Changing trends in sexual behavior, along with tobacco use and infection with the human papillomavirus, could help explain why anal cancer rates are climbing among Americans, particularly gay men, according to two studies that appear in the July 15 issue of the journal Cancer. The first study, by Lisa Johnson of the Public Health Sciences Division at the Fred Hutchinson Cancer Research Center, a nonprofit research institution in Seattle, finds that rates of anal cancer have jumped 160% among men and 78% among women during the past 30 years. The sharpest increase was among African-American men, whose incidence of the disease more than doubled. The five-year survival rate also was lowest for black men.
The second study, by Fred Hutchinson epidemiologist Janet Daling, assessed the impact of various lifestyle factors--such as smoking, sexual orientation, number of sexual partners, and history of anal intercourse--along with HPV infection on anal cancer rates. "We found that infection with HPV is necessary in most if not all cases of anal cancer, as close to 90% of the tumors studied were positive for the virus," said Daling. Two HPV strains linked with cervical cancer--and believed linked to anal cancer as well--were found in most of the anal cancer tumors: HPV-16 was found in 73% of the tumors, and HPV-18 was found in 7% of the tumors. Significantly higher levels of HPV antibodies also were detected in the blood of cancer patients as opposed to cancer-free study subjects.
Other anal cancer risk factors included being a gay or bisexual male, a high number of sexual partners, and a history of receptive anal sex. Having had 15 or more sexual partners or having a history of receptive anal intercourse boosted anal cancer risks sevenfold, according to the study. About half of the men diagnosed with anal cancer were gay or bisexual, Daling reports. "It could be that sexual practices have changed, but it also could be that people are just more likely to discuss their sexual behavior these days," she says. "However, I suspect that increased incidence of anal intercourse among both men and women is most likely to be the primary cause behind the rise in anal cancer."
One of the most surprising findings of the study is that smoking appears to play a significant role in anal cancer development, regardless of sexual orientation or sexual practices. More than half of the anal cancer patients were current smokers at the time of their cancer diagnosis. The study shows smoking boosts anal cancer risks fourfold for both men and women. "Even in controlling for other risk factors, like the number of sexual partners, anal intercourse, and HPV status, smoking was a strong risk factor for squamous-cell anal cancer," Daling said, referring to the most common type of anal cancer, which accounts for about half of all cases. "Therefore, quitting smoking is the best thing a person can do to prevent anal cancer, particularly if they've been exposed to HPV or other risk factors, because it certainly has a promotional effect on these tumors."
The mechanism behind smoking and anal cancer development is unknown, but researchers speculate that smoking interferes with a process called apoptosis, or programmed cell death, which helps rid the body of abnormal cells that could turn cancerous. Another possibility is that smoking suppresses the immune system, which can decrease the body's ability to clear persistent infection or abnormal cells.
Although anal cancer is relatively rare, accounting for about 1% of gastrointestinal malignancies, about 4,000 new cases will be diagnosed this year in the United States, according to the American Cancer Society.