Primary high-grade neuroendocrine carcinoma emerging from an adenomatous polyp in the setting of familial adenomatous polyposis.

Familial adenomatous polyposis (FAP) is a rare inherited syndrome that is characterised by innumerable adenomas of the colon and rectum, a high risk of colorectal cancer and a variety of extracolonic manifestations. FAP presents as hundreds to thousands of colonic adenomas beginning in adolescence. The syndrome is associated with less than 1% of all colorectal cancer cases, but there is a nearly 100% lifetime risk of colorectal cancer in individuals with FAP. This case demonstrates a 60-year-old man with FAP who developed high-grade neuroendocrine carcinoma with glandular and squamous differentiation, and regional lymph node and liver metastases. Early diagnosis of FAP is of the utmost importance to start screening colonoscopies to assess disease burden, perform polypectomies and to make management decisions. Neuroendocrine carcinomas rarely occur in patients with FAP, and awareness of this association among general medical physicians and pathologists is essential for the diagnosis and care of these patients. 2016 BMJ Publishing Group Ltd.