Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN∕SP, São Paulo 05508-000, Brazil.
Med Phys. 2013 May;40(5):051717. doi: 10.1118/1.4802731.
Several studies have reported methodologies to calculate and correct the transit dose component of the moving radiation source for high dose rate (HDR) brachytherapy planning systems. However, most of these works employ the average source speed, which varies significantly with the measurement technique used, and does not represent a realistic speed profile, therefore, providing an inaccurate dose determination. In this work, the authors quantified the transit dose component of a HDR unit based on the measurement of the instantaneous source speed to produce more accurate dose values.
The Nucletron microSelectron-HDR Ir-192 source was characterized considering the Task Group 43 (TG-43U1) specifications. The transit dose component was considered through the calculation of the dose distribution using a Monte Carlo particle transport code, MCNP5, for each source position and correcting it by the source speed. The instantaneous source speed measurements were performed in a previous work using two optical fibers connected to a photomultiplier and an oscilloscope. Calculated doses were validated by comparing relative dose profiles with those obtained experimentally using radiochromic films.
TG-43U1 source parameters were calculated to validate the Monte Carlo simulations. These agreed with the literature, with differences below 1% for the majority of the points. Calculated dose profiles without transit dose were also validated by comparison with ONCENTRA(®) Brachy v. 3.3 dose values, yielding differences within 1.5%. Dose profiles obtained with MCNP5 corrected using the instantaneous source speed profile showed differences near dwell positions of up to 800% in comparison to values corrected using the average source speed, but they are in good agreement with the experimental data, showing a maximum discrepancy of approximately 3% of the maximum dose. Near a dwell position the transit dose is about 22% of the dwell dose delivered by the source dwelling 1 s and reached 104.0 cGy per irradiation in a hypothetical clinical case studied in this work.
The present work demonstrated that the transit dose correction based on average source speed fails to accurately correct the dose, indicating that the correct speed profile should be considered. The impact on total dose due to the transit dose correction near the dwell positions is significant and should be considered more carefully in treatments with high dose rate, several catheters, multiple dwell positions, small dwell times, and several fractions.
已有多项研究报道了用于计算和校正高剂量率（HDR）近距离治疗计划系统中移动辐射源传输剂量分量的方法。然而，这些研究大多采用平均源速度，该速度因所使用的测量技术而异，且无法代表实际的速度分布，因此提供的剂量确定并不准确。在这项工作中，作者基于瞬时源速度的测量来量化 HDR 单位的传输剂量分量，以产生更准确的剂量值。
根据 TG-43U1 规范对 Nucletron microSelectron-HDR Ir-192 源进行了特性描述。通过使用蒙特卡罗粒子输运代码 MCNP5 计算每个源位置的剂量分布并通过源速度进行校正，来考虑传输剂量分量。在之前的工作中，使用连接到光电倍增管和示波器的两根光纤来进行瞬时源速度测量。通过将相对剂量分布与使用放射性铬膜获得的实验结果进行比较，来验证计算剂量。
计算了 TG-43U1 源参数以验证蒙特卡罗模拟。这些模拟结果与文献相符，大多数点的差异小于 1%。未校正传输剂量的计算剂量分布也通过与 ONCENTRA®Brachy v.3.3 剂量值进行比较进行了验证，差异在 1.5%以内。使用瞬时源速度分布校正后的 MCNP5 计算的剂量分布与使用平均源速度校正后的结果相比，在驻留位置附近的差异高达 800%，但与实验数据吻合良好，最大差异约为最大剂量的 3%。在驻留位置附近，传输剂量约占源驻留 1 s 时输送的驻留剂量的 22%，在本研究中考虑的一个假设临床案例中，每个照射达到 104.0 cGy。
本研究表明，基于平均源速度的传输剂量校正无法准确校正剂量，表明应考虑正确的速度分布。在驻留位置附近，由于传输剂量校正而导致的总剂量的影响是显著的，因此在高剂量率、多个导管、多个驻留位置、短驻留时间和多个分数的治疗中应更加仔细地考虑。
