S510 Early Colon Adenoma Development Following Immune Checkpoint Inhibitor Colitis: Implications for Prompt Surveillance Colonoscopy

Introduction: Immune checkpoint inhibitors (ICIs) have transformed cancer therapy but are associated with gastrointestinal toxicities, including ICI-mediated diarrhea and colitis (IMDC). While prolonged IMDC has been linked to improved cancer outcomes, its long-term impact on colon polyps remains underexplored. A preliminary study on a small sample size suggested a higher incidence of adenomas in patients with IMDC, warranting a larger investigation to refine strategies on surveillance colonoscopy. Methods: This retrospective study, conducted at a tertiary cancer center from 2010 to 2025, included adult patients who developed IMDC confirmed by colonoscopy within 90 days of onset, and underwent a follow-up colonoscopy at any time point (n = 248). A historical control group included ICI-treated patients without IMDC, who underwent 2 colonoscopies within 1 year without histological abnormalities. Results: The IMDC cohort was 57.9% male and 90.3% White, with a median age of 63.1 years. Most received either PD-1/L1 inhibitors (43.7%) or combination therapy including CTLA-4 inhibitors (42.5%), and the predominant cancer types were melanoma and genitourinary malignancies. 71 patients developed adenomas on follow-up colonoscopies. Notably, over 50% of these adenomas developed rapidly within 7.5 months of IMDC onset, after which the rate declined over the subsequent 6 years. Among the 210 patients who underwent follow-up colonoscopy within one year of IMDC onset, the adenoma incidence was 15.9% in those without a prior history of polyps, compared to 33.9% in those with baseline polyps (P = 0.004). In patients with baseline histological inflammation, those with baseline adenomas were significantly more likely to develop new adenomas on follow-up (32.6% vs 15.8%, P = 0.014). Within this subset, patients without a prior history of polyps (n = 139) were compared to the control group without IMDC (n = 31), revealing a significantly higher cumulative incidence of adenoma development in the IMDC group (P = 0.002). Colitis was significantly associated with adenoma development in Cox regression analysis (HR = 3.6, P = 0.004), indicating a time-dependent risk. Conclusion: These findings provided evidence that the presence of baseline adenomas and IMDC increased the risk of subsequent adenoma formation. Surveillance colonoscopy within 1 year of IMDC diagnosis is warranted to ensure early detection of adenomas and mitigate the risk of developing subsequent colon cancer.