The utility of p16 immunostaining in cutaneous keratinocytic lesion

Several kinds of atypical keratinocytic lesions including seborrheic keratosis (SK), actinic keratosis (AK), Bowen’s disease (BD) and squamous cell carcinoma (SqCC) are common lesion in pathologic practice. While most of them show characteristic histological feature, occasionally we meet the diagnostic difficulty by overlapping or mixed histologic features. The aim of this study was to evaluate the immunostaining pattern of p16 (INK4a) in SK, AK, BD and SqCC, and identify the utility of p16 immunohistochemical staining in differential diagnosis of above cutaneous keratinocytic lesions. We studied 50 cases of keratinocytic lesions (20 examples of SK and 10 cases of each of AK, BD and SqCC). The cases were stained for p16. Ninety percent of BD cases showed strong block-positivity and typical SK not show reactivity with p16, just only one case showed patch staining pattern. In AK, only one case showed block-positivity and another cases showed patchy staining pattern. In SqCC, only 3 out of 10 cases showed strong staining pattern. This study demonstrated that the expression of p16 is strongly associated with the progression of BD (p < 0.001). This staining pattern was not seen in AK and SK. Accordingly, immunohistochemistry for p16 is a useful test in the differential diagnosis of intra-epidermal atypical keratinocytic lesion. However, despite the statistical significance (p < 0.001), our small series suggests that as not all typical cases of BD are positive with p16, negative p16 staining alone cannot be used to classify atypical keratinocytic lesions, and the finding of hematoxylin and eosin staining remains paramount.