Anal neoplasia and its precursors: facts and controversies.

The normal anal mucosa shows a complex histology, and a large variety of tumors and lesions with more or less malignant potential may arise in this region. Several of these seem to be related to venereal diseases, homosexual behavior, and anal intercourse, and to immunosuppression. Human papilloma virus (HPV) has been identified in many cases, but the relation to the histological picture is not constant and the significance is unclear. However, these observations have helped us to identify risk groups. Histological classification is often difficult and it is likely that the reproducibility is limited. This may, in part, explain different observations with regard to frequency and biological behavior. A reliable diagnosis still largely depends on exact clinical information on location and appearance, detailed knowledge of the normal histology, and careful handling of the specimen. The revised World Health Organization (WHO) classification represents an improvement, but more variants are still identified and a classification based on conventional histology alone seems in many cases unsatisfactory. Immunohistochemistry and hybridization and other techniques hopefully will provide us with improved typing of these lesions.