Body mass index, dose to organs at risk during vaginal brachytherapy, and the role of three-dimensional CT-based treatment planning

Purpose: To assess the effect of body mass index (BMI) on dose to organs at risk (OARs) during high-dose-rate vaginal brachytherapy and evaluate the role of three-dimensional dose evaluation during treatment planning. Methods and Materials: Three-dimensional dosimetric data for rectum, bladder, sigmoid colon, and small bowel for 125 high-dose-rate vaginal brachytherapy fractions were analyzed. Dose-volume histograms were generated for D0.1 ccand D2 ccof each OAR. Contributing factors including the use of urinary catheter and cylinder size were also recorded. As different dose fractionations were used, the OAR doses were tabulated as a percent dose prescribed to 0.5cm. All patients were treated to 4cm of the vaginal length. Results: Median BMI in this cohort was 31.7kg/m2. The BMI values had a weak inverse correlation with D0.1 ccto sigmoid colon (rs=-0.18, p=0.047) and D0.1 ccto bladder (rs=-0.19, p=0.038). There was a strong inverse correlation of D2 ccand increasing BMI (rs=-0.64, p=0.003). The median D2 ccwas 25.1% for BMI higher than 31 and 61.9% for BMI of 31 or lower. For D0.1 cc, there was also a strong inverse correlation with increasing BMI (rs=-0.57, p<0.001). Median D1 ccwas 33.5% for BMI >31 and 84.4% for BMI ≤ 31. On multivariate analysis higherBMI remained a significant predictor of lower small bowel D2 cc(p<0.001) and D0.1 cc(p<0.001). Conclusions: Women with a lower BMI receive higher doses to the bladder and small bowel compared with those with a higher BMI. Three-dimensional dose evaluation should be considered in patients with low BMI, particularly when combined with external beam radiation.