(Reuters Health) - Radioactive seeds that are sometimes used to treat early prostate cancer may do widespread damage to the DNA in a man's sperm, a small study finds.

The damage, researchers say, would seem to be enough to make a man infertile. And they should be aware of that going into treatment.

Known as brachytherapy, the treatment involves implanting radiation-emitting pellets into the prostate gland to kill off cancer cells.

Studies have found that brachytherapy is less likely to cause erectile dysfunction than either traditional external radiation or surgical removal of the prostate gland -- two other treatment options for prostate cancer.

The prostate gland produces fluid for semen, so surgically removing it leads to infertility. But not as much has been known about the effects of brachytherapy, according to Dr. Neil Fleshner, the senior researcher on the new study.

Studies have suggested that men who undergo brachytherapy often become infertile. But there are case reports of patients who have gone on to be fathers -- planned or not.

Many men now have prostate cancer diagnosed at an earlier stage -- and at an earlier age -- through screening with prostate-specific antigen (PSA) blood tests.

More and more prostate cancers are being diagnosed in younger men, and there are more and more older men who still want to have children, said Fleshner, who heads urology at the University Health Network in Toronto, Canada.

So it's important to give men a clearer picture of what to expect after brachytherapy, Fleshner told Reuters Health.

To help do that, he and his colleagues studied semen samples from five men who'd undergone brachytherapy at least one year earlier, all of whom were younger than 55.

They compared the samples with published data on healthy, fertile men and with information on more than 7,600 infertile men who were part of a large database.

Overall, 46 percent of sperm from the brachytherapy patients had DNA fragmentation -- genetic damage that, in great enough proportion, would render a man infertile.

By comparison, 13 percent of sperm from fertile men and 20 percent from infertile men had such damage.

All five brachytherapy patients had an abnormally high amount of genetically damaged sperm -- indicating likely infertility in all, Fleshner's team reports in the Journal of Urology.

Men who are having their prostates surgically removed and still want to have children can opt to bank their sperm ahead of treatment.

Similarly, Fleshner said, if a man is going for brachytherapy, then sperm-banking may be a good idea.

But in weighing treatment options, any man has to discuss the benefits and risks of each with his doctor, according to Fleshner.

In general, any prostate cancer therapy can have lasting side effects, like urinary incontinence and erectile dysfunction.

And because brachytherapy is used for early-stage prostate cancer, some men who are candidates for it may also be able to delay having any treatment at all.

That's because prostate cancer is often slow-growing and may never advance to the point of threatening a man's life. So many men with early-stage tumors can opt for active surveillance -- which means monitoring the prostate cancer over time to see if it's progressing.

One study found that more than 120,000 American men diagnosed with prostate cancer every year are ideal candidates for active surveillance.

In reality, though, the majority of those men end up having surgery, radiation or other treatment instead.

SOURCE: bit.ly/wxNaA6 Journal of Urology, online January 19, 2012.

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