Merkel cell carcinoma: a challenge.

A 70-year-old man presented with an enlarging, recurrent tumor of the left buttock which, on initial biopsies, was interpreted as a histiocytic malignant lymphoma. Further biopsies showed numerous monomorphic, hyperchromatic, noncohesive cells, arranged in compact sheets, throughout the dermis, the fat, and the underlying muscle. Neuron specific enolase stain for neuroendocrine granules was positive. Electron microscopy showed cytoplasmic perinuclear microfilaments and round membrane-bound dense core granules. The histologic, enzymatic, and ultrastructural findings confirmed the diagnosis of Merkel cell carcinoma. Early diagnosis and treatment are imperative for a favorable outcome.