P16 Expression in Primary Vulvar Extramammary Paget Disease.

P16 immunohistochemistry has been widely used in facilitating the diagnosis of human papillomavirus (HPV)-related usual type vulvar intraepithelial neoplasm. However, studies of p16 expression in primary vulvar extramammary Paget disease (EMPD) are limited. We assessed the p16 expression by immunohistochemistry in 40 cases of primary vulvar EMPD, including 34 cases of intraepithelial vulvar EMPD and 6 cases of invasive vulvar EMPD and correlated p16 expression patterns with disease progression. Overall, p16 expression was present in 36 cases (90%), including 20 cases (50%) with focal staining pattern and 16 cases (40%) with diffuse staining pattern. All 20 cases with focal p16 staining pattern were intraepithelial vulvar EMPD. Diffuse p16 staining pattern was present in 10/30 cases (33.3%) of intraepithelial EMPD and in 6/6 cases (100%) with invasive vulvar EMPD. Negative p16 staining was present in four intraepithelial EMPD cases. Using a highly sensitive RNA in situ hybridization method, we did not detect high-risk HPV in the selected 10 cases with diffuse p16 staining pattern, including 6 cases of intraepithelial EMPD and 4 cases of invasive EMPD. We also observed that intraepithelial EMPD had predominantly cytoplasmic p16 immunoreactivity, whereas nuclear p16 immunoreactivity was mainly seen in invasive EMPD components. Our study demonstrated that the p16 positive immunostaining was seen in the majority of primary vulvar EMPD which is not related to HPV infection. Therefore, knowing the overlapping p16 immunostaining patterns in vulvar EMPD and usual type vulvar intraepithelial neoplasm is important to render the correct diagnosis.