CHRONIC RADIATION-DAMAGE IN THE RAT RECTUM - AN ANALYSIS OF THE INFLUENCES OF FRACTIONATION, TIME AND VOLUME

Purpose: Analysis of four different sets of experiments performed by t he G.S.F. group in Munich investigating the late tolerance of the rat rectum to external or intracavitary irradiation. Material and methods: The endpoint was late rectal stenosis in female Wistar rats. The raw data were fitted to the linear-quadratic model by means of a likelihoo d maximization method (Direct Analysis). The model was altered to allo w for repopulation, incomplete repair, and varying irradiated lengths of the rectum. Results: Fractionation sensitivity was high or intermed iate (alpha/beta ratio values [95% confidence limits] ranging from 2.6 7 [0.86, 4.80] to 6.65 [2.21, 11.73] Gy). Significant repopulation occ urred when treatments were longer than 5 days (D-prolif,,,,il equal tb 0.61 [0.20, 1.47] and 1.08 [0.58, 1.90] Gy/day, in fractions of 4 Gy) . Another interpretation is that radiosensitivity changed during treat ment. Repair half-time estimates ranged between 1.84 [1.52, 2.34] and 5.02 [2.83, 21.7] h. Finally, the present analysis indicated that the smallest surviving compartment capable of tissue rescue was about 1/50 to 1/100 of a 1 cm high cylinder of the rectum wall. Conclusions: The radiobiological features of late stenosis in the rats are consistent with combined injuries of early and late responding components of the rectal wall. This raises some concerns about the possible danger of hy perfractionated treatments, where the beneficial impact of fraction si ze reduction may be obviated for interfraction intervals that are too short. Also, accelerated irradiation may result in more late complicat ions because of increased early reactions.