Efficacy and Safety of S-1 and Oxaliplatin with Radiotherapy for Anorectal Cancer: A Retrospective Study

Introduction Neoadjuvant chemoradiotherapy (CRT) is standard for locally advanced rectal adenocarcinoma, but the optimal regimen remains unclear. For squamous cell carcinoma (SCC), 5-fluorouracil (5-FU) plus mitomycin C (MMC) with radiotherapy (RT) is standard, though alternatives are needed for patients unable to receive MMC or those avoiding prolonged infusion. S-1 and oxaliplatin (SOX) with RT has been explored for rectal and anal cancer, but its efficacy and safety remain uncertain. This study retrospectively evaluates SOX + RT outcomes. Methods This single-center retrospective study analyzed 20 patients with anorectal adenocarcinoma and squamous cell carcinoma who received SOX + RT at Osaka University Hospital between March 2012 and June 2024. SOX + RT was given as neoadjuvant or curative treatment. Tumor regression and adverse events were assessed per standard grading criteria. Survival analysis was performed using the Kaplan-Meier method. Results Among 20 evaluable cases, 14 were adenocarcinomas and 6 were SCCs. Twelve patients received the treatment as neoadjuvant, and eight underwent it as curative treatment. The local response rate was 60%, with all SCC cases achieving complete response. R0 resection was achieved in 83%, and anal preservation in 88%. The 3-year progression-free survival (PFS) rate was 39.2%, and overall survival (OS) was 70.6%, with SCC showing better PFS (80.0%) than adenocarcinoma (28.6%). Grade 3 adverse events occurred in 50%, but no treatment-related deaths were reported. Conclusion SOX + RT was safe and effective, particularly for SCC, suggesting its potential as an alternative when standard therapy is infeasible. Prospective studies are needed to confirm its efficacy.