Department of Physics and Mathematics, FFCLRP, University of São Paulo, 14040-901 Ribeirão Preto-SP, Brazil.
Med Phys. 2010 Oct;37(10):5407-11. doi: 10.1118/1.3483780.
Several attempts to determine the transit time of a high dose rate (HDR) brachytherapy unit have been reported in the literature with controversial results. The determination of the source speed is necessary to accurately calculate the transient dose in brachytherapy treatments. In these studies, only the average speed of the source was measured as a parameter for transit dose calculation, which does not account for the realistic movement of the source, and is therefore inaccurate for numerical simulations. The purpose of this work is to report the implementation and technical design of an optical fiber based detector to directly measure the instantaneous speed profile of a 192Ir source in a Nucletron HDR brachytherapy unit.
To accomplish this task, we have developed a setup that uses the Cerenkov light induced in optical fibers as a detection signal for the radiation source moving inside the HDR catheter. As the 192Ir source travels between two optical fibers with known distance, the threshold of the induced signals are used to extract the transit time and thus the velocity. The high resolution of the detector enables the measurement of the transit time at short separation distance of the fibers, providing the instantaneous speed.
Accurate and high resolution speed profiles of the 192Ir radiation source traveling from the safe to the end of the catheter and between dwell positions are presented. The maximum and minimum velocities of the source were found to be 52.0 +/- 1.0 and 17.3 +/- 1.2 cm/s. The authors demonstrate that the radiation source follows a uniformly accelerated linear motion with acceleration of [a] = 113 cm/s2. In addition, the authors compare the average speed measured using the optical fiber detector to those obtained in the literature, showing deviation up to 265%.
To the best of the authors' knowledge, the authors directly measured for the first time the instantaneous speed profile of a radiation source in a HDR brachytherapy unit traveling from the unit safe to the end of the catheter and between interdwell distances. The method is feasible and accurate to implement on quality assurance tests and provides a unique database for efficient computational simulations of the transient dose.
已有文献报道了几种尝试确定高剂量率（HDR）近距离放射治疗单位传输时间的方法，但结果存在争议。源速度的确定对于准确计算近距离放射治疗中的瞬态剂量是必要的。在这些研究中，仅测量源的平均速度作为传输剂量计算的参数，这不能说明源的实际运动情况，因此对于数值模拟来说是不准确的。本工作的目的是报告一种基于光纤的探测器的实现和技术设计，该探测器可直接测量 Nucletron HDR 近距离放射治疗单位中 192Ir 源的瞬时速度分布。
为了完成这项任务，我们开发了一种设置，该设置使用光纤中诱导的切伦科夫光作为在 HDR 导管内移动的辐射源的检测信号。当 192Ir 源在具有已知距离的两根光纤之间移动时，会使用感应信号的阈值来提取传输时间，从而得出速度。探测器的高分辨率使得能够在光纤短距离分离的情况下测量传输时间，从而提供瞬时速度。
呈现了从安全位置到导管末端以及在驻留位置之间移动的 192Ir 辐射源的精确和高分辨率速度分布。发现源的最大和最小速度分别为 52.0 ± 1.0 和 17.3 ± 1.2 cm/s。作者证明，辐射源遵循具有加速度[a] = 113 cm/s2 的匀加速直线运动。此外，作者将使用光纤探测器测量的平均速度与文献中获得的速度进行比较，显示出高达 265%的偏差。
据作者所知，这是首次直接测量 HDR 近距离放射治疗单位中从单位安全位置到导管末端以及驻留间隔之间移动的辐射源的瞬时速度分布。该方法在质量保证测试中是可行且准确的，并为瞬态剂量的有效计算模拟提供了独特的数据库。
