National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan.
Med Phys. 2010 Oct;37(10):5376-81. doi: 10.1118/1.3490086.
Respiration-gated irradiation for a moving target requires a longer time to deliver single fraction in proton radiotherapy (PRT). Ultrahigh dose rate (UDR) proton beam, which is 10-100 times higher than that is used in current clinical practice, has been investigated to deliver daily dose in single breath hold duration. The purpose of this study is to investigate the survival curve and relative biological effectiveness (RBE) of such an ultrahigh dose rate proton beam and their linear energy transfer (LET) dependence.
HSG cells were irradiated by a spatially and temporally uniform proton beam at two different dose rates: 8 Gy/min (CDR, clinical dose rate) and 325 Gy/min (UDR, ultrahigh dose rate) at the Bragg peak and 1.75 (CDR) and 114 Gy/min (UDR) at the plateau. To study LET dependence, the cells were positioned at the Bragg peak, where the absorbed dose-averaged LET was 3.19 keV/microm, and at the plateau, where it was 0.56 keV/microm. After the cell exposure and colony assay, the measured data were fitted by the linear quadratic (LQ) model and the survival curves and RBE at 10% survival were compared.
No significant difference was observed in the survival curves between the two proton dose rates. The ratio of the RBE for CDR/UDR was 0.98 +/- 0.04 at the Bragg peak and 0.96 +/- 0.06 at the plateau. On the other hand, Bragg peak/plateau RBE ratio was 1.15 +/- 0.05 for UDR and 1.18 +/- 0.07 for CDR.
Present RBE can be consistently used in treatment planning of PRT using ultrahigh dose rate radiation. Because a significant increase in RBE toward the Bragg peak was observed for both UDR and CDR, further evaluation of RBE enhancement toward the Bragg peak and beyond is required.
对于移动目标，呼吸门控照射需要更长的时间来单次分割质子放射治疗（PRT）。超高速率（UDR）质子束比目前临床实践中使用的质子束高 10-100 倍，已被研究用于在单次呼吸暂停期间递送每日剂量。本研究的目的是研究这种超高速率质子束的存活曲线和相对生物效应（RBE）及其线性能量转移（LET）依赖性。
HSG 细胞在两个不同的剂量率下接受空间和时间均匀的质子束照射：在布喇格峰处为 8 Gy/min（CDR，临床剂量率）和 325 Gy/min（UDR，超高剂量率），在平台处为 1.75（CDR）和 114 Gy/min（UDR）。为了研究 LET 依赖性，将细胞置于布喇格峰处，那里的吸收剂量平均 LET 为 3.19 keV/μm，以及在平台处，那里的 LET 为 0.56 keV/μm。细胞暴露和集落分析后，用线性二次（LQ）模型拟合测量数据，并比较 10%存活时的存活曲线和 RBE。
在两个质子剂量率下，存活曲线没有观察到显著差异。在布喇格峰处，CDR/UDR 的 RBE 比值为 0.98 +/- 0.04，在平台处为 0.96 +/- 0.06。另一方面，UDR 的布喇格峰/平台 RBE 比值为 1.15 +/- 0.05，CDR 的为 1.18 +/- 0.07。
在使用超高剂量率辐射的 PRT 治疗计划中，可以始终如一地使用当前的 RBE。由于 UDR 和 CDR 对布喇格峰的 RBE 都有显著增加，因此需要进一步评估对布喇格峰及其以外的 RBE 增强的评估。
