Anorectal Cancer: Critical Anatomic and Staging Distinctions That Affect Use of Radiation Therapy

MRI anatomy and pathology of the anal canal

“…The dentate line demarcates a transition from a glandular mucosa to squamous epithelium, is located 2–3 cm proximal to the anal verge, and is not visible on magnetic resonance imaging (MRI) . 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 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 …”

“…Chronic infection with human papilloma virus, especially types 16 and 18, is one of the major predisposing factors for anal SCC . Additional risk factors include immunosuppression, history of cervical or vulvar cancer, certain sexual practices (including receptive anal intercourse), history of sexually transmitted disease, smoking, and multiparity . Since surgical resection is not performed in the majority of patients with anal SCC, staging is based on a combination of clinical and imaging findings .…”

“…Additional risk factors include immunosuppression, history of cervical or vulvar cancer, certain sexual practices (including receptive anal intercourse), history of sexually transmitted disease, smoking, and multiparity . Since surgical resection is not performed in the majority of patients with anal SCC, staging is based on a combination of clinical and imaging findings . Unlike tumors in the GI tract elsewhere, staging of anal SCC is based on the size of the tumor rather than depth of invasion .…”

“…Tumors invading adjacent organs, such as the prostate, urethra, bladder, or vagina, and excluding rectum, are considered T4 . N stage for anal cancer is determined by location of the involved lymph nodes rather than the number of positive nodes . Regional node involvement depends on the location of the primary tumor: tumors arising below the dentate line drain into inguinal lymph nodes, while those located above the dentate line drain into internal iliac and mesorectal chains .…”