p16 Expression in Keratoacanthomas and Squamous Cell Carcinomas of the Skin: An Immunohistochemical Study

Abstract Context.—Distinguishing between keratoacanthoma (KA) and squamous cell carcinoma (SCC) is not an uncommon histologic diagnostic dilemma. Objective.—To determine if p16 expression is useful in the differential diagnosis of SCC and KA. Design.—We studied the expression of p16 by immunohistochemistry in 24 KAs, 24 infiltrating SCCs of the skin, 4 histologically indeterminate lesions, and 8 nonmalignant keratoses. Results.—A range of immunohistochemical p16 expression was seen in KAs and SCCs in terms of the thickness of lesional staining and the percentage of cells staining. No significant difference in measures of p16 expression was identified among the KAs, the SCCs, the indeterminate lesions, or the benign keratoses. Conclusions.—These findings suggest that p16 is not a useful marker to distinguish between KA and SCC, supporting the similarity between the 2 lesions; p16 alterations appear to play a role in the pathogenesis of both KA and SCC.