field sizes. Multiple methods were used to quantify the effect of charge imbalance with good agreement between Monte Carlo simulations and experimental results obtained with modified detectors. When this correction is applied to the Monte Carlo data, the discrepancy from experimental data is eliminated. Based on the detailed component analysis using an adapted Monte Carlo user-code, it has been demonstrated that the effect of charge imbalance can be minimized by modifying the design of the detector's contacts.

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 2017b 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.