Immunohistochemistry and in situ hybridization for the diagnosis and classification of squamous lesions of the anogenital region

Distinguishing anogenital squamous intraepithelial lesions from benign conditions and mimics may be problematic. Immunohistochemistry for surrogate markers of HPV infection, such as Ki-67, p16, and ProEx™ C, may aid the diagnosis in equivocal cases. The main diagnostic pitfall in the diagnosis of LSIL is the occurrence of “pseudokoilocytes” in benign squamous mucosa, which may lead to overdiagnosis. When interpreted correctly, Ki-67 is a sensitive and specific marker for dysplasia in mature squamous epithelium and is therefore useful for confirmation of LSIL and condyloma. A Ki-67 positive result is defined as the presence of a cluster of at least two strongly stained epithelial nuclei in the upper two-thirds of the epithelial thickness. With such a definition, there is almost complete concordance between consensus diagnosis of LSIL/condyloma confirmed by detection of HPV DNA and positive Ki-67. A related proliferation marker, ProEx™ C, has similar staining patterns and utility for the diagnosis of low grade dysplasia. The differential diagnosis of HSIL includes atypical immature squamous metaplasia and atrophy. A marker with high sensitivity and specificity for the detection of HSIL in cervical, vulvar, and anal mucosa is p16. A 2-tier scoring system is used to evaluate p16 staining. No staining or a discontinuous, patchy nuclear and cytoplasmic staining pattern is considered as a negative result. A positive result is defined as diffuse and strong staining of cells of the basal and parabasal layers of the squamous epithelium, with or without staining of superficial cell layers. New markers that are undergoing evaluation for their clinical utility include stathmin-1, phosphorylated S6, and SOX2. Confirmation of the diagnosis of dysplasia by HPV detection in tissue sections using HPV capsid protein immunohistochemistry, HPV DNA or HPV RNA in situ hybridization offers lower sensitivity as compared to immunohistochemistry for surrogate markers and therefore has more limited utility in this context.