Proton therapy for prostate cancer: An analysis of rectal toxicity outcomes at University of Florida Proton Therapy Institute (UFPTI).

Abstract

161 Background: We aimed to characterize gastrointestinal effects of proton therapy (PT) in patients treated for prostate cancer, identify factors associated with rectal bleeding (RB), and compare RB between patients on investigational protocols (IP) versus outcome tracking protocols (OTP). Methods: 1,290 patients treated consecutively with PT between August 2006 and June 2010 were studied. Pre-existing potential risk factors for RB (hemorrhoids, diabetes, aspirin, other anticoagulants) were recorded. Common terminology criteria for adverse events (CTCAE version 4.0) were used to grade toxicity. Median follow up was 3.5 years. Results: The majority (94.6%) of Grade 2 or higher gastrointestinal toxicity (GR2+) events after PT were rectal bleeding. GR1 RB occurred in 390 (30.2%) of patients, including those requiring no prescription intervention (82.6%) and those placed on a vitamin A regimen (16.3%). GR2 RB occurred in 14.7% of patients, including patients requiring prescription rectal suppositories (9.5%), cautery (4.9%), or hyperbaric oxygen (0.3%). Grade 3 (GR3) occurred in 0.8% of patients, including those requiring transfusion (0.6%) or colostomy (0.1%). Multivariate analyses showed that pretreatment daily aspirin (p=0.03), other anticoagulation (p=0.001), and the volume of rectum exposed to radiation dose levels (V70) (<.0001) were associated with an increased risk of Gr2+RB. Patients treated on investigational protocols (IP) had similar toxicity rates as those treated on an outcomes tracking protocol (OTP). Reduced rectal toxicity was noted in latter years of the study, which correlated with changes in treatment technique, pretreatment evaluation, and post-treatment supportive care regimens. Conclusions: PT was associated with low rates of GI toxicity, the most common toxicity being transient RB. GR2+RB is strongly associated with patient use of aspirin, other anti-coagulation and radiation dose volume parameters.