The Effect of Radiation Therapy on Malignant Fistulae of Anorectal Cancers

Abstract

Though fistula formation is a well-studied late toxicity of radiation therapy (RT), malignant fistulae (MF) are often present at the time of diagnosis of locally-advanced anorectal cancers (ARC) and negatively impact quality of life. The effect of RT in the setting of MF has not been evaluated, with practitioners concerned with RT potentially worsening MF. Thus, it may be hypothesized that RT could worsen MF. To test this hypothesis, we report our institutional series. A single institutional retrospective IRB-approved analysis of all patients with primary diagnosis of an ARC receiving RT from 2006-present was performed. These patients were screened for MF, defined as a fistula clearly associated with tumor invasion, including chronic fistula from which cancer developed. To adequately assess effect of RT on MF, patients with MF resected prior to RT, RT not directed at the site of MF, and patients with poor/no follow-up were excluded to yield the final cohort for detailed analysis. Effect was assessed by review of follow-up examination and imaging. Six-hundred-and-thirty-six patients with ARC treated at our institution from 2006-2019 were retrospectively reviewed. Forty-four were found to have MF, yielding an overall MF rate of 7.4% among all patients with ARC referred for RT [24/44 (54.5%) with anal/anal margin squamous cell carcinoma and 20/44 (45.5%) with rectal adenocarcinoma]. In 6/44 (13.6%), cancer arose from pre-existing chronic fistula, most often from long-standing Crohn’s disease (3/6). MF response could be assessed adequately in 27 patients. These patients received a median dose of 50.4 Gy (range 26-59.4 Gy), with 25/27 (93%) also receiving concurrent chemotherapy. With a median follow-up of 30.5 months (range 1.6-123 months), RT improved MF in 14/27 (52%), with 9/27 (33.3%) experiencing complete resolution; median time to improvement was 50 days (range 25-117 days). MF persisted in 13/27 (48%). MF resolved in 1/3 (33.3%) evaluable patients with chronic fistula. RT did not appear to worsen any MF. In this series, 7.4% of a large cohort of anorectal cancer patients presented with malignant fistula. RT led to improvement or resolution of these in more than half of patients. Persistence of malignant fistula was only observed in patients with refractory disease, and RT did not appear to worsen any malignant fistula. Based on our findings, symptomatic malignant fistula is not a contraindication to RT and may be considered as an indication for palliative RT.