KVI-Center for Advanced Radiation Technology, University of Groningen, The Netherlands.
Siemens Medical Solutions, USA, Inc, Knoxville, TN, United States of America.
Phys Med Biol. 2020 Dec 11;65(24):245013. doi: 10.1088/1361-6560/aba504.
Compared to photon therapy, proton therapy allows a better conformation of the dose to the tumor volume with reduced radiation dose to co-irradiated tissues. In vivo verification techniques including positron emission tomography (PET) have been proposed as quality assurance tools to mitigate proton range uncertainties. Detection of differences between planned and actual dose delivery on a short timescale provides a fast trigger for corrective actions. Conventional PET-based imaging of O (T = 2 min) and C (T = 20 min) distributions precludes such immediate feedback. We here present a demonstration of near real-time range verification by means of PET imaging of N (T = 11 ms). PMMA and graphite targets were irradiated with a 150 MeV proton pencil beam consisting of a series of pulses of 10 ms beam-on and 90 ms beam-off. Two modules of a modified Siemens Biograph mCT PET scanner (21 × 21 cm each), installed 25 cm apart, were used to image the beam-induced PET activity during the beam-off periods. The modifications enable the detectors to be switched off during the beam-on periods. N images were reconstructed using planar tomography. Using a 1D projection of the 2D reconstructed N image, the activity range was obtained from a fit of the activity profile with a sigmoid function. Range shifts due to modified target configurations were assessed for multiples of the clinically relevant 10 protons per pulse (approximately equal to the highest intensity spots in the pencil beam scanning delivery of a dose of 1 Gy over a cubic 1 l volume). The standard deviation of the activity range, determined from 30 datasets obtained from three irradiations on PMMA and graphite targets, was found to be 2.5 and 2.6 mm (1σ) with 10 protons per pulse and 0.9 and 0.8 mm (1σ) with 10 protons per pulse. Analytical extrapolation of the results from this study shows that using a scanner with a solid angle coverage of 57%, with optimized detector switching and spot delivery times much smaller than the N half-life, an activity range measurement precision of 2.0 mm (1σ) and 1.3 mm (1σ) within 50 ms into an irradiation with 4 × 10 and 10 protons per pencil beam spot can be potentially realized. Aggregated imaging of neighboring spots or, if possible, increasing the number of protons for a few probe beam spots will enable the realization of higher precision range measurement.
与光子疗法相比，质子疗法可以更好地将剂量适形于肿瘤体积，同时减少对共同照射组织的辐射剂量。已经提出了包括正电子发射断层扫描（PET）在内的体内验证技术作为质量保证工具，以减轻质子射程不确定性。在短时间内检测计划剂量与实际剂量输送之间的差异，可以为校正措施提供快速触发。O（T = 2 分钟）和 C（T = 20 分钟）分布的常规基于 PET 的成像排除了这种即时反馈。我们在这里通过 PET 成像 N（T = 11 毫秒）展示了一种接近实时的射程验证演示。PMMA 和石墨靶材用由一系列 10 ms 束流开启和 90 ms 束流关闭的 150 MeV 质子铅笔束照射。两台经过修改的西门子 Biograph mCT PET 扫描仪的两个模块（每个模块为 21×21 cm），间隔 25 cm 安装，用于在束流关闭期间对束流诱导的 PET 活性进行成像。这些修改使探测器能够在束流开启期间关闭。使用二维重建 N 图像的平面断层扫描来重建 N 图像。通过对二维重建 N 图像的一维投影，从活性轮廓与 sigmoid 函数的拟合中获得活性范围。评估了由于目标配置改变而引起的射程偏移，这些偏移与每个脉冲 10 个质子（大约等于在立方 1 l 体积上施予 1 Gy 剂量时铅笔束扫描输送中的最高强度点）的倍数有关。从 PMMA 和石墨靶材上进行的三次照射获得的 30 个数据集确定，每个脉冲 10 个质子时，活性范围的标准偏差为 2.5 和 2.6mm（1σ），每个脉冲 10 个质子时为 0.9 和 0.8mm（1σ）。从这项研究的结果进行的分析外推表明，使用具有 57%立体角覆盖范围的扫描仪，优化探测器切换和斑点输送时间远小于 N 半衰期，可以在 50 ms 内实现 4×10 和 10 个质子每个铅笔束点的活性范围测量精度为 2.0mm（1σ）和 1.3mm（1σ）。对相邻斑点进行聚合成像，或者如果可能的话，增加几个探针束点的质子数，将能够实现更高精度的射程测量。
