Histomorphological and Molecular Features of Colonic Large-Cell Neuroendocrine Carcinoma in a Patient With Familial Adenomatous Polyposis: A Case Report and Review of Literature.

Colorectal large-cell neuroendocrine carcinoma, a rare and aggressive type of cancer, accounts for <0.6% of all colorectal cancers. Neuroendocrine carcinomas are associated with hereditary conditions such as Lynch syndrome; however, their co-occurrence with familial adenomatous polyposis (FAP) is poorly documented. To date, only 1 patient of colorectal neuroendocrine carcinoma in a patient with FAP has been reported. This report presents a patient with FAP. Large-cell neuroendocrine carcinoma with lymph node metastasis was discovered during right colectomy. Histopathological and immunohistochemical assessments confirmed neuroendocrine differentiation with a high Ki-67 index (>90%). Genetic analysis revealed a pathogenic germline mutation and somatic alterations in , and . Adjuvant chemotherapy commenced postoperatively. No evidence of recurrence was observed for 18 months postoperatively. This case report highlights the rare presentation of colorectal large-cell neuroendocrine carcinoma in a patient with FAP, thereby contributing to the limited literature on this association. mutations have been characterized in adenomatous polyposis and colorectal adenocarcinomas; however, their role in the pathogenesis of neuroendocrine carcinoma remains unclear. Additional mutations of , , , , , and suggest a unique molecular profile that may contribute to the development of neuroendocrine carcinoma in patients with FAP. This is the second reported patient of colorectal large-cell neuroendocrine carcinoma in a patient with FAP. Further studies must be conducted to elucidate the role of mutations in the pathogenesis of neuroendocrine tumorigenesis.