Figure 51.

Diagnostic utility and pitfalls of cytokeratin immunostaining. Top panels (a,b): Spindle cell carcinoma of the skin and Bottom panels (c,d): anaplastic large cell (Ki-1) lymphoma. Left panels (a,c): H&E and Right panels (b,d): immunostaining with wide-spectrum anti-cytokeratin antiserum. Cytokeratin expression confirms the epithelial nature of the spindle cells in the skin. Because of the epithelioid morphology and cytokeratin expression in Ki-1 lymphoma, diagnostic confusion with metastatic undifferentiated carcinoma may occur. The nuclei in panel (b) are counterstained with methyl green.