Definition

The abnormal growth of cells in the lining of the anal canal or in the skin around the outside of the anus called the perianal area. Anal dysplasia is usually caused by certain types of human papillomavirus (HPV). It can be low grade or high grade, depending on how abnormal the cells look under a microscope and how much of the anal tissue is affected. Anal dysplasia is not cancer but may become cancer and spread to nearby normal tissue. It is sometimes called anal intraepithelial neoplasia (AIN) or anal squamous intraepithelial lesion.