Two large new studies being reported on Wednesday add to earlier evidence that a vasectomy may increase a man's risk of developing prostate cancer.
The findings are not definitive, but experts not directly involved in the studies call the studies scientifically well designed, adding that they underscore the need for further research to assess the possible hazards of one of the most effective and widely used methods of contraception.
The studies found that men who had a vasectomy more than 20 years earlier faced up to an 89 percent greater risk of prostate cancer than men who had not had a vasectomy. The studies also found that the more time that had passed since a vasectomy, the greater was the risk of developing prostate cancer. 'High' Quality of Studies
The studies, reported in The Journal of the American Medical Association, were largely financed by the National Institutes of Health. They were conducted by a team headed by Dr. Edward Giovannucci from the Brigham and Women's Hospital and Harvard University in Boston.
Experts said the findings did not establish a cause and effect relationship, merely a statistical association that required further study. No plausible biological cause for the association is yet evident. The experts also noted that the studies might have been influenced by factors that could not be detected with the methods used, and that they conflicted with other studies that did not find a link between vasectomies and prostate cancer. Four previous studies found such a link, and three others found none.
"The data are far too preliminary to consider vasectomy reversal to reduce the risk of prostate cancer," an editorial in the same issue of the journal said. The editorial was written by Dr. Herbert B. Peterson of the Centers for Disease Control and Prevention in Atlanta and Dr. Stuart S. Howards, a urologist at the University of Virginia in Charlottesville who also heads a committee on vasectomy and prostate cancer for the American Urological Association. Monitoring Is Urged
The editorial urged men who have had a vasectomy to undergo annual checkups for prostate cancer, which is the recommendation made by the urological group and the American Cancer Society for men 50 to 70 years old.
Doctors should inform men seeking a vasectomy about the new findings, the editorial said.
The World Health Organization, a United Nations agency based in Geneva, said the studies were not definitive but "of high scientific quality" and that the scientific design of the studies avoided many of the methodological problems of earlier studies, some of which did not find a link between vasectomy and prostate cancer.
Vasectomy is a minor surgical sterilization procedure that can be performed quickly under local anesthesia. The World Health Organization estimates that more than 42 million couples throughout the world rely on vasectomy for family planning.
In the United States, more than 15 percent of men over the age of 40 have had a vasectomy.
Dr. Olaf Meirik of the W.H.O. said "couples seeking family planning should be informed of the benefits and risks of different contraceptive methods, and also the risks and consequences of not using any family planning method at all."
Because vasectomy is such a popular contraceptive method throughout the world, any questions about its safety are of major public health concern.
Experts in the United States and at the W.H.O. said that additional studies were urgently needed because confirmation of the latest findings would have a critical effect on family planning programs. Crucial information could come from at least six ongoing studies in the United States and Europe, the W.H.O. said. The agenc also said it was planning to begin pilot studies later this year in four developing countries where vasectomies are widely used. Panel Is Convened
Indications of a link between vasectomy and prostate cancer from recent studies led the World Health Organization to convene a committee of 23 experts from 10 countries in 1991. They concluded that no changes in family planning policies concerning vasectomies were justified on the basis of the biological and epidemiological evidence available then.
W.H.O. officials said today that they did not know why reports from the meeting had not been published, as it had been announced they would be, in The European Journal of Cancer.
A major problem in assessing the health risks of vasectomy is that both the procedure and prostate cancer are common and have been increasing among American men. But the incidence of prostate cancer is much less frequent in developing countries than in developed countries. The incidence is 50 times as high in the United States as in China, for example.
Even if future studies establish a firm link between vasectomy and prostate cancer, the procedure could not explain all cases of prostate cancer. One in 11 men in the United States will develop prostate cancer, and the overwhelming majority will not have had a vasectomy.
The unanswered question is whether vasectomy changes a man's odds of developing prostate cancer, and by how much.
Prostate cancer is now the second leading cause of cancer deaths among American men. But the causes of prostate cancer are not known and no risk factors have been firmly established.
The two studies being reported on Wednesday were derived from two larger studies that were undertaken several years ago to examine the causes of all types of cancer and heart disease.
One of the original studies involved more than 121,000 married female nurses from 30 to 55 years old; the derivative study involved their husbands.