Tumors and Tumorlike Conditions of the Anal Canal and Perianal Region: MR Imaging Findings

Abstract

Tumors and tumorlike conditions of the anus and perianal region originate from the anal canal and anal margin or result from direct extension of tumors from adjacent organs. The anatomy of the anal canal is complex, and its different histologic characteristics can lead to diverse pathologic conditions. The anal canal extends from the anorectal junction to the anal verge. The World Health Organization classification of anal canal tumors includes (a) anal intraepithelial neoplasia, the precursor of squamous cell carcinoma (SCC), and (b) invasive tumors. Invasive tumors are further classified on the basis of cell type as epithelial tumors (SCC, adenocarcinoma, mucinous adenocarcinoma, small cell carcinoma, and undifferentiated carcinoma), nonepithelial tumors, carcinoid tumors, melanoma, and secondary tumors (direct spread from rectal, cervical, or prostate carcinoma). The anal margin, or perianal skin, lies outside the anal verge and encompasses a radius of 5 cm from the anal verge. Tumors in the anal margin are classified according to the World Health Organization classification of skin tumors. Anal margin tumors include SCC, anal intraepithelial neoplasia, also known as Bowen disease, adenocarcinoma and its precursor Paget disease, basal cell carcinoma, and verrucous carcinoma (Buschke-Löwenstein tumor), which is a rare variant of SCC. Imaging plays an important role in the evaluation, staging, and follow-up of patients with anal and perianal tumors. However, because of the overlap in imaging features among these diverse entities, a definitive diagnosis is best established at histopathologic examination. Nevertheless, familiarity with the pathogenesis, imaging features, and treatment of these tumors can aid radiologic diagnosis and guide appropriate patient treatment. (©)RSNA, 2016.

MRI anatomy and pathology of the anal canal

“…The IAS typically has intermediate to minimally high SI, while the EAS has slightly lower SI, similar to the skeletal muscles elsewhere (Fig. ) . The intersphincteric space and the ischioanal and ischiorectal fossae follow the SI of the gross fat on all sequences.…”

“…THE ANAL REGION, the most distal part of the gastrointestinal (GI) tract, includes the anal canal, the anal verge, and the anal margin . Anatomically, the anal canal extends from the dentate line to the anal verge .…”

“…Functionally, the accepted boundaries of the anal canal span from the proximal aspect of the internal anal sphincter (IAS) to the anal verge, with a length of ~3–6 cm . The anal verge is a band of squamous epithelial tissue that extends from the intersphincteric groove onto the perianal skin . The anal margin, or perianal skin, extends from the anal verge and encompasses a radius of ~5–6 cm …”

“…The anal verge is a band of squamous epithelial tissue that extends from the intersphincteric groove onto the perianal skin . The anal margin, or perianal skin, extends from the anal verge and encompasses a radius of ~5–6 cm …”

“…Squamous cell carcinoma (SCC) is the most common type of neoplasm of the anal canal, constituting 80–85% of all anal canal neoplasms . Chronic infection with human papilloma virus, especially types 16 and 18, is one of the major predisposing factors for anal SCC .…”