The pathology of anal dysplasia

There has been a recent increase in the incidence of anal intraepithelial neoplasia (AIN) among high-risk patient groups. Histology remains the gold standard in the diagnosis and grading of this clinical entity, although interobserver and intraobserver variation exists among histopathologists. In this review article, the histological features of AIN I, AIN II and AIN III are defined together with the histopathological and molecular features of viral infection. Classification systems are discussed and immunohistochemical stains (p16 and Ki67) which may assist in the classification of intermediate grade AIN are considered. As high-risk human papillomavirus (HPV) infection is a well established risk factor for the development of AIN and anal squamous cell carcinoma, high and low risk subtypes are referred to and methods of detection discussed. Finally, guidelines for anal biopsy specimen processing and reporting are presented.