Cancers of the anus are rare problems with diverse histology. While squamous cell carcinoma (SCC) of the anal canal remains by far the most common of these neoplasms and the main focus of the chapter, the anus may also harbor tumors such as adenocarcinoma, melanoma, and basal cell carcinoma. The treatment of anal cancer has undergone dramatic changes in the last 25 years. Multimodality treatment consisting of radiation and chemotherapy has replaced abdominoperineal resection or wide local excision as the mainstay of therapy. Five-year survival rates now exceed 80% and radical surgery is reserved for cancers of the anal canal that do not respond to chemoradiation or that recur locally. Our understanding of the etiology and epidemiology of anal SCC and its precursor lesions has also profoundly changed in the past few decades yielding new initiatives in both therapy and prevention that may further alter the future treatment of this disease. Anal cancer is clearly a disease that benefits from multispecialty intervention. Because of this, the treatment of anal cancer serves as a paradigm for the multimodality treatment of cancer.
ANAL CANAL ANATOMY AND HISTOLOGY
Until recently, discrepancies in anatomic definitions and tumor locations in the anorectal region have made comparisons of therapeutic outcomes difficult. In addition, the evolution of anal canal cancer treatment has resulted in management differences between anal canal and margin tumors that make precise anatomic localization important. In 2000, the World Health Organization refined their definitions of “anal canal” and “anal margin” in the context of histology, the American Joint Committee on Cancer (AJCC)/Union Internationale Contre le Cancer (UICC) staging system, and traditional anatomic landmarks. This standardized definition is currently used and endorsed by surgeons, pathologists, and radiologists.1
The anal canal extends from the top of the anorectal ring (a palpable convergence of the internal sphincter, deep external sphincter, and puborectalis muscle) to the anal verge (the junction of the anal canal and the hair-bearing keratinized skin of the perineum). The lining of the anal canal is comprised of transitional epithelium as well as non−hair-bearing squamous epithelium. Tumors distal or beyond the verge are termed anal margin or perianal tumors (Fig. 55-1).
Anatomy of the anal canal and margin.
The anal canal is divided by the anal transition zone (ATZ) into three histologically distinct areas. The ATZ is a circumferential band that extends above and below the dentate line in fingerlike projections that vary in length. Fenger defined the relationship of the ATZ to the dentate line by staining surgically excised specimens with alcian blue—a dye that renders mucin-rich columnar epithelium dark blue, mucin-poor transitional epithelium light blue, and squamous mucosa colorless.2 He found that the dentate line ranges from 5 to 19 mm above the distal end of the anal canal. ...