Clear definitions of histological groups are essential for studies of liver and intrahepatic bile duct cancers. Thus, we developed a classification system based on abstracted information on histologies of liver and intrahepatic bile duct cancers diagnosed during 1978-2007 within all Surveillance, Epidemiology, and End Results (SEER) registries. Of 61,990 reported primary liver and intrahepatic bile duct cancers, 108 distinct ICD-O histology codes were identified. During the 5 recent years of diagnosis, 2003-2007, the leading histological groups were hepatocellular carcinoma (75%) and cholangiocarcinoma (12%). The remaining categories were other specified (3%) and poorly specified carcinomas (3%), hepatoblastomas (1%), sarcomas (1%), embryonal sarcomas (0.1%), other specified malignancies (0.05%), and poorly specified malignancies (5%). During 2003-2007, only 68% of diagnoses were microscopically confirmed. Factors contributing to incomplete histological classification may include reluctance to obtain diagnostic specimens from late stage cases and administration of therapy in lieu of histological confirmation after positive diagnostic imaging.
Conclusion: The proposed histological classification in this report may facilitate studies of primary liver cancers. This is of value because the inconsistent characterization of some cancers, particularly cholangiocarcinomas, may affect interpretation of incidence trends. Incomplete histological characterization of hepatocellular carcinomas was noted in this report. It is likely to be explained by guidelines affirming the use of non-invasive diagnostic and treatment procedures for this cancer.