Anal intraepithelial neoplasia

How is anal intraepithelial neoplasia (AIN) graded and what is the risk of progression to squamous cell carcinoma (SCC)? 

AIN is graded as low grade (AIN I or mild dysplasia) and high grade (encompasses AIN II and AIN III or moderate and severe dysplasia or carcinoma in situ, respectively).

The term Bowen disease is used for lesions with severe dysplasia (carcinoma in situ) seen at the anal verge or perianal skin.

The high-grade lesions are associated with high-risk human papillomavirus 16 and 18, among others. These lesions are known to recur after local treatment. The risk of progression to SCC is low (approximately 5%).


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