Analysis of Radiation Dose/Volume Effect Relationship for Anorectal Morbidity in Children Treated for Pelvic Malignancies.

PURPOSE: To examine dose-volume effect relationships for anorectal morbidity in children treated with image-guided brachytherapy for pelvic tumors. METHODS AND MATERIALS: Medical records of all consecutive children with pelvic tumors treated in our center and receiving image-guided pulsed-dose-rate brachytherapy with or without external beam radiation therapy (EBRT) between 2005 and 2019 were reviewed. The effect of the minimal doses to the most exposed 0.5 cm3, 1 cm3, and 2 cm3 of the anorectum (respectively: D0.5cm3, D1cm3, and D2cm3), total reference air kerma (TRAK), and volume of 100% isodose was examined for anorectal toxicities. RESULTS: Seventy-eight consecutive children were included. Median age was 2.9 years (range, 0.8-14.9 years). Most of the tumors were bladder or prostate (67%) or vaginal (22%) rhabdomyosarcoma. Six patients received EBRT in addition to brachytherapy. Median follow-up was 21.3 months. At last follow-up, 30 children (38%) had experienced Common Terminology Criteria for Adverse Events version 5 grade ≥1 acute or late anorectal events: 24% had grade 1 events, 7.7% had grade 2 events, and 6.4% had grade 3 events. No toxicity greater than grade 3 was observed (eg, fistula or stricture). In univariate analysis, the D0.5cm3 and D1cm3 were significant for probability of grade 1 to 3 (P = .009 and P = .017, respectively) and grade 2 to 3 anorectal morbidity (P = .007 and P = .049, respectively). There was no significant correlation for D2cm3 (P = .057 for grade 1-3; P = .407 for grade 2-3). A 10% probability (95% confidence interval, 4%-20%) for anorectal toxicity of grade 2 or greater was reached for a D0.5cm3 = 52 Gy. The age, EBRT use, TRAK, and treated volume values were not significant. CONCLUSIONS: To our knowledge, this study is the first to show a significant dose-volume effect relationships for anorectal morbidity in children undergoing treatment with brachytherapy. Integrating these data into brachytherapy treatment planning could help to optimize the therapeutic index in these young patients.