Cytology of metastatic neuroendocrine (Merkel-cell) carcinoma in pleural fluid. A case report.

A case of Merkel-cell (neuroendocrine) carcinoma of the skin with extensive metastases, including pleural effusion, occurring over 20 years after primary resection and treatment, is reported. The histologic appearance of the primary neoplasm was identical to that seen in the biopsy specimens of the metastatic carcinoma involving the great toe and inguinal lymph nodes and to that of the residual neoplasm tissue found at necropsy. Electron microscopic examination of a lymph node metastasis demonstrated cytoplasmic microfilaments and numerous dense-core, peripheral, neurosecretory granules, as previously described in Merkel-cell carcinoma. Cytologic examination of a pleural fluid specimen demonstrated numerous small malignant cells closely resembling the cells seen in the histopathologic sections from the surgical and necropsy tissues involved by metastatic carcinoma. This is the first report of the cytologic findings in a patient with Merkel-cell carcinoma metastatic to the pleural cavity.