Prostate-specific antigen (PSA) is a substance made by normal prostate cells and can be measured in blood. The higher the PSA level, the more likely the chance of prostate cancer, in which cells multiply uncontrollably. While PSA levels tell how likely a man is to have prostate cancer, the results do not provide a definite diagnosis. Men with a high PSA result are advised to have a biopsy to find out whether or not they have cancer.

"The study shows that screen-detected cancers, or a proportion of them, have aggressive features. This is a necessary condition for effective screening, but not sufficient," explains co-author Anssi Auvinen, MD, PhD, professor of epidemiology at the University of Tampere, in Tampere, Finland. "It indicates that there is probably also over-diagnosis — detection of tumors with a low malignant potential," he adds.

The question of whether to screen men for prostate cancer is not a simple one. The authors report that a main criticism against PSA-based screening is that it leads to substantial over-diagnosis and, thus, unnecessary treatment that has substantial side effects. Prostate cancer has been found in autopsies in men who never had any symptoms, and in prostates removed for reasons other than for suspected cancer.

Because of these findings, which have been well documented for many years, there is the possibility that the PSA test will detect some cancers that would never have caused any problems, explains Durado Brooks, MD, program director for prostate and colorectal cancer at the American Cancer Society (ACS). These cancers, which are found incidentally at autopsy or surgery, are generally less aggressive; and because the PSA does not distinguish between these "latent" cancers and those that will eventually cause serious problems, there is a danger of over-treatment of cancers that never would have caused symptoms or other difficulty, Brooks says.

To predict how likely the cancer is to cause symptoms, doctors consider a man’s PSA level, his physical exam findings, the appearance of his cancer under the microscope, and his age. Although these predictions are not perfect, Brooks adds, they are very useful in helping men and their doctors make informed decisions about whether or not to undergo treatment right away, or to wait until symptoms develop ("watchful waiting").

The Finnish authors write that in previous studies, cancer has been detected in the prostate in nearly 40% of men between the ages of 50 and 60 when the prostate was studied at autopsy or after it was surgically removed during treatment for bladder cancer. Further, the authors describe that men with some types of prostate cancer [especially elderly men with low-grade tumors] have a high survival rate by using a "watchful waiting" approach, instead of surgical or hormonal treatment; but with time, there is also the observation that some prostate cancers appearing to be early stage and nonaggressive will develop more aggressive features.

PSA testing will detect both aggressive and non-aggressive cancers, but the test in isolation does not differentiate between these cancers, says Brooks.

Still, the test has value, Brooks says. The ACS recommends that:

• beginning at age 50, all men who have at least a 10-year life expectancy should be offered both the PSA blood test and a digital rectal exam;
• men in high-risk groups should begin screening starting at age 45.
  (High-risk groups include African-American men, and men whose brothers, fathers, or sons had prostate cancer diagnosed under age 60.)

"This study represents additional support for the idea that PSA testing is contributing to the decrease in prostate cancer mortality that has been seen in some recent studies. Randomized screening trials — the best way to prove or disprove this — are currently in progress; but so far, most of the evidence supports the view that early detection is saving lives," Brooks comments.

According to the authors, the main finding in this study was the observation of biologic aggressiveness in as many as 35% of the cancers detected by PSA screening. Brooks explains that these findings suggest tumors found by PSA screening are more likely to show aggressive features, and that these tumors were found at an earlier — and presumably more treatment-responsive — stage than tumors detected clinically [by digital rectal examination or the appearance of other symptoms].

"This is important because tumors demonstrating these aggressive features are more likely to progress to clinically-significant disease," Brooks notes.

"The goal of any disease-screening program is to find disease that is, or will become, clinically significant. These results provide further evidence of the potential benefit of population-based PSA screening programs," emphasizes Brooks.