Proliferating cell nuclear antigen distribution in keratoacanthoma and squamous cell carcinoma

Histologic differentiation of keratoacanthoma (KA) and squamous cell carcinoma (SCC) is often difficult despite well‐delineated histopathologic criteria. This has prompted a search for more objective methods to differentiate these two lesions. In the present study, we immunohistochemically examined the distribution of proliferating cell nuclear antigen (PCNA)‐positive cells in 11 cases of KA, 7 cutaneous SCC, and 2 atypical squamous proliferations (for which a definitive diagnosis could not be made on routine histology) using a commercially prepared anti‐PCNA monoclonal antibody. We found PCNA‐positive cells predominantly in the periphery of squamous nests in KA. In contrast, SCC showed a diffuse staining pattern with PCNA‐positive cells seen throughout squamous nests. Determining the pattern of PCNA‐positive cells is easy, does not require cell counting, and may provide additional histochemical data facilitating the distinction between KA and SCC.