Infrequent HPV Infection in Colorectal Neuroendocrine Carcinoma: Molecular and Histologic Characteristics

Background/Objectives: Colorectal neuroendocrine carcinomas (NECs) are rare, aggressive tumors with poorly defined clinicopathologic and molecular features. Their biological behavior and optimal treatment strategies remain unclear. Additionally, a subset of anorectal NECs may be associated with high-risk human papillomavirus (HPV) infection, suggesting potential heterogeneity in pathogenesis. Methods: We retrospectively reviewed 12 cases of colorectal NECs. Clinical outcomes, histologic morphology, immunohistochemistry, molecular profiling, including common oncogenic mutations, and HPV status were analyzed. Results: Seven cases demonstrated small cell NECs, and five showed large cell NECs. The majority of NECs (n = 9) arose in the rectum. TP53 mutations were the most common (75%), followed by KRAS, RB1, FBXW7, and BRAF mutations. One case demonstrated mismatch repair (MMR) deficiency. High-risk HPV was detected in one rectal NEC, which lacked common oncogenic mutations and was the only long-term survivor (54 months). p16 expression did not correlate consistently with HPV in situ hybridization (ISH) status. Among small cell NECs with follow-up, platinum-based chemotherapy resulted in significantly longer survival than FOLFOX (13.5 vs. 4 months, p = 0.0209). Conclusions: Colorectal NECs display histologic and molecular heterogeneity. The tumors of small cell NECs potentially benefit more from platinum-based chemotherapy. HPV-associated NECs may represent a distinct subset with better prognosis, but p16 is not a reliable surrogate marker. Routine subclassification into small vs. large cell types and comprehensive molecular profiling, including HPV testing, may aid clinical decision-making and prognostication.