Histology and stage-specific incidence trends in appendiceal adenocarcinoma in the United States, 2000–2022: A SEER analysis.

e15701 Background: Appendiceal adenocarcinoma is rare, but recent population-based studies suggest a rising incidence. We examined whether this increase occurred and, if so, whether it was driven by specific histologies, stage migration, or early-onset disease. Methods: We conducted a population-based trend analysis using SEER 17 registries (Nov 2024 submission), 2000–2022. Microscopically confirmed malignant appendiceal adenocarcinomas were identified and grouped as mucinous (ICD-O-3 8480/3, 8481/3), signet ring (8490/3), and non-mucinous adenocarcinoma (selected adenocarcinoma histologies excluding mucinous/signet). Annual age-adjusted incidence rates (per 100,000; 2000 U.S. standard) were calculated. Temporal trends were summarized using log-linear average annual percent change (AAPC) with 95% CIs. Stage-specific patterns were evaluated using the Combined Summary Stage (2004+) and summarized as both distribution and stage-specific incidence trends. Sensitivity analyses excluded 2022. Results: Overall incidence increased from 2000–2022 (AAPC 3.15%, 95% CI 2.58–3.72) with a sharp uprise in the year 2022. Rising incidence was observed for mucinous (AAPC 3.51%, 95% CI 2.71–4.32) and non-mucinous histologies (AAPC 3.21%, 95% CI 2.46–3.97), while signet ring showed no significant increase (AAPC −0.05%, 95% CI −2.24 to 2.19). From 2004–2022, stage-specific incidence increased across localized, regional, and distant disease, indicating that the rise is not solely attributable to stage migration. In sensitivity analyses excluding 2022, trends were similar (overall AAPC 2.95%, 95% CI 2.38–3.51), suggesting a true rise in disease burden. Crude incidence increased in both < 50 and ≥50 age groups, with higher absolute rates in ≥50. Conclusions: Appendiceal adenocarcinoma incidence rose steadily in SEER from 2000–2022, driven primarily by mucinous and non-mucinous histologies, with increases observed across all stages. A sharp rise in the incidence in 2022 motivated evaluation with a sensitivity analysis excluding 2022. However, the findings remained similar suggesting a true rise in disease burden.