Durable local control following salvage interstitial brachytherapy for anorectal cancer pelvic recurrence: a retrospective series

Salvage surgery is the standard of care for patients with pelvic recurrences from anorectal cancers, including rectal adenocarcinoma and anal squamous cell carcinoma (ASCC). Given the associated morbidity, some patients decline surgery, while others are considered unresectable, and controlling pelvic disease in these patients remains a major challenge. Interstitial brachytherapy (ISBT) has demonstrated efficacy in recurrent urologic and gynecologic malignancies, yet its role in the setting of recurrent anorectal cancer is undefined. A retrospective analysis on a series of consecutive patients with anorectal cancer pelvic recurrences treated with ISBT between 2018 and 2025 at a single Canadian tertiary center was performed. Cumulative incidence function was used to estimate in-field local, out-of-field pelvic and distant progression, and Kaplan–Meier was used to estimate overall survival. Fourteen patients (10 adenocarcinoma and 4 ASCC) were included, with a median target lesion size of 3.7 cm. Six (42.9%) patients were treated with external beam radiation therapy (EBRT; median 45 Gy) and ISBT (median 17.3 Gy), while 8 (57.1%) received ISBT alone (median 30 Gy). The overall response rate was 57.1% and median duration of in-field target control was 20.3 months. Median overall survival was 31 months. Out-of-field pelvic and/or distant progression were the predominant patterns of failure, with only one (7.1%) patient demonstrating isolated in-field progression and 3 (21.4%) patients dying with isolated pelvic disease. Grade 3 toxicity, including fibrotic and necrotic complications, was observed in 6 (42.9%) patients, five of whom subsequently developed progressive disease at the site of toxicity. Salvage ISBT results in durable control for anorectal cancer pelvic recurrences with acceptable toxicity profile. Distant relapse often precedes local progression in this high-risk population and death from isolated uncontrolled pelvic disease is rare using this strategy.