Novel anal lesions not to overlook

Although the anal canal comprises only a small portion of the gastrointestinal tract, it can be affected by a variety of inflammatory and neoplastic processes. Inflammation of the anal mucosa can result from trauma, infections, and inflammatory bowel disease. Non-neoplastic lesions such as inflammatory cloacogenic polyps, hemorrhoids, and fibroepithelial polyps are relatively common, but may harbor other lesions, necessitating careful histologic assessment. The anogenital mammary-like glands and perianal skin can give rise to neoplasms including hidradenoma papilliferum and basal cell carcinoma. Human papillomavirus (HPV) is an important cause of squamous dysplasia and squamous cell carcinoma, as well as several recently described non-squamous neoplasms. Adenocarcinoma of the anus is divided into intestinal, anal gland, and fistula-associated types, which have unique morphologic features. Anal Paget's disease can occur as a primary neoplasm, or may be secondary to underlying adenocarcinoma. Less common neoplasms of the anus include neuroendocrine neoplasms, mucosal melanoma, and mesenchymal neoplasms such as granular cell tumor and gastrointestinal stromal tumor (GIST). This review will cover the clinical presentation, key histologic features, differential diagnosis, and relevant ancillary studies to consider for each condition. Recognition of inflammatory and neoplastic diseases of the anus is essential to guide appropriate treatment of patients.