An aggressive, high-grade and poorly differentiated carcinoma with neuroendocrine differentiation that arises from the ampulla of Vater and the periampullary region. The mitotic count is more than 20 per 10 HPF. According to the size of the malignant cells, the prominence of the nucleoli, and the amount of cytoplasm, it is classified either as small or large cell neuroendocrine carcinoma. Patients usually present with advanced stage disease. [from
NCI]