Impact of BMI on Organs at Risk Dose for Cervical Cancer Treated With Definitive Radiation Therapy Followed by Brachytherapy Boost.

Objective Higher body mass index (BMI) is identified as a protective factor in intracavitary brachytherapy (BT) for cervical cancer (CC) treated with definitive radiation therapy, particularly for gastrointestinal toxicity. The impact of BMI on organs at risk (OAR) dose remains unknown as hybrid intracavitary-interstitial applicators are used. Methods Patients with CC who received definitive chemoradiation followed by BT boosts from 2023 to 2025 were included. BT doses to OAR of 0.1cm3, 1cm3, and 2cm3 were collected as % of the prescription dose (%Rx). A linear mixed effects regression model was fitted to evaluate the relationship between BMI and dose to OARs. Results Fifty-six patients with 221 BT plans were included. The majority of patients included were 2018 International Federation of Gynecology and Obstetrics (FIGO) stage IIIC (n = 29, 51.8%). The mean ± standard deviation (SD) of BMI for the cohort was 29.4 ± 7.6 kg/m2, with 36 (64.3%) patients receiving hybrid BT with a median (interquartile range (IQR)) of 2 (2-3) needles. The mean ± SD of D0.1cm3, D1cm3, and D2cm3 %Rx for rectum were 55.4% ± 18.6, 45% ± 15.4, and 41.4% ± 13.9; for sigmoid were 63.9% ± 17.2, 52.5% ± 14.3, and 47.8% ± 13.3; for bladder were 83.3% ± 13.6, 71.0% ± 11.2, and 65.6% ± 10.6; and for small bowel (SB) were 51.4% ± 22.9, 42.1% ± 19.6, and 38.0% ± 17.5, respectively. While controlling for other variables, each one-point increase in BMI decreased the small bowel D0.1cm3, D1cm3, and D2cm3 by 0.91, 0.78, and 0.72, respectively (p = 0.008 for D0.1cm3, and 0.006 for D1cm3 and D2cm3). The bladder D1cm3 and D2cm3 decreased by 0.31 (p=0.030) and 0.32 (p=0.017), respectively. The same was not true for the rectum and sigmoid. Conclusion We found an inverse relationship between BMI and dose to the small bowel and bladder during BT.