EARLY AND LATE INJURIES IN MOUSE RECTUM AFTER FRACTIONATED X-RAY AND NEUTRON-IRRADIATION

Purpose: to assess mouse rectum tolerance to fractionated X-ray and neutron irradiation. Materials and methods: doses per fraction ranged between 0.25 and 35 Gy for X-rays, 0.05 and 12 Gy for neutrons. Neutron top-up doses were added when the fractionated irradiation was given in fractions less than 2 Gy of X-rays or 0.35 Gy of neutrons in order to bring the damage into the detectable range. The early endpoints were the nadir of weight loss occurring within the first 2-3 weeks following irradiation and lethality by 2 months. The late endpoints were the peak of weight reached at maturity of the mice, the proportion of short feces in the daily fecal output at 10 months and lethality by 12 months. The linear-quadratic (LQ) model was fitted to the data (direct ''one-step'' analysis) and the estimated parameters were used to calculate relative biological effectiveness (RBE) values. Results: alpha/beta ratio estimates were for X-rays: 19.9 Gy [95 % confidence limits: 15.2, 27.0] for weight nadir, 13.4 Gy [9.3, 19.5] for early lethality, 6.4 Gy [3.6, 11.0] for peak weight, and 6.9 Gy [4.2, 10.8] for late lethality, for neutrons: 19.9 Gy [9.5, 61.01 for peak weight. The fecal-deformity data were poorly fitted by the LQ model. The RBE was slightly higher for acute endpoints than for the late ones when X-ray fraction sizes were equal to or larger than 10 Gy. However, the change in RBE with decreasing X-ray dose per fraction was much steeper for the late endpoints, so that it became equal to or even higher than for acute reactions at doses per fraction of 5 Gy or less. Conclusion: Our results were consistent with those obtained from previously published studies using the same experimental system but larger doses per fraction.