Parkins C S, Whitsed C A, Fowler J F
Gray Laboratory, Mount Vernon Hospital, Middlesex.
Int J Radiat Biol. 1988 Sep;54(3):429-43. doi: 10.1080/09553008814551811.
The potential advantage for sparing normal tissue damage by hyperfractionation of low-LET radiation may be limited by the repair kinetics of tissues in the irradiated field. Tissues with slow repair kinetics will limit the number of fractions that may be given on the same day. Results are presented for mouse lung treated with a range of doses per fraction using either two or three fractions per day in multiple X-ray fractionation schedules. The results are analysed to determine whether the repair kinetics follow a single exponential function of time. The calculated repair rate (T1/2) was about 1.2 h for two fractions per day of 2 Gy (10F/5d) but slightly less (T1/2 = 0.8 h) for two fractions of 9 Gy (2F/1d). For smaller doses per fraction of 1.1 Gy, given three times per day (39F/13d), the T1/2 was not significantly less (T1/2 = 0.3-0.7 h). For three fractions per day of 1.1 Gy per fraction an unsatisfactory fit is achieved using a single exponential function of time, and a better fit is obtained using two components of repair. The repair kinetics are slow for lung, in comparison to acute reacting tissues (except skin), and may require that 6-8 h (i.e. four or five half-times) should be allowed between fractions on the same day so that more than 95 per cent of the repairable dose is repaired. At present the variation in repair kinetics with doses per fraction between 1.1 and 9 Gy are not significantly different, so no reduction of interfraction interval should be proposed.
低线性能量传递辐射超分割照射对正常组织损伤的潜在保护优势，可能会受到照射野内组织修复动力学的限制。修复动力学缓慢的组织会限制同一天内可给予的分次照射次数。本文给出了在多种X射线分次照射方案中，采用每天两次或三次分次照射，对小鼠肺给予不同分次剂量后的结果。对这些结果进行分析，以确定修复动力学是否符合时间的单指数函数。对于每天两次2 Gy的照射（10次/5天），计算得到的修复率（T1/2）约为1.2小时，但对于两次9 Gy的照射（2次/1天），修复率略低（T1/2 = 0.8小时）。对于每次1.1 Gy的较小分次剂量，每天照射三次（39次/13天），T1/2并没有显著降低（T1/2 = 0.3 - 0.7小时）。对于每次1.1 Gy、每天三次的照射，用时间的单指数函数拟合效果不理想，而用两个修复成分进行拟合效果更好。与急性反应组织（皮肤除外）相比，肺的修复动力学较慢，同一天内分次照射之间可能需要间隔6 - 8小时（即四到五个半衰期），以便超过95%的可修复剂量得到修复。目前，在1.1至9 Gy的分次剂量范围内，修复动力学的变化没有显著差异，因此不建议缩短分次间隔时间。
