Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2011 May;38(5):2542-51. doi: 10.1118/1.3572229.
The goal of the present work was to evaluate the accuracy of a plastic scintillation detector (PSD) system to perform in-phantom dosimetry during 192Ir high dose rate (HDR) brachytherapy treatments.
A PSD system capable of stem effect removal was built. A red-green-blue photodiode connected to a dual-channel electrometer was used to detect the scintillation light emitted from a green scintillation component and transmitted along a plastic optical fiber. A clinically relevant prostate treatment plan was built using the HDR brachytherapy treatment planning system. An in-house fabricated template was used for accurate positioning of the catheters, and treatment delivery was performed in a water phantom. Eleven catheters were inserted and used for dose delivery from 192Ir radioactive source, while two others were used to mimic dosimetry at the rectum wall and in the urethra using a PSD. The measured dose and dose rate data were compared to the expected values from the planning system. The importance of removing stem effects from in vivo dosimetry using a PSD during 192Ir HDR brachytherapy treatments was assessed. Applications for dwell position error detection and temporal verification of the treatment delivery were also investigated.
In-phantom dosimetry measurements of the treatment plan led to a ratio to the expected dose of 1.003 +/- 0.004 with the PSD at different positions in the urethra and 1.043 +/- 0.003 with the PSD inserted in the rectum. Verification for the urethra of dose delivered within each catheter and at specific dwell positions led to average measured to expected ratios of 1.015 +/- 0.019 and 1.014 +/- 0.020, respectively. These values at the rectum wall were 1.059 +/- 0.045 within each catheter and 1.025 +/- 0.028 for specific dwell positions. The ability to detect positioning errors of the source depended of the tolerance on the difference to the expected value. A 5-mm displacement of the source was detected by the PSD system from 78% to 100% of the time depending on the acceptable range value. The implementation of a stem effect removal technique was shown to be necessary, particularly when calculating doses at specific dwell positions, and allowed decreasing the number of false-error detections-the detection of an error when it should not be the case--from 19 to 1 for a 5% threshold out of 43 measurements. The use of the PSD system to perform temporal verification of elapsed time by the source in each catheter--generally on the order of minutes--was shown to be in agreement within a couple of seconds with the treatment plan
We showed that the PSD system used in this study, which was capable of stem effect removal, can perform accurate dosimetry during 192Ir HDR brachytherapy treatment in a water phantom. The system presented here shows some clear advantages over previously proposed dosimetry systems for HDR brachytherapy, and it has the potential for various online verifications of treatment delivery quality.
本研究旨在评估一种塑料闪烁探测器（PSD）系统在 192Ir 高剂量率（HDR）近距离治疗中进行体模内剂量测量的准确性。
我们构建了一个能够去除茎效应的 PSD 系统。一个红色-绿色-蓝色光电二极管与双通道静电计相连，用于探测从绿色闪烁组件发出并沿塑料光纤传输的闪烁光。使用 HDR 近距离治疗计划系统为前列腺建立了一个临床相关的治疗计划。使用内部制作的模板对导管进行精确定位，在水模体中进行治疗输送。11 根导管被插入并用于 192Ir 放射性源的剂量输送，而另外两根导管用于使用 PSD 模拟直肠壁和尿道的剂量测量。将测量的剂量和剂量率数据与计划系统的预期值进行比较。评估了在 192Ir HDR 近距离治疗中使用 PSD 去除体模内剂量中的茎效应的重要性。还研究了用于检测驻留位置误差和治疗输送的时间验证的应用。
在不同的尿道位置和插入直肠的 PSD 处，对治疗计划的体模内剂量测量得到了与预期剂量的比值为 1.003 +/- 0.004 和 1.043 +/- 0.003。在每个导管内和特定驻留位置测量的尿道内输送剂量的验证导致平均测量到预期的比值分别为 1.015 +/- 0.019 和 1.014 +/- 0.020。在直肠壁处，每个导管内的比值为 1.059 +/- 0.045，特定驻留位置的比值为 1.025 +/- 0.028。源定位误差的检测能力取决于与预期值的差异的容限。PSD 系统能够检测到源的 5mm 位移，时间从 78%到 100%，具体取决于可接受的范围值。实施茎效应去除技术被证明是必要的，特别是在计算特定驻留位置的剂量时，可以减少假错误检测的数量-在不应该出现错误的情况下检测到错误-从 43 次测量中的 19 次到 1 次，阈值为 5%。使用 PSD 系统在每个导管中执行源经过时间的时间验证-通常在几分钟的量级-与治疗计划的结果在几秒钟内一致。
我们表明，在本研究中使用的能够去除茎效应的 PSD 系统可以在水模体中对 192Ir HDR 近距离治疗进行精确的剂量测量。与以前提出的 HDR 近距离治疗剂量测量系统相比，本研究中的系统具有明显的优势，并且具有对治疗输送质量进行各种在线验证的潜力。
