CHICAGO (CBS) — After a man completes treatment for prostate cancer, what are the odds that cancer comes back? A new Harvard Medical School study might have the answer.

The study followed more than 150 prostate cancer patients over 16 years. Researchers examined the results of a blood test that looked for the prostate specific antigen (PSA).

If the PSA level is zero, the patient is in the clear; if it’s above 0.5, there’s a good chance the cancer will come back.

Dr. Edward Schaeffer, chair of the department of urology at Northwestern University’s Feinberg School of Medicine, said the study allows doctors to catch problems much earlier than in the past.

“By doing that, you may be able to help patients be more proactive about their follow-up, and their subsequent treatment after radiation therapy,” he said.

Schaeffer said men whose PSA levels are above 0.5 after treatment need closer monitoring, due to the increased chance the cancer will return.