About two months ago, Dr. Elizabeth Stier was shocked to learn that she would lose a vital credential, board certification as a gynecologist, unless she gave up an important part of her medical practice and her research: taking care of men at high risk for anal cancer.

The disease is rare, but it can be fatal and its incidence is increasing, especially among men and women infected with HIV. Like cervical cancer, anal cancer is usually caused by the human papillomavirus, or HPV, which is sexually transmitted.

Though most of her patients are women, Stier, who works at Boston Medical Center, also treated about 110 men last year, using techniques adapted from those developed to screen women for cervical cancer.

But in September, the American Board of Obstetrics and Gynecology insisted that its members treat only women, with few exceptions, and identified the procedure in which Stier has expertise as one that gynecologists are not allowed to perform on men. Doctors cannot ignore such directives from a specialty board, because most need certification to keep their jobs.

Now Stier’s studies are in limbo, her research colleagues are irate, and her male patients are distraught. Other gynecologists who had translated their skills to help male patients are in similar straits.

And researchers about to start a major clinical trial that is aimed at preventing anal cancer, with $5.6 million from the National Cancer Institute, say the board’s decision will keep some of the best qualified, most highly skilled doctors in the United States from treating male patients in the study. The director of the planned study and Stier have asked the gynecology board to reconsider its position.

But the board, based in Dallas, has not budged.

"We haven’t heard of any compelling reason to change anything," said Dr. Kenneth L. Noller, the board’s director of evaluation. He said there were plenty of other doctors available to provide the HPV-related procedures that some gynecologists had been performing on men.

Dr. Larry C. Gilstrap, the group’s executive director, said the specialty of obstetrics and gynecology was specifically designed to treat problems of the female reproductive tract and was "restricted to taking care of women." Of the 24 medical specialties recognized in the United States, he said, it is the only one that is gender-specific, and it has been that way since 1935.

Stier said that she, like many other doctors, had not understood the definition of their field to be quite so absolute.

The board had always regarded the treatment of women as its mission, Gilstrap said, but felt a particular need to emphasize it now because the specialty’s image was being tarnished by members who had strayed into moneymaking sidelines, like testosterone therapy for men, and liposuction and other cosmetic procedures for both women and men.

Dr. Mark H. Einstein, a gynecologic oncologist at Montefiore Medical Center in the Bronx, N.Y., said, "The board’s approach is to be rather dogmatic and to draw a line in the sand."

On Sept. 12, the board posted on its website a stringent and newly explicit definition of obstetrician-gynecologists, limiting the proportion of time they could spend on nongynecologic procedures and noting that, with few exceptions, members must not treat men. The notice specifically prohibited gynecologists from performing an examination called anoscopy on men.

Anoscopy involves using a tube and a light to examine the anal canal, which is about 1.5 inches long. The procedure is the same in men and women. A "high-resolution" version adds a magnifier to look for abnormal growths that may be cancers or precancers. Cancers usually require surgery, but doctors can burn off precancers in hopes of preventing cancer.

A similar approach led to a tremendous decline in cervical cancer in the United States, and doctors hope to accomplish the same for anal cancer. About 7,000 new cases of anal cancer, and 880 deaths, are expected in 2013 in the United States; the incidence has been increasing by 2.2 percent a year for the last 10 years.

Anoscopy and other techniques aimed at preventing anal cancer have not been rigorously tested, and the purpose of the planned clinical trial is to determine whether they work. Five thousand patients, men and women, are to be studied for eight years, ultimately costing tens of millions of dollars.

If the trial shows that cancers can be prevented, it could change the standard of care, Dr. Douglas R. Lowy, a deputy director of the Center for Cancer Research at the National Cancer Institute, said in an interview.

Doctors planning to participate in the trial have had extensive training in high-resolution anoscopy. People with various types of medical training can learn the procedure, but experts say gynecologists are the quickest to master it because of their experience in screening women.

"We need as many trained people as possible," said Dr. Joel Palefsky, an infectious disease specialist at the University of California, San Francisco, who will direct the study. "The assumption all along has been that many of the gynecologists we trained would participate in the study and would see both men and women."

Stier had been treating men for more than 10 years, and expected to enroll about 100 in the study. Now, she will be able to enroll only women. She is the only person with the special training at her hospital, so now another hospital will have to sign up more men.

But what really worries her is what will become of the men she has been treating. Those who had precancers need to be examined once or twice a year, because the growths tend to recur. Stier said the procedures are embarrassing and uncomfortable for patients, and it takes time for a doctor to gain their trust. Many of her patients are poor, from minority groups and infected with HIV. Some live in shelters, some have histories of drug use. And anal disorders add more stigma. "My main issue here is that I don’t think my patients are going to get the follow-up that they need, and I think they’re going to be lost to care, and we take care of a very vulnerable patient population," Stier said.

Einstein had also been treating some male patients and had planned to enroll men in the new trial. Like Stier, he was blindsided by the gynecology board’s notice.

He said only three doctors at his hospital had special training in high-resolution anoscopy, and that was nowhere near enough. Now two of those doctors, including himself, have to stop treating men.

"I think we’ll see significant setbacks," Einstein said.