EFFECTS OF CONTINUOUS LOW-DOSE-RATE BRACHYTHERAPY ON THE RECTUM OF THE RAT

The dose-limiting factor in essentially all curative radiotherapy is d amage to normal tissue. In spite of this, our understanding of radiati on-induced damage to normal tissue leaves much to be desired. Damage t o the rectum is often observed after clinical brachytherapy of pelvic malignancies. A model in which the rectum of the rat is irradiated wit h a uniform and reproducible dose of brachytherapy irradiation was dev eloped. This system consists of an intracavitary applicator, which is used in conjunction with methods that reduce fecal flow, a jacket that restrains rat movement and a lead-shielded chamber to contain rats du ring low-dose-rate brachytherapy. Irradiation was applied in 48-h inte rvals separated by 48-h breaks during which the applicator was removed . Methods to analyze the sequelae of brachytherapy irradiation of the rectum were also developed. The total dose of 0.75 Gy/h continuous low -dose-rate brachytherapy required to produce obstruction in the rat re ctum has been measured. The ED(50) was 70.6 +/- 4.2 Gy. The length of time between irradiation and obstruction was also correlated with tota l dose. Other effects on the rectum that were related to dose were dia rrhea and rectovaginal fistula. This irradiation system can produce un iform and reliable brachytherapy doses to the rat rectum. The late eff ects in normal tissues observed in this model are similar to those obs erved in humans after irradiation of the rectum. The model should be a ppropriate for analyzing relative changes in the rectum resulting from different dose rates and fractionation schemes relevant to brachyther apy. (C) 1996 by Radiation Research Society