National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
Phys Med Biol. 2020 Jun 15;65(12):125006. doi: 10.1088/1361-6560/ab8059.
In heavy-ion therapy, the stopping position of primary ions in tumours needs to be monitored for effective treatment and to prevent overdose exposure to normal tissues. Positron-emitting ion beams, such as C and O, have been suggested for range verification in heavy-ion therapy using in-beam positron emission tomography (PET) imaging, which offers the capability of visualizing the ion stopping position with a high signal-to-noise ratio. We have previously demonstrated the feasibility of in-beam PET imaging for the range verification of C and O ion beams and observed a slight shift between the beam stopping position and the dose peak position in simulations, depending on the initial beam energy spread. In this study, we focused on the experimental confirmation of the shift between the Bragg peak position and the position of the maximum detected positron-emitting fragments via a PET system for positron-emitting ion beams of C (210 MeV u) and O (312 MeV u) with momentum acceptances of 5% and 0.5%. For this purpose, we measured the depth doses and performed in-beam PET imaging using a polymethyl methacrylate (PMMA) phantom for both beams with different momentum acceptances. The shifts between the Bragg peak position and the PET peak position in an irradiated PMMA phantom for the O ion beams were 1.8 mm and 0.3 mm for momentum acceptances of 5% and 0.5%, respectively. The shifts between the positions of two peaks for the C ion beam were 2.1 mm and 0.1 mm for momentum acceptances of 5% and 0.5%, respectively. We observed larger shifts between the Bragg peak and the PET peak positions for a momentum acceptance of 5% for both beams, which is consistent with the simulation results reported in our previous study. The biological doses were also estimated from the calculated relative biological effectiveness (RBE) values using a modified microdosimetric kinetic model (mMKM) and Monte Carlo simulation. Beams with a momentum acceptance of 5% should be used with caution for therapeutic applications to avoid extra dose to normal tissues beyond the tumour when the dose distal fall-off is located beyond the treatment volume.
在重离子治疗中，需要监测初级离子在肿瘤中的停止位置，以实现有效治疗并防止对正常组织的过量辐射。正电子发射离子束，如 C 和 O，已被提议用于重离子治疗中的束内正电子发射断层扫描 (PET) 成像中的射程验证，该成像提供了以高信噪比可视化离子停止位置的能力。我们之前已经证明了用于 C 和 O 离子束射程验证的束内 PET 成像的可行性，并在模拟中观察到，根据初始束能散度，束停止位置和剂量峰值位置之间存在轻微的偏移。在这项研究中，我们专注于通过用于 C（210 MeV u）和 O（312 MeV u）正电子发射离子束的 PET 系统实验确认布拉格峰位置和最大检测到的正电子发射碎片位置之间的偏移，该系统的动量接受度分别为 5%和 0.5%。为此，我们测量了深度剂量，并使用不同动量接受度的 PMMA 体模对两种束进行了束内 PET 成像。对于 O 离子束，在辐照的 PMMA 体模中，布拉格峰位置和 PET 峰位置之间的偏移分别为 1.8 毫米和 0.3 毫米，动量接受度分别为 5%和 0.5%。对于 C 离子束，两个峰位置之间的偏移分别为 2.1 毫米和 0.1 毫米，动量接受度分别为 5%和 0.5%。我们观察到两种束的动量接受度为 5%时，布拉格峰和 PET 峰位置之间的偏移更大，这与我们之前研究中的模拟结果一致。还使用修正后的微剂量动力学模型 (mMKM) 和蒙特卡罗模拟从计算的相对生物效应 (RBE) 值估计了生物剂量。对于治疗应用，当剂量远落位置超出治疗体积时，应谨慎使用动量接受度为 5%的束，以避免肿瘤以外的正常组织受到额外的剂量。
