[Merkel cell carcinoma. Clinical and histological differential diagnosis, diagnostic approach and therapy].

Merkel cell carcinomas are rare tumors of the skin with an aggressive behavior and frequent regional and distant metastases. Typically, the primary is a fast-growing, painless, reddish nodule with an iceberg-like effect, broadening in the depth. On the trunk and the buttocks, deep clinically rather inconspicuous nodules can occur. The clinical differential diagnosis of the Merkel cell carcinoma includes skin metastases, malignant lymphomas, malignant adnexal tumors and cysts when the tumor is located deep in the soft tissue (e.g. on the buttocks). Histological and immunohistochemical analysis is necessary for the diagnosis. The demonstration of cytokeratin 20 in the typical globular distribution pattern is of main importance in the diagnosis of Merkel cell carcinoma. Because they are very rare, Merkel cell carcinomas are infrequently diagnosed clinically, in spite of the rather characteristic picture. Diagnostic excision with a safety margin of 3 cm is recommended followed by an adjuvant radiotherapy. The radiation field should include the area of the draining vessels and the first regional lymph nodes. There are some reports concerning the advantage of sentinel lymph node biopsy. In distant metastases, the therapy is multimodal and palliative including surgery, radiation and chemotherapy. Because of the high incidence of regional and distant metastases, regular follow-up is important.