Skin, Merkel cell carcinoma, CK20 stain

Merkel cell carcinoma is a rare aggressive tumour that usually occurs in the sun-damaged skin of elderly patients. The male to female ratio is 2.5:1, and the tumour usually occurs in sun-exposed areas such as the skin of the head, neck, and upper extremities. Approximately half of the patients will have lymph node metastases at presentation, 34% will have distant metastases, and 34% will die from the disease. The cell of origin of Merkel cell carcinoma is not agreed upon, however most believe it is derived specialized neuroendocrine cells of the epidermis that act as mechanoreceptors.

On microscopy, the tumour is composed of confluent sheets and trabeculae of small round cells with high N:C ratio. The nuclei are vesicular with salt and pepper chromatin and inconspicuous nucleoli. Mitotic figures and apoptotic bodies are usually abundant.

Immunohistochemical studies for Merkel cell carcinoma should include CK20 which stains in a perpendicular dot-like pattern. Merkel cell carcinoma is also positive for CAM-5.2, NSE, chromogranin and synaptophysin.

This slide shows CK20 stain. See Related Content for H&E stain.