Repair kinetics in mouse lung after multiple X-ray fractions per 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. Results are analysed to determine whether repair kinetics follow a single exponential function of time. Calculated repair rate (T1/2) was about 1.2 h for two fractions per day of 2 Gy (10F/5d) but slightly lesss (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 lesss (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 obtained using two repair components. Repair kinetics are slow for lung, in comparison to acute reacting tisssues (except skin), and may require that 6-8 h should be allowed between fractions on the same day so that more than 95% repairable dose is repaired. (author)