ABSTRACT
PURPOSE: To determine the effect of intracavitary brachytherapy (ICBT) fractionation schedule on biologically effective dose to organs at risk. METHODS AND MATERIALS: We reviewed records from 26 patients who had CT imaging during ICBT for International Federation of Gynecology and Obstetrics stage IB2-IVA cervical cancer. Using α/ß=10, we calculated hypothetical nominal doses to achieve a biologically effective dose at 2 Gy per fraction (EQD2α/ß=10) of 40 Gy to Point A for high-dose-rate ICBT with 1-15 fractions. Corresponding minimum EQD2α/ß=3s to the maximally irradiated 2 cc of rectum, bladder, and sigmoid were calculated for each fractionation scheme and added to EQD2α/ß=3 from external beam radiotherapy. Total EQD2α/ß=3s were compared with American Brachytherapy Society suggested dose constraints (rectum/sigmoid, ≤75 Gy; bladder, ≤90 Gy). RESULTS: Except for rectal EQD2α/ß=3 in three patients, the rectal, bladder, and sigmoid EQD2α/ß=3s decreased with increasing fractionation in all patients. Although the total rectal EQD2α/ß=3s were less than the American Brachytherapy Society rectal dose constraint in all patients at all fractionation schedules, the total bladder EQD2α/ß=3s routinely exceeded the bladder dose constraint, even at maximum fractionation. By contrast, increasing fractionation decreased the number of patients with doses exceeding the sigmoid dose constraint by 48%. CONCLUSIONS: The relationship between ICBT fractionation schedule and relative EQD2α/ß=3s to rectum, bladder, and sigmoid depends on individual anatomy. Fractionation optimization can improve therapeutic ratios by minimizing the risk or severity of toxic effects. For patients in whom many fractions optimize the therapeutic ratio, low-dose-rate or pulsed-dose-rate brachytherapy may be preferred.
