Anal cancer is on the rise in both sexes, and changing trends in sexual behavior - combined with current tobacco use and infection by a specific strain of the human papillomavirus - may help explain the increase. These findings, from two separate studies by researchers at Fred Hutchinson Cancer Research Center, are reported in the journal Cancer. The first study, by Lisa G. Johnson of Fred Hutchinson's Public Health Sciences Division, found that incidence rates of anal cancer have increased significantly in the past 30 years, jumping 160 percent in men and 78 percent in women.
The second study, led by epidemiologist Janet Daling of Fred Hutchinson's Public Health Sciences Division, set out to better understand the underlying biological and lifestyle causes behind the rising incidence of anal cancer. The study tested for the presence of human papillomavirus, or HPV - a well-known sexually transmitted viral marker of anal cancer - in both blood and tumor tissue.
It also assessed the impact of various lifestyle factors associated with the disease, such as smoking, sexual orientation, number of sexual partners and history of anal intercourse.
"We found that infection with HPV is necessary in most if not all cases of anal cancer, as close to 90 percent of the tumors studied were positive for the virus," said Daling, a pioneer in studying the link between HPV and anogenital-cancer risk.
Overall, regardless of the patient's sexual orientation or gender, the researchers found that 88 percent of the tumors were positive for HPV DNA. Of these, 73 percent tested positive for a strain called HPV-16, and 7 percent contained a strain called HPV-18. Both strains of HPV are implicated in cervical-cancer development as well. The researchers also found significantly higher amounts of circulating HPV antibodies in the blood of anal-cancer patients. Approximately 40 percent of the women and 50 percent of the men with anal cancer tested positive for HPV antibodies as compared to about 15 percent of the men and women in the control group. Since the blood test can only detect HPV antibodies in a subset of those who are exposed to the virus, the overall HPV exposure among the control group was probably much higher, Daling cautions.
One of the most surprising findings, Daling said, was that smoking appears to play a significant role in anal-cancer development that's independent of other behavioral risk factors, such as sexual activity. More than half of the anal-cancer patients studied were current smokers at the time of diagnosis, as compared to a smoking rate of about 23 percent among the controls.
"Current smoking is a very important promoter of the disease," said Daling, also a professor of epidemiology at the University of Washington School of Public Health and Community Medicine. "There's a fourfold increase in risk if you're a current smoker, regardless of whether you're male or female."
"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."
Other risk factors associated with increased anal-cancer risk included gay or bisexual orientation among men, a high number of lifetime sexual partners and a history of receptive anal sex. The study also suggested that the overall increase in anal cancer rates might be partially attributable to an increase in the average number of lifetime sexual partners and an increase in the number of people engaging in anal sex, particularly among women.
Among the female control group studied, 21.5 percent had reported practicing anal sex, a significant increase from a previous case-control study by Daling and colleagues, published in 1987, in which 11 percent of female controls had reported ever having anal sex. Similarly, 40 percent of the women in the control group of the current study reported having five or more lifetime sexual partners as compared to 9 percent of the female control group in the 1987 study.
"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," Daling said. "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."