Prostate cancer screening aims for better accuracy
The current method of screening for prostate cancer is both valuable and problematic. It’s the only way to detect the early stages of the disease, but the test’s results are often inaccurate.
The inadequacy of the screening, which measures the levels of prostate specific antigen (PSA) in the blood, has pushed scientists to search for an alternative. One test, for a protein called early prostate cancer antigen (EPCA), looks especially promising.
In a study published recently in the Journal of Urology, researchers found that the new test might be able to detect prostate cancer five years earlier than it is currently diagnosed.
“Patients want to know if they have prostate cancer at a time when treatment might render a cure,” says Dr. Mark Kawachi, director of the Prostate Cancer Center at City of Hope Cancer Center. “The issue here is not only early detection but avoiding unnecessary biopsies.”
About 1.2 million men undergo biopsies each year in the United States to check for cancer of the prostate, a gland near the urethra that produces some of the seminal fluid. Many of these men do so because their PSA test looks abnormal. But only about a quarter of those men have tumors, says Robert Getzenberg, a professor of urology, pathology and pharmacology at the University of Pittsburgh School of Medicine.
“We do a lot of biopsies to identify the one man who has prostate cancer,” says Getzenberg, senior author of the recent study on the new test. “These other men [with positive PSA tests but negative biopsies] are worrying about whether they really have prostate cancer. In those men, we should be able to find out if they really have it or not.”
In Getzenberg’s lab, the quest for a better way to detect prostate cancer began several years ago with studies on the structure of a cancer cell. When cancer is present, says Getzenberg, the nucleus of the cell changes. “We thought that if it looks different, maybe there is something different in the underlying structure,” he says. Getzenberg identified a protein with the acronym of EPCA that, in most cases, appears in the prostate tissue of men with cancer but doesn’t appear in tissue free of disease.
An unusual aspect of the protein is that, when cancer is present, the protein is found throughout the prostate gland, not just in the tumor, Getzenberg says. Researchers aren’t sure why, but it suggests that “there is something going on in the entire prostate of someone with prostate cancer,” he says.
That the protein can be found elsewhere in the gland is important when it comes to testing because a biopsy sample taken from any part of the gland -- not just the area containing cancer cells -- should suggest that cancer is present somewhere in the prostate.
The discovery of the protein has led to a test for biopsied tissue. But a blood test that would reveal evidence of the protein would be more useful. That may take another year or two to develop.
The blood test would probably be used when the PSA test’s results are positive, Getzenberg says, to determine who should undergo a biopsy.
Currently, the EPCA tissue test produces false positives -- results that incorrectly suggest cancer -- in about 5% to 10% of cases. Thus the test shouldn’t be used “to decide who should have their prostate taken out and who shouldn’t,” Getzenberg says.
The PSA test, in contrast, can produce false positives in up to 75% of men who don’t have cancer and may miss as many as 20% of tumors.
“In the future, if these testing methods are found to be extremely reliable, this may be the way all prostate cancer will be diagnosed,” Kawachi said.
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Advance warning
Beginning next month, pathologists will be able to test for the presence of early prostate cancer antigen in prostate tissue samples.
Typically, a pathologist examines a biopsied tissue sample under a microscope to determine whether cancer cells are present.
Such biopsies, however, do not always confirm the absence of cancer; some cancers are missed on a first biopsy.
The ProstaMark EPCA test can be used as a secondary test after the visual examination of the tissue. The test may be especially useful if the biopsied tissue appears to be free of cancer but a high level PSA score indicates cancer may be present.
The presence of EPCA, even when the biopsy is negative may mean that the patient should be followed closely to see if a tumor develops, says Robert Masterson, president of Tessera Diagnostics Inc., the manufacturer of the product.
