41 Dose discrepancy between conventional and ICRU No. 38 rectal reference points: Caveat in using ICRU definition as rectal tolerance dose

Abstract

INTRODUCTION: The mosl frequent source loading optimization in brachytherapy for cancer of cervix is to limit the dose to the rectum. Since ICRU No. 38 recommends uniform rectal dose reporting, many institutions are beginning to utilize this referencc dose as the rectal tolerance dose bcforc clinical confirmation. This study is In compare differences in dose between the convcntional and the ICRU rectal reference points and emphasize the need for caution in using thc ICRU defined rectal dose as the tolerance dose. MATERIALSAND METHODS: We reviewed orthogonal radiographs of 30 patients who were treated with low-dose-rate intracavitary brachytherapy for cancer of Ih¢ cervix. Comparative analysis of the difference in dose between the convcmional rectal point (defined on the anterior rectal mucosa visualized with contrast from the middle of the intracavitary intravaginai sources) and the ICRU No. 38 rectal point (5 n;m behind the posterior vaginal wall from the middle of the intravaginal sources) were performed. The influence of age, clinical stage, and longitudinal position of colpostats relative to symphysis pubis on the dose specified at the two rectal reference points wcrc cvaluated. RESULTS: In 21 of 30 cases (70%), the ICRU reference doses were higher on average 33% (8% to 186%) than the conventional reference doses. In 9 of the 30 cases (30%), the dose at the conventional and the ICRU rectal reference points were similar. The average dose to the rectal point defined by the ICRU definition was 73% (range 41 to 108%) of the dose prescribed at Point A compared with 55% (range 25% to 88%) for the conventional rectal poinl. Age, stage, and posilion of colpostats made no difference to definitions of either rectal point. CONCLUSION: The ICRU definition results in significantly higher doses to the rectal reference point compared to the conventional definition for the majority of patients. In the current era of 3D treatment phmning, cxccssivc oplimization hascd on the ICRU definition '.,.'ill iiiiidlfy Ih¢ desirable rch:rcn,.'e i~car-shapcd isodcls¢ vtvlunl¢ which it2suhs ill reduced ctlVCl;igc ol Iht: t.t.'l',tcal IUIIIAII thlo[ ~.llIttt.d confirmation based on thc ICRU dcfthition is available, the conventional definition that sets the tolerancc limit of the rectum should be used.