The treatment of anal squamous cell cancer with definitive chemoradiation is cemented as the gold-standard therapy for localized anal cancer, mainly due to its sphincter-saving and colostomy-sparing potential. Over the course of the past 2 decades, several studies have addressed different chemoradiation regimens in hopes of improving on the standard Nigro protocol of fluorouracil, mitomycin, and radiation. While these studies failed to reveal any superiority of alternative regimens to the Nigro protocol, important conclusions were derived regarding the continuity of radiation as well as the role of induction (pre-chemoradiation) and maintenance chemotherapy (post-chemoradiation) in patients with anal cancer.
Before we continue with a consideration of treatment, some background on anal cancer is in order. Part 1 of this review will provide an overview of anal cancer epidemiology, risk factors, screening, prevention, and diagnosis. Part 2, which will appear in the next issue of ONCOLOGY, will focus on treatment, highlighting the current status of chemoradiation for anal cancer and potential areas for future improvement.
Anal cancer affected an estimated 5,290 patients (2,100 men and 3,190 women), and led to approximately 710 deaths in 2009. Although it is a rare disease, the incidence of anal cancer is increasing in men and women worldwide.[2-6] The incidence rates according to the Surveillance Epidemiology and End Results (SEER) database are 1.4 per 100,000 in men and 1.7 per 100,000 in women. Women have a higher incidence rate than men for age groups over 50 years, whereas men dominate for the ages of 20 to 49 years. Over the period 1973 to 2000, black men had the sharpest increase in anal cancer incidence rates, followed by white men, white women, and black women in decreasing order.
The median age at diagnosis of anal cancer ranges from 60 to 65 years. The overall 5-year survival rates for anal cancer are 60% in men and 78% in women, based on SEER data analysis for the modern era of therapy, 1994 to 2000. During the same period, the 5-year overall survival of black men decreased to 28%, presumably due to complicating human immunodeficiency virus (HIV) infection.
Risk Factors for Anal Cancer
The risk factors associated with anal cancer are summarized in Table 1.[1-30] The most important risks are behavioral factors that predispose individuals to human papillomavirus (HPV) infection or immunosuppression.
Behavioral Risk Factors
Specific sexual practices have been associated with an increased risk of anal cancer. The risk of anal cancer appears to be the highest among men having sex with men[7-10] (odds ratio [OR] = 17.3; 95% confidence interval [CI] = 8.2–36.1). These risks are increased in the setting of men having sex with men who are HIV-positive (relative risk [RR] = 59.5). It is now recognized that the increased risk of anal cancer in this population is due to the increased HPV infection rate rather than isolated HIV infection.
Risk factors common to both men and women are receptive anal intercourse, lifetime number of sexual partners, cigarette smoking, and a history of genital warts. For women, additional risk factors include history of high-grade vulvar intraepithelial neoplasia, and vulvar cancer or cervical cancer.[7,9,11-14]
Squamous cell cancer of the anal region is similar to that of the uterine cervix, vagina, and vulva, and shares the common association with high-risk HPV infection.[10,12,16-21] While the prevalence of cervical HPV infection in women declines after age 30 years, anal HPV in HIV-negative men who have sex with men (MSM) remains high and constant throughout life.[22,23]
The prevalence of anal HPV infection is greater than cervical HPV infection in women who are HIV-positive or have a high risk of HIV infection.[24-26] In a population of healthy Hawaiian women, anal HPV infection was as common as cervical HPV. In immunocompetent heterosexual men, anogenital and anal HPV infections were documented in 65.4% and 24.8% of patients, respectively.[28,29] History of multiple sexual partners in homosexual or heterosexual individuals or of unprotected anal intercourse were predictive of greater risk of developing anal intraepithelial neoplasia (AIN) and invasive anal cancer.
Several subtypes of HPV are linked to anal cancer and its precursor lesions. HPV 16 has the highest degree of association and, to a lesser extent, types 18, 31, 33, 35 and others have been connected. The prevalence of high-risk HPV is about 85% in patients with squamous cell cancer of the anal canal, depending on the sensitivity of the assay[15,31,32] and the geographic variations. In one series, high-risk HPV was identified in 90% of anal squamous cell carcinomas in women and 63% of such cancers in men. HPV-negative anal cancers were similar to HPV-positive anal cancers in terms of patient age, adjacent dysplasia, ductal differentiation, and prognosis.
Immunosuppression and HIV
Immunosuppression probably impairs the body’s ability to clear HPV after sexual exposure.[35-38] Patients undergoing organ transplantation have a 10- to 100-fold risk of anal cancer compared to the general population.[39-44] The rate of high-grade squamous intraepithelial neoplasia is higher in HIV-positive patients than HIV-negative patients and is inversely related to the CD4 lymphocyte count.[45,46] However, HIV-related immunosuppression has not been clearly established as an independent risk factor for anal cancer.
Since the introduction of highly active antiretroviral therapy (HAART), HIV patients are living longer and consequently, have an increased lifetime risk of developing anal cancer. Although there have been reductions in the incidence of Kaposi’s sarcoma and lymphoma, no significant change has been seen in the incidence of anal carcinoma. In a study matching a cancer database to AIDS databases, the relative risk of developing anal cancer among HIV-positive men with a history of homosexual contact was 59.5, while the relative risk for anal cancer was 6.8 in HIV-positive women in comparison to the general population.
Benign Lesions and Anal Cancer
Benign lesions in the anal canal such as fistulas or fissures do not appear to predispose to cancer. Similarly, inflammatory bowel disease does not appear to predispose to anal squamous cell cancer.[50,51] A Danish population-based cohort study of 6,334 patients with ulcerative colitis and 2,723 patients with Crohn’s disease who were followed for 10 years showed no increased risk of anal cancer.
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a Tx = primary tumor cannot be assessed;T0 = no evidence of primary tumor; T1 = tumor size 2 cm or less in greatest dimension; T2 = tumor > 2 cm but < 5 cm; T3 = tumor > 5 cm; T4 = tumor of any size invading adjacent organs.
b Nx = regional lymph nodes cannot be assessed; N0 = no regional lymph node metastastsis; N1 = metastasis in perirectal lymph node; N2 = metastasis in unilateral internal iliac or unilateral inguinal lymph node; N3 = metastasis in perirectal and inguinal lymph node and/or bilateral inguinal lymph node and/or bilateral internal iliac lymph nodes.
c Mx = distant metastasis cannot be assessed; M0 = no distant metastasis; M1 = distant metastasis.