Study: No single PSA level ensures safety from prostate cancer risk
Ian M. Thompson Jr., M.D., professor and chair of urology at the Health Science Center, is looking for a better test – or set of tests – to diagnose prostate cancer and to distinguish between cancers that will kill men if left untreated and those that will not threaten health and do not need treatment.
The widely used PSA test (prostate-specific antigen) is not getting the job done by itself, and more than ever the onus is on men to discuss risks and options with their physicians.
That is strongly confirmed in a new study reported in the July 6 issue of The Journal of the American Medical Association. Dr. Thompson is lead author among collaborators from the Fred Hutchinson Cancer Research Center, the University of Colorado, the National Cancer Institute, Cancer Research and Biostatistics (Seattle) and the Southwest Oncology Group.
Dr. Thompson spoke at a press briefing July 5 at San Antonio’s Cancer Therapy & Research Center (CTRC), where he is a member of the medical staff. Others in attendance included study co-author Charles A. Coltman Jr., M.D., of the Southwest Oncology Group, the San Antonio-based national clinical trials group, and Karen Fields, M.D., president and chief executive officer of the CTRC. Co-author Howard L. Parnes, M.D., of the National Cancer Institute, spoke by teleconference.
The study, which draws conclusions from the Prostate Cancer Prevention Trial (PCPT) conducted under the sponsorship of the National Cancer Institute, examines the performance characteristics of the PSA test. Dr. Thompson and his colleagues analyzed PCPT data taken from regular PSA screenings in participants and the outcomes of end-of-study prostate biopsies of more than 8,500 men who received a placebo rather than active medication.
“We once thought a PSA level of 4.0 (ng/ml) or lower was a magic figure for men, ensuring they were not at risk for prostate cancer,” Dr. Thompson said. “Now we are thinking ‘outside the 4.0 box.’ There is no level of PSA that gives us high enough sensitivity, meaning the ability to identify the man with cancer, while at the same time providing high enough specificity, meaning the ability to tell a man without cancer that he is fine. The bottom line is this: Each man needs to talk with his physician about his own risk factors – age, family history of prostate cancer, ethnicity and even weight – and decide where to go from there. PSA readings provide significant information but cannot be interpreted alone.”
The authors wrote in their paper that lowering the PSA cutoff value – the value that if exceeded would lead to further testing – to 1.1 ng/ml would result in detection of 83.4 percent of prostate cancers “but would subject 61.1 percent of men without cancer to prostate biopsy.” They also wrote that lowering the threshold, while it would result in finding more aggressive cancers earlier, would lead to the “unavoidable tradeoff [of] increased detection of biologically inconsequential cancers.”
Between 1993 and 2003, the PCPT enrolled 18,882 men with PSA levels less than or equal to 3.0 ng/ml and normal digital rectal exam results. The men were randomized to receive either the medication finasteride or a placebo for seven years. The final study results, reported in 2003, showed that the group taking finasteride developed almost 25 percent fewer cases of prostate cancer than the group taking a placebo, although more cases of serious prostate cancers were reported in the men taking finasteride.
Last year, Dr. Thompson and his colleagues reported that 15 percent of PCPT participants with so-called “normal” PSA levels less than or equal to 4.0 ng/ml actually were found to have prostate cancer upon biopsy, and 15 percent of those had serious disease.
This year, Dr. Thompson and investigators from the Health Science Center’s San Antonio Center of Biomarkers of Risk for Prostate Cancer (SABOR), a National Cancer Institute-supported program that has enrolled more than 3,000 men from South Texas, reported that obesity was one factor that may lower PSA and lead to a later diagnosis of prostate cancer. The SABOR program continues to enroll men from San Antonio and South Texas and has a goal of developing a better test for the early detection of prostate cancer.
Dr. Thompson occupies the Henry B. and Edna Smith Dielmann Memorial Chair at the Health Science Center and is a member of the San Antonio Cancer Institute (SACI), one of only two NCI-designated Cancer Centers in Texas. The SACI is a partnership of the Health Science Center and the Cancer Therapy & Research Center.