Adenosquamous carcinoma in situ of the anus: a case series.

While human papillomavirus (HPV)-associated adenocarcinoma of the anus resembling endocervical adenocarcinoma has recently been described, anal adenocarcinoma in situ/AIS has not. Moreover, to the best of our knowledge, truly biphasic anal adenosquamous carcinoma in situ (ADSQ-IS) is essentially non-existent in the literature. Here, we report four cases of anal ADSQ-IS including one with associated invasive adenosquamous carcinoma. Histologically, all cases of ADSQ-IS showed a basal/peripheral population of stratified, p40-positive dysplastic squamous cells adjacent to luminal, columnar, p40-negative dysplastic glandular cells, bearing a striking resemblance to the normal anal transitional epithelium. Both lesional components were diffusely and strongly positive for p16 and positive for high-risk HPV by RNA in situ hybridization. These cases expand the morphological spectrum of high-risk HPV-associated pre-invasive lesions and underscore the plasticity of HPV-associated neoplasia.