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Men who have had a vasectomy, the simple operation for sterilization, face greater risk of prostate cancer, according to a Roswell Park Cancer Institute study scheduled for release today.

And the earlier the operation was performed, the greater the risk, the study shows.

But it's difficult to draw conclusions on the study's implications for public health because it's unclear if vasectomies cause increased cancer risk or if vasectomies are associated with some other factor that is the true source of the risk.

"We've established an association, but it's uncertain if vasectomy is a risk factor," said Dr. Curtis J. Mettlin, co-author of the study and director of Roswell Park's Department of Cancer Control and Epidemiology. "There's no strong biological rationale for increased prostate cancer risk after a vasectomy. We've got to find out if something else -- a specific pattern of sexual activity, for instance -- is a factor for increased risk of cancer," he said.

Nonetheless, the study in today's issue of the American Journal of Epidemiology is the first and largest to date to find a pattern of significant increased cancer risk for men who have had a vasectomy. The same journal also includes another study by Boston researchers that shows the identical pattern.

The Roswell Park study included 610 prostate cancer patients, 27 of whom received vasectomies, and 2,580 patients with other cancers, 115 of whom had vasectomies.

"We're opening a door to a whole series of questions that will have to be answered," Mettlin said.

The key question is that the true risk factor for prostate cancer is still unknown. Mettlin said further research will have to study whether frequency of intercourse and numbers of partners play any part in increased cancer risk.

Some experts said the recent studies
are too small and limited and only raise questions about a possible link between vasectomies and prostate cancer.

"By contrast, other studies of vasectomy have examined large numbers of men and have been consistently reassuring about the safety of vasectomy. There are no known biological or medical reasons for a relationship between vasectomy and prostate cancer," said Janel Halpern, spokeswoman for the Association for Voluntary Surgical Contraception.

Ms. Halpern added that experts consider vasectomies safe and that no change in the practice is warranted on the basis of the recent studies.

A vasectomy is a minor surgical procedure that consists of cutting the two ducts that carry sperm from the testes to the seminal vesicles. The operation -- an estimated 455,000 are performed annually in the United States -- blocks the passage of sperm from the testes but does not prevent the prostate and other glands from secreting the fluids that form the milky fluid of semen.

During the past decade, as the number of men who had received a vasectomy increased since the 1960s when the procedure began to become popular, experts have looked into whether the operation increased risk of prostate cancer. But past studies found no clear pattern of risk.

Prostate cancer is the second most common cancer in men, after skin cancer, with 106,000 cases expected this year, American Cancer Society statistics show. An estimated 30,000 men will die from prostate cancer in 1990, making it the second leading cause of cancer deaths in men after lung cancer.

More than 80 percent of prostate cancer cases occur in men age 65 and older. For reasons not currently known, black Americans have the highest incidence rate in the world.

Until these studies, few risk factors, except age, race and testicular function, have been associated with prostate cancer.

Mettlin and his colleagues found that risk for prostate cancer is twice as high for men who had a vasectomy performed more than 12 years before their cancer was diagnosed. They found no significant risk for men with vasectomies performed less than 13 years earlier.

Overall risk was increased 70 percent for men with a history of vasectomy and 120 percent for men who had a vasectomy 13 to 18 years earlier.

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