Incidence trends and misclassification patterns of hepatocholangiocarcinoma: A SEER database analysis.

603 Background: Hepatocholangiocarcinoma (H-ChC), or combined hepatocellular–cholangiocarcinoma, is a rare primary liver malignancy with mixed hepatocytic and cholangiocytic features. True incidence remains uncertain due to misclassification as hepatocellular carcinoma (HCC) or intrahepatic cholangiocarcinoma (ICC). Understanding coding patterns is critical for accurate epidemiology and outcomes research. Methods: We queried the SEER 18 Registries (2000–2020) using ICD-O-3 histology codes for H-ChC (8180/3), HCC (8170–8175), and ICC (8160). Incidence rates were age-adjusted per 100,000 population. Temporal trends were assessed using Joinpoint regression. Survival outcomes were compared across groups using Kaplan–Meier and multivariable Cox regression. Misclassification was inferred by examining cases initially registered as HCC or ICC but recoded to H-ChC, and by assessing histology versus primary site discordance. Results: A total of 3,248 patients were coded as H-ChC, representing 0.8% of primary liver cancers. The age-adjusted incidence of H-ChC rose from 0.03/100,000 in 2000 to 0.07/100,000 in 2020 (annual percent change +2.1%, p