Health Graduate School, Aarhus University, Aarhus, Denmark.
Department of Oncology, Aarhus University Hospital, Aarhus, Denmark.
Med Phys. 2021 Nov;48(11):7382-7398. doi: 10.1002/mp.15257. Epub 2021 Oct 20.
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.
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 mm (HDR) or a 1.0 × 0.4 × 0.4 mm (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.
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.
ZnSe:O detectors can be used effectively for in vivo dosimetry and with high accuracy for HDR and PDR brachytherapy applications.
高剂量率（HDR）和脉冲剂量率（PDR）近距离放射治疗需要一种独立的治疗验证系统，以便实时监测治疗输送并检测治疗中的错误。本文对一种基于光纤耦合的高 Z 无机闪烁体的探测器进行了特性描述，并提供了其不确定度预算，该探测器能够在 HDR 和 PDR 近距离放射治疗中进行时间分辨的体内剂量测定。
探测器由探测器探头和光学读取器组成。探测器探头由 0.5×0.4×0.4mm（HDR）或 1.0×0.4×0.4mm（PDR）的长方体 ZnSe:O 晶体粘在光纤电缆上构成。探测器探头的外径为 1mm，可以安装在标准的近距离放射治疗导管内。探测器探头的信号由光学读取器内的光电二极管和数据采集设备以 20Hz 的频率读取。为了构建探测器的不确定度预算，确定了六个特性：（1）探测器探头的温度依赖性，（2）在二维空间中探头-源位置的能量依赖性（使用机器人臂以 2mm 的分辨率确定），（3）信噪比（SNR），（4）8 小时内的短期稳定性，（5）在 HDR 和 PDR 治疗中用于体内监测的三个光学读取器和四个探头的长期稳定性，在 21 个月内进行了 196 次治疗和 189 次探测器校准，（6）剂量率依赖性。
在 20mm 的探头-源距离处，探测器的总不确定度分别为 HDR 和 PDR 版本的<5.1%和<5.8%。关于上述特性，（1）探测器探头温度从 22°C 到 37°C变化时，探测器的灵敏度平均下降了 1.4%/°C；（2）探测器的能量依赖性是非线性的，并且取决于探头-源距离和探头与近距离放射源之间的角度；（3）在 20mm 的探头-源距离处，HDR 和 PDR 版本的中位数 SNR 分别为 187 和 34（分别对应 4.8 和 0.56Ci 的中位数源活度）；（4）在 8 小时内，11 次相同的辐照中探测器的响应变化了 0.6%；（5）在 HDR 和 PDR 中，四个探测器探头的灵敏度分别系统地下降了 0-1.2%/100Gy 的剂量，并且在用于 PDR 的三个探头中观察到灵敏度的随机波动为 4.8%，用于 HDR 的探头的灵敏度波动为 1.9%；（6）探测器的响应与剂量率呈线性关系。
ZnSe:O 探测器可有效地用于体内剂量测定，并可高精度应用于 HDR 和 PDR 近距离放射治疗。
