Patterns and Predictors of Relapse Following Radical Chemoradiation Therapy Delivered Using Intensity Modulated Radiation Therapy With a Simultaneous Integrated Boost in Anal Squamous Cell Carcinoma

Purpose: To describe the patterns and predictors of treatment failure in patients receiving definitive chemoradiotherapy (CRT) for anal squamous cell carcinoma (ASCC), delivered using intensity modulated radiotherapy (IMRT). Materials and methods: A retrospective cohort analysis of consecutive patients treated with curative intent for ASCC using CRT delivered with a standardised IMRT technique in five UK cancer centres. Patients were included from the start of UK IMRT guidance in February 2013 to 31st October 2017. Collected data included baseline demographics, treatment details, tumour control, sites of relapse and overall survival. Statistical analysis to calculate outcomes and predictive factors for outcome measures were performed using SPSS and R. Results: The medical records of 385 consecutive patients were analysed. Median follow-up was 24.0 months. 86.7% of patients achieved a complete response (CR) within 6 months of completing chemoradiotherapy. 3yr disease free survival (DFS) and overall survival (OS) were 75.6% and 85.6% respectively. Of all relapses, 83.4% occurred at the site of primary disease. There were two isolated relapses in regional nodes not involved at outset. Predictive factors for cancer recurrence included male sex, high N-stage and failure to complete radiotherapy as planned. Conclusions: The treatment results compare favourably to published outcomes from similar cohorts using 3D conformal CRT. The observed patterns of failure support the current UK IMRT voluming guidelines and dose levels, highlighting our prophylactic nodal dose is sufficient to prevent isolated regional relapse in uninvolved nodes. Further investigation into strategies to optimise CR should remain a priority in ASCC, as the site of primary disease remains the overwhelming site of relapse