Feasibility study of using fall-off gradients of early and late PET scans for proton range verification.
Feasibility study of using fall-off gradients of early and late PET scans for proton range verification.
复制标题
DOI：
10.1002/mp.12191
复制
发表时间：
2017-05
期刊：
影响因子：
3.8
通讯作者：
El Fakhri G
中科院分区：
文献类型：
--
作者：
Cho J;Grogg K;Min CH;Zhu X;Paganetti H;Lee HC;El Fakhri G
虽然正电子发射断层扫描 (PET) 允许对质子束激活的组织进行成像，以监测所给予的治疗，但大多数内源性组织成分是由相对高能的质子激活的。因此，剂量下降和活性下降之间存在相对较大的距离偏移。然而， 16O(p,2p,2n)13N 具有相对较低的能量阈值，峰值约为 12 MeV，并且剩余质子范围约为 1 至 2 mm。在这项模型研究中，我们测试了利用 13N 产生峰值的可行性以及早期和晚期 PET 扫描之间活性下降的差异来验证质子范围。这项研究的主要目的之一是开发一种不需要蒙特卡罗模拟的质子范围验证方法。单能和分散的布拉格峰光束都被传递到两个模型——水状凝胶和组织状凝胶，其中质子射程分别约为 9.9 和 9.1 厘米。照射后延迟 1 分钟后，使用室内 PET 扫描模型 30 分钟。使用两种不同的照射后延迟和采集时间重建两个单独的（早期和晚期）PET 图像；早期 PET：延迟 1 分钟，采集 3 分钟；晚期 PET：延迟 21 分钟，采集 10 分钟。然后将 PET 信号的深度梯度标准化并绘制为深度的函数。从晚期 PET 图像的归一化梯度中减去早期 PET 图像的归一化梯度，以观察 13N 活性随深度的分布。蒙特卡罗模拟也使用与前面所述的测量相同的设置进行。减去的梯度显示原始光束和 SOBP 光束在水凝胶和组织凝胶中分别在 9.4 cm 和 8.6 cm 处出现峰值。这些峰的产生与 13N 信号随深度的突然变化有关，并且始终出现在 13N 信号最丰富的上游 2 mm 处（水凝胶和组织凝胶中分别为 9.6 cm 和 8.8 cm）。蒙特卡罗模拟提供了与测量类似的结果。水凝胶和组织凝胶的减去 PET 信号梯度峰值和质子范围显示 4 至 5 毫米的距离偏移。该偏移量可用于仅使用 PET 测量数据而不进行蒙特卡罗模拟的质子范围验证。需要更多的研究来克服各种限制，例如灌注驱动的冲洗，以确保该技术在活体患者中的可行性。
While positron emission tomography (PET) allows for the imaging of tissues activated by proton beams in terms of monitoring the therapy administered, most endogenous tissue elements are activated by relatively high energy protons. Therefore, a relatively large distance off-set exists between the dose fall-off and activity fall-off. However, 16O(p,2p,2n)13N has a relatively low energy threshold which peaks around 12 MeV and also a residual proton range that is approximately 1 to 2 mm. In this phantom study, we tested the feasibility of utilizing the 13N production peak as well as the differences in activity fall-off between early and late PET scans for proton range verification. One of the main purposes for this research was developing a proton range verification methodology that would not require Monte Carlo simulations. Both mono-energetic and spread-out Bragg peak beams were delivered to two phantoms – a water-like gel and a tissue-like gel where the proton ranges came to be approximately 9.9 and 9.1 cm, respectively. After 1 min of post-irradiation delay, the phantoms were scanned for a period of 30 min using an in-room PET. Two separate (Early and Late) PET images were reconstructed using two different post-irradiation delays and acquisition times; Early PET: 1 min delay and 3 min acquisition, Late PET: 21 min delay and 10 min acquisition. The depth gradients of the PET signals were then normalized and plotted as functions of depth. The normalized gradient of the early PET images was subtracted from that of the late PET images, to observe the 13N activity distribution in relation to depth. Monte Carlo simulations were also conducted with the same set-up as the measurements stated previously. The subtracted gradients show peaks at 9.4 cm and 8.6 cm in water-gel and tissue-gel respectively for both pristine and SOBP beams. These peaks are created in connection with the sudden change of 13N signals with depth and consistently occur 2 mm upstream to where 13N signals were most abundantly created (9.6 cm and 8.8 cm in water-gel and tissue-gel, respectively). Monte Carlo simulations provided similar results as the measurements. The subtracted PET signal gradient peaks and the proton ranges for water-gel and tissue-gel show distance off-sets of 4 to 5 mm. This off-set may potentially be used for proton range verification using only the PET measured data without Monte Carlo simulations. More studies are necessary to overcome various limitations, such as perfusion-driven washout, for the feasibility of this technique in living patients.