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 this 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 past 30 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 subsequently 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. The importance of the surgeon's role in the detection and diagnosis of anal cancer remains undiminished. The surgeon is the clinician most likely to diagnose the disease, delegate treatment, and provide follow-up care. Anal cancer is clearly a disease that benefits from multidisciplinary intervention. Because of this, the treatment of anal cancer serves as a paradigm for the multimodality treatment of cancer.
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 columnar cells, transitional epithelium, and non–hair bearing squamous epithelium. Tumors distal or beyond the verge have been historically been termed anal margin tumors (Fig. 42-1A).
A. Anatomy of the anal canal and margin, classic description. Modern Classification system of anal cancers. B. Coronal section. a-c: intra-anal (anal canal) lesions. C. Perianal view. d: perianal (anal margin) lesion; e: skin lesions.
The anal transition zone, or transformation zone (ATZ) is a unique anatomic region, which has a variable histologic makeup. It is a 1- to 2-cm region which begins at the dentate line and extends proximally. This zone, similar to the transformation zone of the cervix, contains a transitional epithelium containing columnar cells with variable amounts of squamous metaplasia. These metaplastic cells may be found as high as 6–10 cm proximal to the dentate line. This may, in part, explain the existence of rare “intra-anal” SCCs that have been found in the mid-low rectum. Tumors arising in the anal canal above and within the ATZ are typically nonkeratinizing SCCs. Those originating below this level are generally keratinizing.1