Anorectal Cancer

The most common anorectal cancer is adenocarcinoma. Squamous cell (nonkeratinizing squamous cell or basaloid) carcinoma of the anorectum accounts for 3 to 5% of distal large-bowel cancers. Basal cell carcinoma, Bowen's disease (intradermal carcinoma), extramammary Paget's disease, cloacogenic carcinoma, and malignant melanoma are less common. Other tumors include lymphoma and various sarcomas. Metastasis occurs along the lymphatics of the rectum and into the inguinal lymph nodes.

Risk factors include infection with human papillomavirus (HPV), chronic fistulas, irradiated anal skin, leukoplakia, lymphogranuloma venereum, and condyloma acuminatum. Gay men practicing receptive anal intercourse are at increased risk. Patients with HPV infection may manifest dysplasia in slightly abnormal or normal-appearing anal epithelium ("anal intraepithelial neoplasia," histologically graded I, II, or III). These changes are more common in HIV-infected patients, particularly gay men. Higher grades may progress to invasive carcinoma. It is unclear whether early recognition and eradication improve long-term outcome; hence, screening recommendations are unclear.

Wide local excision is often satisfactory treatment of perianal carcinomas. Combination chemotherapy and radiation therapy result in a high rate of cure when used for anal squamous and cloacogenic tumors. Abdominoperineal resection is indicated when radiation and chemotherapy do not result in complete regression of tumor and there are no metastases outside of the radiation field.

Frequency

United States

Colorectal cancers are the most common GI cancer and the second most common cause of cancer death in developed countries. In 2005, there were an estimated 145,290 new cases of colorectal cancer in the United States; 104,950 were in the colon and 40,340 rectal (only marginally less than lung cancer), and 56,300 related deaths were reported (47,700 colon cancer, 8,600 rectal), accounting for 11% of all cancer deaths. The highest GI cancer rates are in the Northeast and North Central states, and the lowest rates are in the southern and western states (except for the San Francisco Bay area and Hawaii, which have the highest incidences in the United States).

International

More than 940,000 new cases of colorectal cancer and nearly 500,000 related deaths are reported each year worldwide (World Health Organization, 2003). The incidence rate of rectal cancer is highest in the westernized countries of North America, northern Europe, Australia, and New Zealand. Intermediate rates are found in southern Europe, and there are low rates in Africa, Asia, and South America. Rectal cancer shows less international variation than colon cancer. Although there is a 60-fold difference in the incidence rates of colon cancer between countries with the highest incidence and those with the lowest incidence, there is only an 18-fold difference in the incidence rates for rectal cancer. High colon-to-rectal cancer ratios (3-4:1) prevail in North America, northern Europe, Australia, and New Zealand. Ratios equalling less than 1 are typical in Asia and Africa.