A rare, usually well-differentiated neuroendocrine neoplasm that arises from the testis. It usually affects adults, and a minority of patients have carcinoid syndrome. It may occur in isolation or, less frequently, be associated with testicular teratoma. The majority of cases are not associated with isochromosome 12p or germ cell neoplasia in situ. The clinical course is usually benign. [from
NCI]