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Still, "it may be that circumcision should be considered as an option for men at higher risk, such as black men and men with a family history of prostate cancer, but we need more research to confirm this," said study co-author Marie-Elise Parent. She is an associate professor of epidemiology at the University of Quebec's INRS-Institut Armand-Frappier Research Center in Laval, Canada.
Prostate cancer is the second-leading cause of cancer death in U.S. men, behind only lung cancer, according to the American Cancer Society. About one in seven men will be diagnosed with the disease in his lifetime, and prostate cancer will kill one in 36, the cancer society estimates.
"We still know very little about the potential causes of prostate cancer. There are probably many factors associated with it, but so far we have not been able to pin down what factors could be modified in order to prevent this cancer," Parent said.
"All that we know for sure is that risk of developing it increases with age, that having a father, brother or son with prostate cancer increases one's risk, and that this cancer is more frequent among men of African ancestry," Parent added.
There's no way to use these risk factors to prevent the disease, she said.
Previous research has suggested that circumcision could slightly reduce the risk of prostate cancer, Parent said. Circumcision has also been linked to lower rates of sexually transmitted diseases like HIV, the virus that causes AIDS. However, circumcision remains controversial, with critics calling it barbaric, unnecessary and an impediment to sexual pleasure.
In the new study, published May 29 in the journal BJU International, researchers examined the medical records of 1,555 men treated for prostate cancer at a Montreal hospital from 2005-09. They compared them to 1,586 similar men who didn't have prostate cancer.
The researchers couldn't find a statistically significant difference in prostate cancer rates between circumcised and uncircumcised men.
However, the researchers did find a 60 percent lower risk of prostate cancer among circumcised black men. They also found evidence linking circumcision after age 35 to a 45 percent lower risk of prostate cancer.
But the findings merely point to an association between circumcision and lower risk in these two groups, not a direct cause-and-effect relationship.
The researchers reached their conclusions after adjusting their statistics for traits such as age at diagnosis and family history of prostate cancer. Due to the design of the study, researchers cannot offer simple numbers to describe the levels of risk for various types of men.
Parent cautioned that the black men in the study, mainly of French descent, may not reflect black men as a whole. And she said the study included few men who were circumcised at a later age, so that finding is potentially questionable.
Dr. Stephen Freedland, a urologist and associate professor of surgery and pathology at the Duke University School of Medicine in Durham, N.C., pointed out that men circumcised after age 35 are unusual. "They're usually older guys who are sick and have medical problems," he said.
The study findings as a whole aren't convincing, Freedland added, especially since it included relatively few black men -- just 178 of more than 3,100 participants.
"I don't think we'll be recommending massive circumcisions to prevent prostate cancer," he said. "And men shouldn't go around thinking, 'I'm circumcised, therefore I'm safe from prostate cancer.'"
If circumcision does have a protective effect against prostate cancer -- and that's not proven -- why would that be so?
Circumcision at an early age may possibly protect against later sexually transmitted infections, which are thought to increase the risk of prostate cancer, Parent said.
"We tried to take this into consideration into our study," she said. "It did not seem to be the main explanatory factor, but it cannot be ruled out entirely, either."
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SOURCES: Marie-Elise Parent, Ph.D., associate professor, epidemiology, INRS-Institut Armand-Frappier Research Center, Laval, Canada; Stephen Freedland, M.D., urologist and associate professor, surgery and pathology, Duke University School of Medicine, Durham, N.C.; May 29, 2014, BJU International