Human papillomavirus, or HPV, is a leading cause of throat cancer, and it is sexually transmitted. But how the timing, number and types of sexual behaviors affect the risk, and why some people develop cancer and others don’t, are still open questions. Researchers are beginning to suggest some possible answers.
HPV causes about 70 percent of oropharyngeal cancers — tumors of the back of the throat, the base of the tongue and the tonsils. According to the Centers for Disease Control and Prevention, there are about 3,500 new cases of HPV-associated oropharyngeal cancers diagnosed in women and 16,200 in men every year in the United States. These cancers are more common among white people than among African-Americans, Asian-Americans, Hispanics, or American Indians and Native Alaskans.
There are many types of HPV, only some of which cause cancer. In addition to throat cancer, HPV is a cause of cervical, vaginal, vulvar, penile and anal cancers. It can take years, even decades, after infection for cancer to develop.
To try to sort out the risk factors for HPV-associated throat cancer, researchers compared 163 patients with cancer with 345 cancer-free controls. Patients and controls ranged in age from 18 to 89, but more than 95 percent of them were over 40. At the start of the study, all of the participants provided a blood sample, and the scientists obtained tumor samples from the patients with cancer. None of the participants had had the HPV vaccine, which was introduced in the United States in 2006 and recommended primarily for preadolescents, teenagers and young adults.
Using a self-interview administered on a computer, the participants also answered detailed questions on lifetime and recent sexual behavior, including number of partners, age of sexual initiation, types of sexual acts, extramarital sex and the use of alcohol and recreational drugs during sex. The scientists also had data on income, education, sexual orientation, and any history of sexually transmitted and other diseases. The study is in the journal Cancer.
Taken together, the various cancers caused by HPV are slightly more common in women, but HPV-associated oropharyngeal cancers are almost five times as common in men. Exactly why is unclear.
“There is some evidence that cunnilingus is more infective than fellatio,” said the senior author, Gypsyamber D’Souza, a professor of epidemiology at Johns Hopkins. “But that’s a surrogate for more nuanced behavior. Many patients have not engaged in high-risk sex behavior and are unlucky enough to still get this. It’s not just sexual partners, but the timing, the kind of practice, the nature of the partners and other factors, plus aspects of our own immunological response that are all involved in this.”
The cancer patients in the study were about 80 percent more likely than those without cancer to have ever performed oral sex on a partner. They were also younger when they first did so — 37 percent of patients were younger than 18 the first time they performed oral sex, compared with 23 percent of controls — and they were more likely to have performed oral sex at their sexual debut.
Why having oral sex at a first sexual encounter would raise the risk is unknown. Is there a different immune response if a person has had other forms of sex before having oral sex? Does the initial site of exposure affect your risk? “We don’t have good answers yet,” Dr. D’Souza said.
Almost 45 percent of patients had had more than 10 sexual partners during their lifetimes, compared with 19 percent of the cancer-free controls. People under 23 who had a sexual partner at least 10 years older were more likely to be infected, possibly because older people have had longer exposure to the virus.
Deep kissing was also associated with increased risk. Those who had 10 or more deep-kissing partners were more than twice as likely to have an HPV-related cancer as those who had none or one.
People who reported that their partners had extramarital affairs, and those who even suspected that their partners had had affairs, also had an increased risk of HPV-associated throat cancer. There was no association of HPV-related throat cancer with smoking, alcohol consumption or substance use.
The study had limitations. It depended on self-reports, which are not always reliable, and because more than 95 percent of the participants described themselves as heterosexual, there was not enough data to draw conclusions about the effects of sexual orientation on HPV and cancer risk. But the analysis had carefully matched controls, HPV tumor data, and a confidential questionnaire, all of which contribute to its strengths.
Dr. Jason D. Wright, an associate professor of gynecologic oncology at Columbia who was not involved in the research, believes the work could be useful in clinical practice. “This is one of the first studies to provide in-depth details for patients about how specific practices influence your long-term risk,” he said. “A higher exposure, more partners, oral sex early on — these are all risk factors. These are important things to think about in talking to patients.”
The lead author, Dr. Virginia E. Drake, a resident physician at Johns Hopkins, said that explaining the infection to patients can be difficult. “If people get this infection, they’re going to ask, ‘Why me?’” she said. “How this information will change things clinically, we don’t know. But we can give patients a better understanding of the disease process and how someone gets it.”
Still, she said, “It’s complex, more complex that just the number of sexual partners. We don’t have the exact answers on this, and we’re still figuring out the complete picture.”