Clonal highlights: Clonal seborrheic keratoses often demonstrates p16 expression

At times, distinguishing Bowen disease (BD) and benign seborrheic keratosis (SK) is histologically challenging, especially when SK shows clonal features (clonal seborrheic keratosis [CSK]). While p16 is often reported as positive in BD and negative in SK, p16 expression in CSK is rarely studied. Here we investigate p16 immunohistochemistry in CSK, SK, and BD.

p16 immunohistochemistry with pattern of expression was noted for 14 CSK, 12 SK, and 18 BD. The degree of inflammation among lesions with respect to p16 expression was also noted.

When examining p16 staining in clonal nests of CSK, 57% showed diffuse or patchy/diffuse positivity, 21% showed patchy positivity, and 21% showed clusters of single positive cells. 67% of BD showed diffuse positivity, 11% showed patchy/diffuse positivity, 17% showed patchy positivity, and 6% were negative. 25% of SK showed focal areas of patchy to full thickness positivity, 25% showed moderate number of positive single cells with or without patchy staining, and 50% showed negative/scattered single cell positivity.

Our findings support that p16 positivity limited to clonal nests in CSK is normal. p16 positivity in clonal nests of CSK in isolation without concurrent atypical histologic features should not be used to support a diagnosis of BD.