The incidence of anal cancer has increased among both men (160%) and women (78%) from 1973 to 2000 in the U.S. The authors conducted a population-based case–control study of anal cancer to examine factors that may account for this increase.
Men (n = 119 patients) and women (n = 187 patients) who were diagnosed with anal cancer between 1986 and 1998 in the Seattle area were ascertained through the local Surveillance, Epidemiology, and End Results registry. Control participants (n = 1700) were ascertained through random-digit telephone dialing. Participants were interviewed in person and provided blood samples. Archival tumor tissue was tested for human papilloma virus (HPV) DNA, and serum samples were tested for HPV type 16 (HPV-16).
Overall, 88% of tumors (all histologies) in the study were found to be positive for HPV. HPV-16 was the most frequent HPV type detected (73% of all tumors), followed by HPV-18 (6.9%), regardless of gender. However, 97.7% of tumors from men who were not exclusively heterosexual contained HPV DNA. The risk of anal cancer increased among men (odds ratio [OR], 5.3; 95% confidence interval [95% CI], 2.4–12.0) and women (OR, 11.0; 95% CI, 5.5–22.1) who had ≥ 15 sexual partners during their lifetime. Among men who were not exclusively heterosexual and women, receptive anal intercourse was related strongly to the risk of anal cancer (OR, 6.8 [95% CI, 1.4–33.8] and OR, 2.2 [95% CI, 1.4–3.3], respectively). Current smokers among men and women were at particularly high risk for anal cancer, independent of age and other risk factors (OR, 3.9 [95% CI, 1.9–8.0] and OR, 3.8 [95% CI, 2.4–6.2], respectively).
The high proportion of tumors with detectable HPV suggests that infection with HPV is a necessary cause of anal cancer, similar to that of cervical cancer. Increases in the prevalence of exposures, such as cigarette smoking, anal intercourse, HPV infection, and the number of lifetime sexual partners, may account for the increasing incidence of anal cancer in men and women. Cancer 2004. © 2004 American Cancer Society.