How quickly a man's PSA level has been rising before a diagnosis of prostate cancer may help doctors predict whether the cancer is more likely to kill him, according to a new study.

Though doctors have known for some time that rising levels of PSA (prostate-specific antigen) in the blood are a marker of aggressive cancer, the recent study quantifies the relationship by giving doctors hard and fast numbers to look for, said Durado Brooks, MD, director of the prostate and colorectal cancer programs for the American Cancer Society.

Prostate cancer is the most common cancer among American men (after skin cancer), affecting about 230,000 men each year. A fraction of those -- about 30,000 a year -- die from it.

That's because many prostate cancers are what doctors call "indolent" -- that is, they grow slowly and may never cause any symptoms. Others, though, are aggressive and deadly. Being able to tell the difference between the two forms could help men and their doctors make smarter decisions about which cancers really need to be treated, and how intensive that treatment should be.

PSA levels have long been used to help diagnose prostate cancer -- anything above 4.0 ng/ml (nanograms per milliliter of blood) generally warrants a prostate biopsy to look for cancer. But the PSA number alone has not been able to tell doctors how aggressive a particular cancer is.

Because prostate cancer treatments can have potentially severe side effects like incontinence and impotence, sparing men unnecessary treatment is an important consideration. Some men with prostate cancer choose to delay treatment -- an option known as "watchful waiting" -- until there are indications the disease might cause problems.

The new research suggests that PSA velocity -- how quickly a man's PSA level goes up over time -- may point to a cancer that should be treated aggressively. The findings were published in the New England Journal of Medicine (Vol. 351, No. 2: 125-135).

Researchers from Boston's Dana-Farber Cancer Institute, Chicago's Northwestern Memorial Hospital, and several other institutions studied 1,095 men who had been treated for prostate cancer with radical prostatectomy. They looked at the men's PSA velocity during the year before their cancer was diagnosed.

Men whose PSA had risen by more than 2 points during the preceding year (PSA velocity higher than 2.0 ng/ml) had a higher risk of dying from prostate cancer within the next 7 years than men whose PSA had risen more slowly, researchers found. Men with a high PSA velocity were also more likely to have advanced disease (cancer that had spread to the lymph nodes or was of a higher stage and higher Gleason score) at the time of surgery.

"The study results indicate that men with a high PSA velocity should not be managed by watchful waiting, and many will require more than a radical prostatectomy to prevent prostate cancer death," study co-author William J. Catalona, MD, of Northwestern Memorial Hospital, said in a statement.

Brooks agreed.

"Knowing the velocity helps the physician identify those patients for whom adjuvant therapy [chemotherapy or radiation after surgery] might at least be considered," he said.

He noted, however, that watchful waiting might still be appropriate for elderly men with other major medical conditions (diabetes or heart disease, for instance) that might make more aggressive prostate cancer treatment riskier.

Because of the new finding, Catalona recommended that men begin PSA testing at age 40 to establish a baseline value that can be used to determine whether subsequent levels have risen dangerously fast.

Brooks disagreed with this conclusion.

"Until we know for sure that we're doing more good than harm with mass prostate cancer screening, I think it's premature to make that recommendation," he said.

Prostate cancer screening is heavily debated in the medical community because it is not yet known whether screening saves lives and because treating prostate cancer unnecessarily can have a negative impact on a man's quality of life. The American Cancer Society recommends that men at average risk discuss the benefits and risks of screening with their doctors to determine if it's right for them.

Several clinical trials are currently assessing the effect of widespread prostate cancer screening -- whether it actually saves lives, how many men are impaired by side effects of treatment, etc. Those results should help the medical community come to better agreement about who should be screened for prostate cancer and the most effective way to screen.

Until then, though, Brooks said the current study is more useful for guiding men who are known to have prostate cancer, or who are suspected of having it. Both he and the study researchers said men in this situation should consider entering a clinical trial that could help determine what course of treatment is most successful.