A new genetic test to gauge the aggressiveness of prostate cancer may help tens of thousands of men each year decide whether they need to treat their cancer right away or can safely monitor it.

The new test, which goes on sale Wednesday, joins another one that recently came on the market. Both analyze multiple genes in a biopsy sample and give a score for aggressiveness, similar to tests used now for certain breast and colon cancers.

Doctors say tests like these have the potential to curb a major problem in cancer care - overtreatment. Prostate tumors usually grow so slowly they will never threaten a man's life, but some prove fatal and there is no reliable way now to tell which ones will. Treatment with surgery, radiation or hormone blockers isn't needed in most cases and can cause impotence or incontinence, yet most men are afraid to skip it.

"We're not giving patients enough information to make their decision," said Peter Carroll, chairman of urology at the University of California, San Francisco. "You can shop for a toaster" better than for prostate care.

A study he led of the newest test - the Oncotype DX Genomic Prostate Score - will be discussed Wednesday at an American Urological Association meeting in San Diego.

The results suggest the test could triple the number of men thought to be at such low risk for aggressive disease that monitoring is a clearly safe option. Conversely, the test also suggested some tumors were more aggressive than doctors had believed.

Independent experts say such a test is much needed but that it's unclear how much information this one adds or whether it will be enough to persuade men with low-risk tumors to forgo treatment, and treat it only if it gets worse. Only 10 percent who are candidates for monitoring choose it now.

One man may view a 15 percent chance that his tumor is aggressive as low risk "but someone else might say, 'Oh my God, let's set the surgery up tomorrow,"' Bruce Roth, a cancer specialist at Washington University in St. Louis. "I don't think it's a slam dunk."

Also unknown: Will insurers pay for the expensive test without evidence it saves lives?

The newest test was developed by Genomic Health Inc., which has sold a similar one for breast cancer since 2004. Doctors at first were leery of it until studies in more groups of women proved its value, and the same may happen with the prostate test, said Len Lichtenfeld, the American Cancer Society's deputy chief medical officer.

The company will charge $3,820 for the prostate test and says it can save money by avoiding costlier, unnecessary treatment.