Dosimetric factors predicting severe radiation-induced bowel complications in patients with cervical cancer: Combined effect of external parametrial dose and cumulative rectal dose

The purpose of this study was to analyze dosimetric factors of radiation-induced severe bowel complications among patients with cervical cancer. We reviewed 297 patients of stage IB-IVA cervical cancer managed by curative-intent radiotherapy from May 1993 through December 1997. Whole-pelvic irradiation of external beam radiation therapy (EBRT) (34.2-48.6 Gy/19-27 fractions) was delivered to all patients. Two hundred and three patients received additional bilateral parametrial boost (3.6-18 Gy/2-10 fractions). High dose-rate (HDR) intracavitary brachytherapy (ICBT), 16-24 Gy/5 fractions to Point A, was given after external irradiation. Cumulative rectal biologically effective dose (CRBED) at rectal reference point was determined by summation of EBRT and ICBT component. The 5-year incidences of Grade 3-4 enterocolitis and proctitis were 10% and 7%, respectively. Both complications were associated with external parametrial dose (PMD) and CRBED. Interaction of CRBED and PMD was noted in multivariate analysis of enterocolitis (P < 0.001) and proctitis (P < 0.001). In CRBED ≥ 100 Gy₃ group, PMD was an independent factor in enterocolitis (P = 0.010) and proctitis (P = 0.039). In PMD ≥ 54 Gy group, CRBED was an independent factor in enterocolitis (P = 0.003) and proctitis (P = 0.036). Patients with both PMD ≥ 54 Gy and CRBED ≥ 100 Gy₃ had higher incidence of 5-year enterocolitis (26%) (P < 0.001) and proctitis (17%) (P < 0.001) than other dose groups. Radiation-induced severe bowel complications are association on both high PMD and high CRBED. We do not suggest both external PMD ≥ 54 Gy and CRBED ≥ 100 Gy₃ for treatment of cervical cancer due to unacceptably high incidence of severe bowel complications.