Tolerance of rat small intestine to localized single dose and fractionated irradiation.

The tolerance of rat small intestine to localized single-dose and fractionated irradiation was assessed. In 168 rats, bilateral orchiectomy was performed and a loop of small intestine was transposed to the left part of the scrotum. Beginning 3 weeks postoperatively, single dose (18-24 Gy) or fractionated (4.2 Gy or 5.6 Gy per fraction) x-irradiation was delivered to the transposed intestine. The animals were observed for complications, and groups of animals were killed 2 and 26 weeks after completion of irradiation for assessment of injury. Mortality (i.e. the occurrence of lethal intestinal complications) and a semiquantitative histopathologic scoring system were used as endpoints to assess the degree of radiation injury. The most frequent intestinal complications were enterocutaneous fistula formation and intestinal obstruction. Logistic regression analysis ov complications data was used to estimate LD50 values and the alpha/beta ratio. There was good correlation between histopathologic scores and the incidence of lethal complications. The estimated LD50 values were 22.1 +/- 0.5 Gy, 37.0 +/- 4.4 Gy and 51.0 +/- 5.3 Gy for the single dose regimen and the fractionated regimens of 5.6 Gy and 4.2 Gy respectively. The estimated alpha/beta ratio was 10.7 +/- 2.4 Gy. The goodness of fit of the linear-quadratic isoeffect model to our data was satisfactory. Our results indicate that acute mucosal damage may be pathogenetically involved in the development of intestinal complications.