A cutaneous APUDoma or Merkel cell tumor? A morphologically recognizable tumor with a biological and histological malignant aspect in contrast with its clinical behavior.

Clinical, microscopic, ultrastructural, and histochemical characteristics of a primary cutaneous tumor, identified as an APUDoma possibly arising from Merkel cells, are presented. The tumor can be diagnosed by means of routine histology. In some cases the argyrophylic staining technique can be helpful. The dermatological and histological aspects of this tumor suggest a highly malignant undifferentiated process. The biological behavior of this tumor, however seems to be of a low-grade malignancy. It is therefore important to recognize it from undifferentiated, highly malignant, mostly metastatic processes in the skin. There is also a summary of the clinical history and microscopic findings of identical tumors in 4 other patients.