A high‐Z inorganic scintillator–based detector for time‐resolved in vivo dosimetry during brachytherapy

Abstract

Purpose High‐dose rate (HDR) and pulsed‐dose rate (PDR) brachytherapy would benefit from an independent treatment verification system to monitor treatment delivery and to detect errors in real time. This paper characterizes and provides an uncertainty budget for a detector based on a fiber‐coupled high‐Z inorganic scintillator capable of performing time‐resolved in vivo dosimetry during HDR and PDR brachytherapy. Method The detector was composed of a detector probe and an optical reader. The detector probe consisted of either a 0.5 × 0.4 × 0.4 mm3 (HDR) or a 1.0 × 0.4 × 0.4 mm3 (PDR) cuboid ZnSe:O crystal glued onto an optical‐fiber cable. The outer diameter of the detector probes was 1 mm, and fit inside standard brachytherapy catheters. The signal from the detector probe was read out at 20 Hz by a photodiode and a data acquisition device inside the optical reader. In order to construct an uncertainty budget for the detector, six characteristics were determined: (1) temperature dependence of the detector probe, (2) energy dependence as a function of the probe‐to‐source position in 2D (determined with 2 mm resolution using a robotic arm), (3) the signal‐to‐noise ratio (SNR), (4) short‐term stability over 8 h, and (5) long‐term stability of three optical readers and four probes used for in vivo monitoring in HDR and PDR treatments over 21 months (196 treatments and 189 detector calibrations, and (6) dose‐rate dependence. Results The total uncertainty of the detector at a 20 mm probe‐to‐source distance was < 5.1% and < 5.8% for the HDR and PDR versions, respectively. Regarding the above characteristics, (1) the sensitivity of the detector decreased by an average of 1.4%/°C for detector probe temperatures varying from 22 to 37°C; (2) the energy dependence of the detector was nonlinear and depended on both probe‐to‐source distance and the angle between the probe and the brachytherapy source; (3) the median SNRs were 187 and 34 at a 20 mm probe‐to‐source distance for the HDR and PDR versions, respectively (corresponding median source activities of 4.8 and 0.56 Ci, respectively); (4) the detector response varied by 0.6% in 11 identical irradiations over 8 h; (5) the sensitivity of the four detector probes decreased systematically by 0–1.2%/100 Gy of dose delivered to the probes, and random fluctuations of 4.8% in the sensitivity were observed for the three probes used in PDR and 1.9% for the probe used in HDR; and (6) the detector response was linear with dose rate. Conclusion ZnSe:O detectors can be used effectively for in vivo dosimetry and with high accuracy for HDR and PDR brachytherapy applications.

Monte Carlo characterization of high atomic number inorganic scintillators for in vivo dosimetry in 192Ir brachytherapy

“…11,18,19,21,65 It may lead to inaccurate characterization of detector response, which is also illustrated in Figure 6. There, a linear absorbed-dose energy correction function for ZnSe:O determined experimentally by Kertzscher and Beddar 16 contradicts the MC results from the present study and the robotic-arm measurement data by Jørgensen et al 20 As the MC results indicate, the absorbed-dose energy response of the detector cannot be described knowing the radial distance r alone. The behavior depends on both r 0 and 𝜃 values, and the discrepancy was most pronounced at distances greater than 3 cm where source anisotropy effect increases.…”

“…The findings lead to the development of a 3D source tracking detector system where CsI:Tl was used in combination with plastic scintillators 18 . Several other high‐ Z inorganic scintillator systems with (Cd,Zn)S:Ag 19 and ZnSe:O 20 were investigated experimentally and shown to be suitable for 192 Ir BT in vivo dosimetry.…”

In Vivo Verification of Treatment Source Dwell Times in Brachytherapy of Postoperative Endometrial Carcinoma: A Feasibility Study

“…Debnath et al reported results for the repeatability test using their inorganic scintillator detector with a variation lower than 0.35% in all measurements, with a maximum deviation of 0.54% [ 27 ]. The detector response reported by Jorgensen et al across 11 irradiations conducted for the short-term stability test was 0.6% [ 28 ]. The values obtained in their work are very similar to the present results regardless of whether we consider the PSD average count rate, the PSD total counts, or the relative standard deviation of measured dwell time.…”

“…A significant decrease in sensitivity with accumulated absorbed dose was observed in a study by Jørgensen et al with inorganic scintillation detectors (ISD) for pulsed dose rate (PDR) but not for HDR brachytherapy [ 28 ]. They reported a standard deviation of the residuals from the linear fit of 1.9% for the probe used in HDR treatments.…”

“…Jorgensen et al obtained a percent deviation of around 3% in absorbed dose linearity for their high-Z inorganic scintillator used in HDR brachytherapy [ 28 ]. The percent deviation of count rate from the predicted of our PSD detector was below 2% for all the range of source activities reported (similar to the source activity range reported by Jorgensen et al [ 28 ]).…”