Comparative study of late rectal toxicity in veterans undergoing low-dose rate prostate brachytherapy treated with or without supplemental external beam radiotherapy.

Abstract

112 Background: Supplemental external beam radiation therapy (sEBRT) is often prescribed in men undergoing low dose rate (LDR) brachytherapy for prostate cancer. A population of patients was analyzed to assess the effect of sEBRT on late rectal toxicity. It was hypothesized that sEBRT+LDR would be associated with a higher risk of late rectal toxicity, compared to LDR brachytherapy alone. Methods: This retrospective cohort study examined LDR brachytherapy patients, treated with or without sEBRT, with a minimum of 5 years follow-up. All were treated at a credentialed Veterans Affairs medical center that enrolled many of these patients on RTOG 0234. Longitudinal assessments were evaluated using the Computerized Patient Record System, and toxicities were coded using the CTCAE v4.0. The Kaplan-Meier method was used for analysis. Results: Median follow-up was 7.5 years for 245 consecutive patients from 2004 and 2007. sEBRT was administered to 33.5%. Follow-up beyond 5 years was available for 89%. Overall rates of grade ≥ 2 and ≥ 3 rectal toxicities were 6.9% and 2.9%, respectively. The risk of grade ≥ 2 rectal toxicity was 2.8-fold higher for patients receiving sEBRT (1.1 - 7.2, 95% CI, p = 0.02). The risk of grade ≥ 3 rectal toxicity was 11.9-fold higher for patients who received sEBRT (1.5 - 97.4, 95% CI, p = 0.003). Six out of 7 patients with a grade ≥ 3 rectal toxicity received sEBRT, including one who required an abdominoperineal resection due to radiation proctopathy. The median post-LDR D90, V150, V200, and R100 values were 103.3%, 59.4%, 30.1%, and 0.5 cc. The minimum R100 associated with a grade ≥ 2 toxicity was 0.2 cc, and the minimum R100 for a grade ≥ 3 toxicity was 0.4 cc. Grade ≥ 2 rectal toxicity was seen in 3% vs 13% of patients who had post-implant dosimetry of R100 < 1 cc vs R100≥ 1 cc, respectively. Conclusions: In a cohort of LDR brachytherapy patients with high rates of follow-up, sEBRT+LDR was associated with significantly higher risk of grade ≥ 2 and ≥ 3 late rectal toxicity. This analysis supports previous findings and maintains concern about the supplemental use of EBRT with LDR brachytherapy while its benefit for tumor control has yet to be prospectively validated.