Neuroendocrine carcinoma within lymph nodes in the absence of a primary tumor, with special reference to Merkel cell carcinoma.

We report eight cases of neuroendocrine carcinomas found within inguinal (five cases), axillary (two cases), and submandibular (one case) lymph nodes. The patients underwent extensive investigations, but no primary tumor was found in any case. Although the existence of an occult or regressed primary cannot be ruled out, the possibility of a lymph node origin should be considered on the basis of epithelial inclusions or anomalous carcinomatous differentiation of stem cells of the lymphoreticular system.