Contributor: Gordon K. Klintworth
Cutaneous neuroendocrine tumor (Merkel cell tumor, primary small cell cutaneous carcinoma, trabecular carcinoma, small cell neuroepithelial tumor, cutaneous APUDoma, cutaneous neuroendocrine carcinoma) is a rare malignant neoplasm of the skin that arises from the Merkel cell. The synonyms reflect the appearance and the function of the cell, which is derived from the neural crest. The cutaneous neuroendocrine tumor most commonly occurs in the elderly, and is found most frequently on the head and neck. It may also occur on the trunk and limbs. Sometimes it arises in the eyelid or eyebrow. In one series of  69 patients with cutaneous neuroendocrine tumors 7 (9%) involved the eyelid. Most cutaneous neuroendocrine tumors of the eyelid affect the upper eyelid more often than the lower eyelid and the tumor occurs more frequently in women than men. Clinically, the tumor appears as a nontender, reddish blue nodule with telangiectasia on its surface. Thus, it resembles an angioma. One case manifested as a recurrent chalazion. The median age of patients is 71 years, but a cutaneous neuroendocrine tumor of the eyelid has been described in a 15-year-old girl with ectodermal dysplasia. Microscopically, the cells may be confused with other small cell neoplasms such as oat cell carcinoma [carcinoma - oat cell] of the lung and lymphoma. Immunohistochemical and ultrastructural studies help to characterize the cell. The cell stains positively for neuron-specific enolase, keratin, and EMA, and met-enkephalin. Peptides such as calcitonin, somatostatin, and ACTH have also been demonstrated in the tumor cells. Cytokeratin 20 is useful in distinguishing Merkel cell tumor from small cell carcinoma [carcinoma - small cell] (Chan et el. Am J. Surg Path 21:226-234, 1997). A fine-needle biopsy with immunohistochemical studies can establish the nature of a metastatic cutaneous neuroendocrine tumor.