Low Body Mass Index As A Risk Factor For Long-Term Proctitis After Prostate Sbrt: Assessing The Dosimetric And Clinical Implications

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