Pathology of Anal Neoplasia

Dr Bryan Warren, Constultant Histopathologist, Oxford

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Epithelial tumours of the anus are rare and diverse in histological type. This is simply due to the many different types of epithelium present in this anatomical region. Distinction should be made between malignant tumours of the anal canal and anal margin. Anal margin tumours are usually keratinised in squamous cell carcinomas with better prognosis and need less aggressive treatment than their non-keratinising variants in the anal canal.

Benign tumours include fibro epithelial polyps, inflammatory keratigenic polyps and squamous hyperplasia (mucoplatia). Anal intraepithelial neoplasia (AIN). Dysplasia of the squamous epithelium of the anal canal is now classified as anal intraepithelial neoplasia Grades 1, 2 or 3 depending on its severity and is a precursor of squamous cell carcinoma and related to human papilloma virus. There is considerable variation in opinion in the diagnosis of AIN 1, 2 and 3. Viral warts may also be seen at the anal verge condylomata acuminata as may be the giant condyloma of Buschke and Loewenstein which may be enormous before it develops invasion. The invasion is by varicous carcinoma which may present considerable difficulties in diagnosis on biopsy. Since the cytology is so bland. A keratin acanthoma is a benign lesion which can be mistaken as squamous cell carcinoma. Bowen's Disease is carcinoma in situ. Bowenoid papulosis is a papulo eruption in the anogenital region in young to middle aged adults and the histology sometimes resembles Bowen's Disease. This can cause considerable confusion if viewed histologically in isolation without the history and knowledge of the macroscopic appearances of this eruption around the anus and this may lead to misdiagnosis with serious consequences.

Sweat gland tumours may also occur, as may extra mammary Paget's disease and metastatic tumours occasionally. The malignant epithelial tumours are squamous cell carcinoma, which in the anal canal is often basaloid or non-keratinising, and malignant melanoma, which unfortunately usually presents quite late due to the site and adenocarcinoma of anal ducts and anal glands. These may sometimes present as a colloid carcinoma causing considerable difficulties and some biopsies will consist of mucous on many occasion s before the epithelium is recognised but the endoscopic appearances and macroscopic appearances are quite characteristic. Mucinous adenocarcinomas may occur within fistulae. In he anal margin squamous cell carcinomas and basal cell carcinomas may be seen other very rare tumours may occur occasionally. Correct pathological diagnosis of anal neoplasia is crucial to the correct management.