CHU de Quebec-Université Laval, Quebec, Canada.
Université Laval, Quebec, Canada.
Med Phys. 2021 May;48(5):2095-2107. doi: 10.1002/mp.14607. Epub 2021 Mar 28.
The aim of this study is to perform three-dimensional (3D) source position reconstruction by combining in vivo dosimetry measurements from two independent detector systems.
Time-resolved dosimetry was performed in a water phantom during HDR brachytherapy irradiation with Ir source using two detector systems. The first was based on three plastic scintillator detectors and the second on a single inorganic crystal (CsI:Tl). Brachytherapy treatments were simulated in water under TG-43U1 conditions, including a HDR prostate plan. Treatment needles were placed in distances covering a range of source movement of 120 mm around the detectors. The distance from each dwell position to each scintillator was determined based on the measured dose rates. The three distances given by the mPSD were recalculated to a position along the catheter (z) and a distance radially away from the mPSD (xy) for each dwell position (a circumference around the mPSD). The source x, y, and z coordinates were derived from the intersection of the mPSD's circumference with the sphere around the ISD based on the distance to this detector. We evaluated the accuracy of the source position reconstruction as a function of the distance to the source, the most likely location for detector positioning within a prostate volume, as well as the capacity to detect positioning errors.
Approximately 4000 source dwell positions were tracked for eight different HDR plans. An intersection of the mPSD torus and the ISD sphere was observed in 77.2% of the dwell positions, assuming no uncertainty in the dose rate determined distance. This increased to 100% if 1σ search regions were added. However, only 73(96)% of the expected dwell positions were found within the intersection band for 1(2) σ uncertainties. The agreement between the source's reconstructed and expected positions was within 3 mm for a range of distances to the source up to 50 mm. The experiments on a HDR prostate plan, showed that by having at least one of the detectors located in the middle of the prostate volume, reduces the measurement deviations considerably compared to scenarios where the detectors were located outside of the prostate volume. The analysis showed a detection probability that, in most cases, is far from the random detection threshold. Errors of 1(2) mm can be detected in ranges of 5-25 (25-50) mm from the source, with a true detection probability rate higher than 80%, while the false probability rate is kept below 20%.
By combining two detector responses, we enabled the determination of the absolute source coordinates. The combination of the mPSD and the ISD in vivo dosimetry constitutes a promising alternative for real-time 3D source tracking in HDR brachytherapy.
本研究旨在通过结合两个独立探测器系统的体内剂量测量来进行三维（3D）源位置重建。
在 HDR 近距离放射治疗 Ir 源照射过程中，使用两个探测器系统在水模体中进行时间分辨剂量测定。第一个探测器系统基于三个塑料闪烁体探测器，第二个探测器系统基于单个无机晶体（CsI:Tl）。在 TG-43U1 条件下模拟了近距离放射治疗，包括一个 HDR 前列腺计划。将治疗针放置在距离探测器 120mm 的范围内，以覆盖源运动的范围。根据测量的剂量率确定每个驻留位置到每个闪烁体的距离。mPSD 给出的三个距离被重新计算为沿着导管的位置（z）和距 mPSD（xy）的距离，用于每个驻留位置（mPSD 周围的一个圆周）。根据与该探测器的距离，从 mPSD 的圆周与 ISD 周围球体的交点导出源的 x、y 和 z 坐标。我们评估了源位置重建的准确性，作为源距离、探测器在前列腺体积内的最可能位置以及检测定位误差的能力的函数。
对八个不同的 HDR 计划进行了大约 4000 个源驻留位置的跟踪。如果剂量率确定距离没有不确定性，则在 77.2%的驻留位置观察到 mPSD 圆环与 ISD 球体的交点。如果添加 1σ 搜索区域，则增加到 100%。然而，如果在 1（2）σ 不确定性下，只有 73（96）%的预期驻留位置在交点带内。对于距离源的范围高达 50mm，源的重建位置与预期位置之间的一致性在 3mm 以内。在 HDR 前列腺计划上的实验表明，与探测器位于前列腺体积外部的情况相比，至少有一个探测器位于前列腺体积的中部，可以大大减少测量偏差。分析表明，在大多数情况下，检测概率远低于随机检测阈值。在距源 5-25（25-50）mm 的范围内，可以检测到 1（2）mm 的误差，真实检测概率高于 80%，而错误概率保持在 20%以下。
通过结合两个探测器的响应，我们能够确定绝对源坐标。mPSD 和 ISD 体内剂量测定的组合为 HDR 近距离放射治疗中的实时 3D 源跟踪提供了一种很有前途的替代方法。
