Primary neuroendocrine (Merkel cell?) carcinoma of the skin. I. A clinicopathologic and ultrastructural study of 43 cases.

The clinical and pathologic features of 43 cases of primary neuroendocrine carcinoma of the skin are reported. These tumors arise in the dermis and subcutaneous tissues of elderly individuals. The head and neck are the most common primary sites followed by the lower and upper extremities and trunk. Characteristic histologic features include round cells with scanty amphophilic cytoplasm and vesicular nuclei with multiple small nucleoli. The cells are arranged in sheets, solid nests, or anastomosing trabeculae. Collections of perinuclear intermediate filaments, cytoplasmic dense-core membrane-bound secretory granules, complex intercellular junctions, and cytoplasmic spinous processes are the principal fine structural features. These ultrastructural findings are similar to those of the normal cutaneous Merkel cell. The natural history of this neoplasm is characterized by local recurrence in 30% of cases, regional lymph node metastases in 65% of cases, and distant metastases in 40%. One-third of the patients were dead because of their tumors. Treatment of extensive local or distant metastatic disease with chemotherapy or radiotherapy resulted in only short-term palliative response.