Merkel cell carcinoma in a cat.

A 17-year-old spayed female Japanese domestic cat presented with a solitary cutaneous mass in the right thoracic area. Histopathologically, the mass consisted mainly of round tumor cells that had infiltrated throughout the dermis and deep subcutaneous tissue. The proliferating pattern of tumor cells was solid but also trabecular or cord-like in some areas, and lined with small cells resembling mature lymphocytes or basal cells. The tumor cells were positive for cytokeratin 20 and synaptophysin. The positive reaction for cytokeratin 20 showed localized to diffuse pattern. This is the first report describing now cytokeratin 20 was clearly beneficial for the differential diagnosis of feline Merkel cell carcinoma.