Amidst growing controversy over prostate-specific antigen screening, a new analysis raises the possibility that more men would be diagnosed with advanced prostate cancer without early testing.
The incidence of men presenting with prostate cancer that has already metastasized is three times lower than would be expected, if incidence figures from the pre-PSA era were extrapolated to today’s U.S. population, according to the report published online July 30 in Cancer.
Based on data for diagnoses made from 1983 to 1985, clinicians today could expect to see approximately 25,000 cases of prostate cancer that has already metastasized at presentation. But the actual number of such cases is three times lower, at approximately 8,000, said Emil N. Scosyrev, Ph.D., of the department of urology, University of Rochester (N.Y.) Medical Center, and his associates.
It would be erroneous to assume a causal relationship from these observational data, however, or to conclude that PSA testing alone contributed to this reduction. Temporal changes in other factors that influence prostate cancer almost certainly have contributed to the decrease, the investigators noted.
"Our current findings must be viewed primarily as a description of observed time trends rather than as definitive tests of causal hypotheses about screening," they said.
Dr. Scosyrev and his colleagues assessed age- and race-specific annual rates of presenting with metastatic prostate cancer from 1983 to 2008, using data from Surveillance, Epidemiology, and End Results (SEER) Program registries in San Francisco, Connecticut, Detroit, Hawaii, Iowa, New Mexico, Seattle, Utah, and Atlanta. The Food and Drug Administration approved PSA testing for clinical use in 1986, so the researchers compared prostate cancer data in the pre-PSA era (1983-1985) to that in the PSA era (1986-2008).
When men of all ages and races were considered together, 2,277 men in the SEER registries would be expected to present with metastatic prostate cancer in 2008, the most recent year for which data are available. However, the actual number who presented with metastatic prostate cancer that year was only 739 in the SEER registries.
These registries are thought to capture approximately 25% of all cases of prostate cancer in the U.S. each year. When the study figures are extrapolated to the entire U.S. population in 2008, only about 8,000 cases actually occurred when about 25,000 would have been expected to occur. This suggests that 17,000 cases of metastatic prostate cancer at presentation were prevented in 2008, the investigators said (Cancer 2012 July 30 [doi:1002/cncr.27503]).
It is important to note that other factors besides PSA testing also changed during this time period, which could not be accounted for in this observational study, the authors noted.
For example, obesity may be associated with an increased risk of prostate cancer, and the prevalence of obesity has certainly increased since 1983. "Hence, if obesity had a major influence on the incidence of presenting with [metastatic prostate cancer] during the study period, then the true benefit of screening may be underestimated in our study," Dr. Scosyrev and his associates said.
Medical imaging techniques also have improved over time, however, enabling earlier diagnosis of prostate cancer in more recent years.
"In particular, in the pre-PSA era, CT and bone scans were not as widespread (or of the same quality) as they are today, MRI and PET-CT were not available, and metastatic prostate cancer often was diagnosed because of symptomatic progression of metastatic lesions. In contrast, in the modern PSA era, metastatic prostate cancer is often diagnosed on staging imaging for high-risk disease," the authors added.
Another issue that must be considered, they said, "is the potential lead-time effect resulting from screening." To that end they offered the hypothetical case of a man who develops organ-confined prostate cancer at 70 years of age, micrometastates at 75, and overt metastatic disease at 80, and then dies of prostate cancer when he is 82 years of age.
"If this man were screened between ages 70 and 75 years, then his prostate cancer potentially could be cured. Conversely, screening between ages 75 and 80 years would result in an earlier diagnosis, but not in a cure," they wrote, going on to note substantial stage shifts in newly diagnosed prostate cancer after mass screening programs were introduced.
Because metastatic prostate cancer is highly symptomatic and "rapidly fatal," the authors said they "believe that many men with this disease would likely benefit from earlier diagnosis, either in terms of improved survival or at least from the palliative perspective (e.g., prevention or delay of skeletal complications)."
It also is important to remember that PSA screening has led to overdiagnosis and overtreatment of nonaggressive prostate cancer, another issue that could not be addressed in this study, they added.