Low Body Mass Index As a Risk Factor for Long-Term Proctitis after Prostate SBRT: Assessing the Dosimetric and Clinical Implications

Abstract

Body Mass Index (BMI) is a parameter used by the World Health Organization and American Cancer Society (ACS) to categorize patient health. Stereotactic Body Radiation Therapy (SBRT) is a standard treatment option for men with localized prostate cancer. Previous studies have described a lower volume of anterior perirectal and mesorectal fat in individuals with lower BMI’s. This study assesses the incidence, dosimetric impact, and clinical implications of low BMI on patients treated with prostate SBRT. 2421 patients with prostate cancer and for whom BMI’s were available were treated at an academic institution with non-coplanar SBRT. Intrarectal amifostine was administered prior to each fraction. The mean age was 67.5 (41-93) years and the mean pre-treatment PSA was 7.55ng/ml (0.33-82.49). Patients were treated for low (23.4%), favorable intermediate (26.7%), unfavorable intermediate (41.3%), and high risk (8.6%) disease. Most (89.1%) patients were treated with a dose of 3500cGy (3500-3625) over 5 fractions. Androgen deprivation Therapy (ADT) was prescribed in 17.2% of cases. Proctitis was scored by RTOG late toxicity scale. The Pearson chi-square test was used to compare patient groupings. Freedom from grade 2+ proctitis was assessed using the Kaplan-Meier method and analyzed using Cox regression. The mean BMI in this series was 28.7 kg/m2 (15.2-58.8). Per ACS stratification, 19.6% of patients were normal weight, while 46.1% and 34.0% of patients were overweight and obese, respectively. 24 (1.0%) patients registered a BMI of ≤20 kg/m2. These patients were more likely to have T2b/T2c stage disease (30.4% vs. 12.7%, p = .039), but were without any difference in initial PSA, Gleason score, or NCCN risk grouping. They were more likely to have a pre-existing diagnosis of HIV (9.5% vs. 0.9%, p<.0001), with similar utilization of blood thinners. There was no difference in ADT use. Patients with a BMI ≤20 kg/m2 had smaller CTV sizes (55.9cc’s vs. 79.0cc’s, p = .002), with a higher Rectal V3600 (1.13cc’s vs. 0.69cc’s, p = .018). There was no difference in bladder dosimetry. They had a higher 6-year cumulative risk of grade 2+ (11.9% vs. 3.0%, p = .002) and grade 3+ (5.6% vs. 0.9%, p = .011) proctitis. There was no difference in bladder outcomes. On multivariate analysis, receiving treatment with a BMI ≤20 kg/m2 (OR 7.68, CI 1.53-38.50, p = .013) and using anti-coagulation use during SBRT (OR 4.16, CI 1.89-9.19, p<.0001) were the lone predictors for grade 2+ proctitis. Long-term rates of proctitis are low in patients receiving prostate SBRT. In a large institutional series, those with BMI ≤20 kg/m2 had an increased risk of grade 2+ and grade 3+ rectal toxicity. The association of lower BMI with higher rectal dosimetry might be a function of decreased perirectal fat. This data is thought provoking and might suggest patient groupings which may derive the greatest benefit to hydrogel placement prior to prostate SBRT.