Fine-needle aspiration cytology of a metastatic Merkel-cell carcinoma.

The cytologic features of Merkel-cell carcinoma metastatic to inguinal lymph nodes are described. The primary tumor was in the buttocks. The cytologic smears from the inguinal lymph node were highly cellular, with small to medium-sized cells in a predominantly discohesive or single-cell pattern, as well as large clusters surrounding segments of capillary-sized blood vessels. The nuclei were uniform, round to oval, with delicate nuclear membranes. The chromatin pattern was fine and "powdery" with multiple small micronucleoli. The cytoplasm was very scanty. Intermediate filament "buttons" were abundant. Rosette formation was very rare. Frequent mitotic figures and individual cell necrosis were observed. These features are helpful in the differential diagnosis from other small-cell tumors that may involve the skin.