The incidence of prostate cancer appears to be dropping. And a screening test may be the reason.
But not in the way you might think.
Researchers at the National Cancer Institute recently found that the number of newly diagnosed prostate cancers, which rose sharply from 1989 to 1992, decreased or stabilized between 1992 and 1993. The number of new cases dropped by 16 percent in white men and stabilized in African Americans, while the number of new cases in older men of both races declined between 17 and 29 percent. But the researchers say the decrease does not necessarily mean that prostate cancer has become less prevalent: It could simply mean that fewer men are being tested for the disease (Journal of the National Cancer Institute, Nov. 20, 1996).
"The decline may have resulted from a fall in the use of PSA [prostate-specific antigen] for screening and detection," says Ray Merrill, Ph.D., the study's lead investigator (Medical Tribune, Dec. 12, 1996).
Controversy has surrounded the PSA test, a blood test that measures the level of an enzyme that usually rises in men with prostate cancer. While the PSA has been shown to predict cancer in some men before a tumor is found, the test poses several problems. PSA tests often produce false-negative results--that is, the results are normal even if cancer is present. In studies, between 20 and 40 percent of men with prostate cancer had normal PSA levels. False-positive results, or high levels of PSA when there's no cancer, are also common. Only one in three men with elevated PSA levels who undergo biopsies are found to have cancer. But even when the test is accurate, experts debate whether its use as a screening test is appropriate.
This is some question whether an early finding of prostate cancer increases a man's life span or just his anxiety level. There is no proof that routine screening for prostate cancer reduces mortality. The cancer reduces mortality. The cancer usually grows slowly; in many cases its progression is so slow that its victims--particularly the older ones--are likelier to die from something else. And radical prostatectomy, the surgical procedure used to treat early-stage prostate cancer, is not without risks and complications that can adversely affect the quality of life.
Merrill and his colleagues speculate that this controversy may have decreased the number of men undergoing the PSA test and, consequently, the number of men being diagnosed. Or, Merrill says, the original increase in incidence rates could have reflected the introduction of the PSA test.
A study reported in the Oct. 23/30, 1996, Journal of the American Medical Association lends credence to the latter theory. In the study, researchers looked at the test results of men 50 or older who underwent PSA screening every six months for four years. They found that as time went on, both the proportion of men with abnormal test results and the prostate cancer detection rate decreased. Eighty-five percent of the prostate cancers detected during the study period were found during the first two years of screening.
"When you screen through a population of men who have never been examined before, there will be a number of asymptomatic cases that were just sitting there undiagnosed." Patrick Walsh, M.D., director of urology at the Johns Hopkins University School of Medicine, told Medical Tribune (Dec. 12, 1996). "Once you catch these cases, then you're picking up only new ones."
In other words, the actual incidence rate of prostate cancer may not have changed much, but the detection rate may have.
So, is the incidence of prostate cancer actually dropping and will it continue to drop? That depends, in part, on whether the PSA and other tests are used for routine screening. To date, experts remain undecided about whether the tests should be widely used. The U.S. Preventive Services Task Force last year recommended against routine screening for prostate cancer--with the PSA test or with the digital examination. But the American Cancer Society recommends that all men over 50 undergo both screening tests, and the American Urological Association recommends routine screening for men over 40.