Clinicopathologic and Molecular Characterization of Inflammatory Bowel Disease-Associated Neuroendocrine Carcinomas and Mixed Neuroendocrine-Non-Neuroendocrine Neoplasms.

The pathogenesis of neuroendocrine carcinomas (NECs) and mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs) in the gastrointestinal tract remains poorly understood. This study aims to characterize the clinicopathologic and molecular features of NEC/MiNEN in patients with inflammatory bowel disease (IBD). Eighteen surgically resected IBD-associated intestinal carcinomas with a minimum of 30% neuroendocrine component were collected from 6 academic centers and compared with a control group of 12 IBD-associated carcinomas lacking neuroendocrine differentiation. Both groups exhibited a male predominance and similar age distribution. The NEC/MiNEN group was more likely to have a higher percentage of Crohn disease (9/18 vs 1/12; P = .024), occur in the rectum (9/18 vs 3/12) and small intestine (4/18 vs 0/12) (P < .01), be diagnosed on resection without a preceding biopsy (6/18 vs 0/12; P = .057), and have unidentifiable precursor lesions (10/18 vs 1/12; P = .018) than the control group. Synchronous carcinoma, advanced tumor stage (pT3 and pT4), and lymph node metastasis occurred at similar rates; however, the NEC/MiNEN group had a higher incidence of angiovascular invasion (14/18 vs 4/12; P = .024), distant metastasis (8/18 vs 1/12; P = .049), mortality (8/18 vs 2/12; P = .058), and worse survival (Kaplan-Meier; P = .023) than the control group. All tested cases were mismatch repair proficient. A Ki-67 proliferation index ranged from 25% to 100%. Next-generation sequencing in 11 NEC/MiNEN cases revealed low tumor mutational burdens but complex genetic abnormalities commonly involving TP53 (9/11; 82%), FBXW7 (4/11; 36%), and APC (3/11; 27%) genes, with the other genetic alterations randomly occurring in 1 or 2 cases. The neuroendocrine component, which shared similar molecular alterations as the nonneuroendocrine component, was subcategorized into intermediate (G3a) and high grade (G3b); the higher grade correlated with more genetic alterations. In conclusion, IBD-associated NEC/MiNEN shows diverse histologic features, variable precursor lesions, intricate genetic abnormalities, and aggressive biologic behavior. The classification and grading of gastrointestinal NEC/MiNEN may be refined for better clinical management.