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Researchers say they’ve found a new way to tell if a man’s prostate cancer will come back and kill him after treatment.

If a blood test called a PSA doesn’t fall to low enough levels after treatment, it means the cancer’s not all gone and will likely come back and spread, the team at Brigham and Women's Hospital and Harvard Medical School reported.

Radiologist Val J. Lowe, director of the cancer imaging program at the Mayo Clinic cancer center, looks at a PET scan of a patient with recurrent prostate cancer Ariana Lindquist / Bloomberg via Getty Images file

PSA tests look for prostate specific antigen, a protein made only by prostate cells. Higher PSA levels suggest that prostate cells are growing — often because of cancer, but sometimes if the prostate is inflamed or because of the harmless enlargement of the prostate that comes with aging.

The important number to know: PSA should fall to 0.5 nanograms (ng per ml) or lower.

That gives doctors a chance to act right away, said Dr. Anthony D’Amico, the senior oncologist on the study.

“Instead of waiting to see if PSA has gone up, this can tell you that somebody has not only failed treatment, but failed so badly that they are going to die of prostate cancer,” D’Amico told NBC News.

“You should know what your PSA is after your treatment. You need to know once it stops going down if that low point is above half a point (0.5)."

"By identifying and enrolling these men in clinical trials immediately, the hope is to take a prostate cancer that appears to be incurable and make it curable" added Dr. Trevor Royce, who led the work on the study.

It’s an important question.

Prostate cancer is very common, showing up in 240,000 U.S. men every year. It kills about 30,000 a year.

In most men, prostate cancer isn't likely to kill them before something else does. But since prostate cancer still kills so many men, it’s important to find out which men are most at risk of dying early.

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Related: All Prostate Cancer Treatments Are About Equally Effective

This new study shows that PSA can tell you.

PSA’s not a very good indication of cancer, but it’s a good measure of how well cancer treatment has worked. PSA should drop to very low levels after surgery or radiation treatment for cancer.

But it doesn’t always, and it often rebounds.

“Normally, a man gets treated for prostate cancer and his PSA is monitored every six months for a few years. In order to call somebody a failure, that the disease has recurred, you need to see a PSA that is going up.”

But not every man whose PSA goes up after treatment dies of cancer. And not every prostate cancer patient is saved by fresh treatment once his PSA rises to a certain level, usually a reading of 10.

Related: Prostate Surgery Rates Fall After Screening Advice

The Harvard team wanted to see if there’s a more precise way to tell who had the more dangerous cancer.

They studied 157 men treated for prostate cancer, watching them for more than 16 years on average. Most — 70 percent — died by then.

“Men were seen in follow-up every three months for two years, every six months for the subsequent three years, and every year thereafter,” the team wrote in their report, published in the Journal of the American Medical Association’s JAMA Oncology.

The main determinant of whether the men would die, the team found, was how low their PSA level fell. If it did not drop to 0.5 after treatment, the men were most likely to have the cancer come back and kill them, D’Amico said.

“If it doesn’t drop below this half point in follow-up … you know that person not only has residual prostate cancer, but the type of prostate cancer that often goes on to kill them,” D’Amico said.

Just having PSA levels rise again was not a very good predictor of whether the men would die, the team found.

Related: Fewer Men Smoking, Fewer Dying From Prostate Cancer

Now doctors need to see if treating these men right away, instead of waiting for their PSA levels to rise more, may save them, D’Amico said. “Before, we saw this number and said ‘gee we are concerned but let’s watch,’” he said. Now doctors can act.

There are many drugs that prostate cancer patients can get, but they’re almost never given until the cancer’s come back and started causing symptoms.

"You know that person not only has residual prostate cancer, but the type of prostate cancer that often goes on to kill them."

“These are treatments that are used when a man has metastatic disease. They have been shown to prolong life but not to cure it,” D’Amico said.

It might be if a patient gets such treatment right away, he could live even longer or perhaps even be cured. But a study will have to be done to show it.

The men in the study had radiation or hormone therapy, but D’Amico said the finding should hold for men who have had their prostates surgically removed, also.

“You should know what your PSA is after your treatment. You need to know once it stops going down if that low point is above half a point (0.5),” D’Amico said.