Molecular Subtyping of Biliary Tract Neuroendocrine Carcinomas

Neuroendocrine carcinoma (NEC) of the biliary tract is an exceptionally rare and highly aggressive malignancy, and its molecular subtype landscape and diagnostic boundaries remain poorly defined. We retrospectively analyzed 42 biliary tract NECs, including 22 surgically resected cases, arising in the intrahepatic bile duct (n=8), extrahepatic bile duct (n=6), gallbladder (n=23), and ampulla of Vater (n=5). Using immunohistochemistry, NECs were classified into ASCL1 (A), NEUROD1 (N), POU2F3 (P), and null subtypes, and their clinicopathological features and disease-specific survival (DSS) were assessed. In the surgical cohort, the incidence of NEC ranged from 0.5% to 2.8% across anatomic sites. Most surgically resected tumors were combined with adenocarcinoma and classified as mixed neuroendocrine-non-neuroendocrine neoplasms, and metastatic lesions were predominantly composed of NEC regardless of the proportion of the NEC component in the primary tumor. Four cases arose in association with intraductal papillary neoplasms. Molecular subtyping identified 21% A-type, 40% N-type, 26% P-type, and 12% null-type tumors. The P type was enriched in gallbladder NECs (39%). The N type was more frequently associated with resectable disease, whereas the P type tended to be associated with unresectable disease. DSS differed significantly among the molecular subtypes, and the null type independently predicted shorter DSS (hazard ratio=3.48; P=0.034). Notably, several P-type tumors lacked expression of conventional neuroendocrine markers, including chromogranin A, synaptophysin, and INSM1. These findings indicate that biliary tract NECs exhibit a distinct, site-dependent molecular subtype distribution and suggest that molecular subtyping may serve as a practical adjunct to diagnosis and prognostic stratification.