Variable impact of intracavitary brachytherapy fractionation schedule on biologically effective dose to organs at risk in patients with cervical cancer

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/β = 10, we calculated hypothetical nominal doses to achieve a biologically effective dose at 2 Gy per fraction (EQD2 α/β=10 ) of 40 Gy to Point A for high-dose-rate ICBT with 1–15 fractions. Corresponding minimum EQD2 α/β=3 s to the maximally irradiated 2 cc of rectum, bladder, and sigmoid were calculated for each fractionation scheme and added to EQD2 α/β=3 from external beam radiotherapy. Total EQD2 α/β=3 s were compared with American Brachytherapy Society suggested dose constraints (rectum/sigmoid, ≤75 Gy; bladder, ≤90 Gy). Results Except for rectal EQD2 α/β=3 in three patients, the rectal, bladder, and sigmoid EQD2 α/β=3 s decreased with increasing fractionation in all patients. Although the total rectal EQD2 α/β=3 s were less than the American Brachytherapy Society rectal dose constraint in all patients at all fractionation schedules, the total bladder EQD2 α/β=3 s 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 EQD2 α/β=3 s 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.