Pagetoid squamous cell carcinoma in situ of the vulva: comparison with extramammary paget disease and nonpagetoid squamous cell neoplasia.

Pagetoid squamous cell carcinoma in situ (PSSCIS) is a variant of squamous cell intraepithelial neoplasia. Although PSSCIS is well-documented in the cutaneous skin and esophagus, cases of vulvar PSSCIS are rare. In the vulva, the main differential diagnosis is extramammary Paget disease (EMPD). We report 2 cases of vulvar PSSCIS along with the immunohistochemical and human papillomavirus (HPV) status of this disease compared with primary cutaneous EMPD of the vulva. Although PSSCIS and EMPD share CK7 and CK19 expression, PSSCIS is consistently mucin and carcinoembryonic antigen negative. In contrast to EMPD, both cases of PSSCIS strongly expressed p16 (INK4A) protein, consistent with RB1 protein dysregulation. However, integration of high-risk HPV was found in only 1 of the 2 PSSCIS cases. Given the morphological and immunohistochemical findings, we suggest that PSSCIS arises from a bidirectional stem cell capable of both squamous and glandular differentiation. Additionally, as with nonpagetoid squamous cell neoplasias of the vulvar, integration of high-risk HPV may occur in some, but not all, cases of PSSCIS.