Merkel cell carcinoma is a rare but aggressive skin cancer. Merkel cells are primary neural cells, found
either alone within the basal layer of the epidermis or in groups as a component of the tactile hair disc of Pinkus. Merkel cell carcinoma was originally called trabecular cell carcinoma and was thought to be derived from sweat glands. However, electron microscopy demonstrated dense-core granules typical of Merkel cells and other neuroendocrine cells within these tumors, suggesting (although not proving) their origin was from Merkel cells. Merkel cell carcinoma is also called neuroendocrine carcinoma of the skin or small cell carcinoma of the skin.

Merkel cell carcinoma is rare, with only approximately 500 new cases each year and an annual incidence based on Surveillance, Epidemiology, and End-Results data of 0.23 per 100,000 for whites. This tumor is typically considered a disease of the elderly, with an average age at diagnosis of 69 years and only 5% of patients below the age of 50 years. The incidence is higher in males than in females. Merkel cell carcinomas most commonly present as an intracutaneous nodule or plaque that has grown rapidly over a few weeks to months. It can be flesh colored, red, or purple, and it sometimes ulcerates. Merkel cell carcinoma appears to be related to sun exposure, and as such is more common on sun-exposed areas, with approximately 50% on the face and neck, 40% on the extremities, and 10% on the trunk.

Merkel cell tumors arise in the dermis and frequently extend into the subcutaneous fat with an intact epidermis. The tumor is composed of small blue cells with hyperchromatic nuclei and minimal cytoplasm. Lymph-vascular invasion is commonly present. There are three variants: intermediate (the most common), small cell, and trabecular. The small cell variant is identical to other small cell carcinomas and must be distinguished from metastatic small cell carcinoma. This can be accomplished through immunohistochemistry. Merkel cell tumors express CAM 5.2 and cytokeratin (CK) 20. CK7 and thyroid transcription factor, which are typically found on bronchial small cell carcinomas, are absent on Merkel cell carcinomas.

Approximately 70% to 80% of patients present with localized disease. Ten percent to 30% have regional lymph node involvement, and 1% to 4% have distant disease at presentation. In addition to the regional lymph nodes, Merkel cell carcinoma also frequently metastasizes to skin; all patients should have a thorough skin and lymph node examination. Other sites of metastasis include the liver, lung, bones, and brain. Patients with clinically involved lymph nodes or symptoms suggestive of distant metastases should have a computed tomography (CT) scan of the chest, abdomen, and pelvis. CT scan of the chest should also be considered in patients with the small cell variant to exclude the presence of a lung mass suspicious for small cell lung cancer. Both positron emission tomography (PET) and octreotide scans have also been described as useful in select situations. Bone scans and magnetic resonance imaging (MRI) of the brain are not indicated in the absence of specific symptoms.