TUESDAY, July 5, 2005 (HealthDay News) -- While still a valuable screening tool for prostate cancer, the prostate-specific antigen (PSA) test should not be used to provide clear-cut guidance for treating the disease, a major new study finds.
"It used to be thought of as a dichotomy -- plus or minus, positive or negative," said study leader Dr. Ian M. Thompson. "But PSA is now more complex than that."
Conventional wisdom has held that a biopsy to obtain a tissue sample was necessary only if the PSA reading was higher than 4.0 milligrams per milliliter of blood. But the new study, which followed 18,882 men for up to seven years, found that there was no specific PSA reading that accurately predicted disease risk.
"There is a continuum of risk," said Thompson, who is chairman of the department of urology at the University of Texas Health Science Center at San Antonio.
The study findings appear in the July 6 issue of the Journal of the American Medical Association.
A screening test, such as the PSA, is judged by what doctors call "specificity" -- its ability to tell whether someone has a disease -- and "sensitivity" -- its ability to exclude false-positive results.
The existing guidelines for PSA tests are inadequate, the study found. For example, biopsies done for a PSA reading of 4.1 detected only 20.5 percent of cancer cases, and mistakenly diagnosed cancer in 6.2 percent of the men. Similar readings were found for other PSA levels, the study found.
"A clear-cut decision rule for prostate biopsy based on PSA value would be challenging to derive from these data," the researchers wrote. "...There is no single cutoff that would simultaneously yield both high sensitivity and high specificity."
But that finding should not be taken to mean that PSA testing is of no value, Thompson said -- an extremely important point, considering that about 75 percent of American men 50 and older have had at least one PSA screening test, and more than half have periodic PSA screening.
What is needed is judgment on the part of doctors and their patients, Thompson said. "The physician has to have more information, and the patient has to get rid of the idea that only a reading above 4 is important," he said.
Other factors can help interpret PSA test results, Thompson said. For example, a biopsy might be justified in a man with a relatively low test reading but whose father died of prostate cancer.
One clear finding that emerged from the study was the increased incidence of prostate cancer in men whose PSA levels rose year after year, he said.
The study "is one of the most important ever performed on prostate cancer," said Dr. Mark H. Kawachi, director of the Prostate Cancer Center at City of Hope Cancer Center in Duarte, Calif. The results are definitive because a biopsy was done on every man in the study. All earlier studies did biopsies only when men had PSA readings of 4 or higher, he noted.
"It shows the importance of watching PSA levels serially," he said. "It also indicates the need to identify other tests that will help tell when a slight rise in PSA means that a patient has prostate cancer."
The standard recommendations for testing still stand, Kawachi said, because "PSA continues to be the most powerful tool in the early detection of prostate cancer." Every man should have an initial PSA test no later than age 50. Men at high risk because of a family history or other factors should have a first test at age 45. Annual tests should be done after 50, he said.
Last fall, the researcher who first identified the PSA test announced it now had questionable value.
Writing in the October issue of the Journal of Urology, Stanford University's Dr. Thomas Stamey said the test was now more likely to spot benign prostate enlargement or very slow-moving malignancies than "significant," aggressive cancers, raising risks for misdiagnosis and unnecessary surgeries.
Another study on prostate cancer, this one done by the American Cancer Society, has found that long-term, regular use of aspirin or other non-steroidal anti-inflammatory (NSAID) drugs such as ibuprofen are associated with a reduced incidence of the cancer.
Men who took 30 or more pills of those medications per month for five years or more were 18 percent less likely to develop prostate cancer, according to the society's Cancer Prevention Study II Nutrition Cohort.
However, more research is needed before aspirin and NSAIDs should be prescribed as a possible treatment for preventing prostate cancer, the researchers said.
"The American Cancer Society does not currently recommend using aspirin or other NSAIDs to prevent cancer," said Eric J. Jacobs, a senior epidemiologist with the society. "They can cause serious side effects, such as gastrointestinal bleeding."
The findings appear in the July 6 issue of the Journal of the National Cancer Institute.
To learn more about prostate cancer, visit the National Library of Medicine.