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The men were young going into the study (about age 36, on average), so few did develop cancer. Among men with no sperm -- what doctors call azoospermia -- just over 2 percent were diagnosed with cancer.
Still, their risk was three times higher than that of the average man their age.
"They have the cancer risk of a man about 10 years older," said lead researcher Dr. Michael Eisenberg, an assistant professor of urology at Stanford University School of Medicine.
About 15 percent of infertile men are azoospermic, according to the study, which was published June 20 in the journal Fertility and Sterility.
"This suggests that it's not male infertility in general, but azoospermia in particular," Eisenberg said.
That's an important piece of information, said a male-infertility expert not involved in the study. If the link between male infertility and cancer is real, you would expect that more severe infertility would be tied to a greater cancer risk, said Dr. Thomas Walsh, of the University of Washington in Seattle.
"This reinforces the idea that this is a real relationship," Walsh said.
He said he doubts anyone would say that infertility is causing cancer. But he and Eisenberg said it's possible that some common genetic factors contribute to both azoospermia and a greater vulnerability to cancer.
"When we see a man with azoospermia, we usually assume there's a genetic cause," Eisenberg said. There are certain gene mutations already tied to the condition, but a minority of azoospermic men turn out to have one of them when they are tested. That means there are likely other, as yet unknown, gene defects involved in azoospermia, Eisenberg said.
And some of those genetic flaws might be involved in cancer susceptibility, he said.
Another infertility expert was cautious about interpreting the findings because of the small numbers: only 10 cases of cancer among the 451 men with azoospermia, and 19 cases among nearly 1,800 men with other types of infertility.
The idea that genetic abnormalities might underlie both azoospermia and cancer risk has merit, said Dr. Frederick Licciardi of NYU Langone Medical Center in New York City. But, he said, "while this is important reasoning and is based in basic science studies, I do not feel they have enough evidence in this paper to bolster this theory."
Another question is whether azoospermia is linked only to certain cancers. Past studies, including one Walsh worked on, have found that infertile men show a higher than average risk of testicular cancer -- a highly curable disease usually diagnosed in young men.
Eisenberg said there were too few cases of each cancer to see whether men with azoospermia were at particular risk for any one type.
For now, he recommended that men with the condition "be aware of the possible risk, and pay attention to your health." That includes not only maintaining a healthy lifestyle, he said, but also doing what most younger men do not -- seeing your doctor for a regular check-up.
"It's too early to make any recommendations about cancer screening," he said. But a routine visit to your doctor for a physical exam -- which can detect testicular cancer, for example -- is wise, Eisenberg said.
Licciardi agreed. "Any man -- very low sperm count or not -- should have regular physical examinations."
Walsh said much more research is needed to dig into the connection between male infertility and cancer, including studies that follow men over a long period since cancer rates climb with age, as well as basic lab research to try to uncover the reasons for the link.
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