By LEE BOWMAN
Scripps Howard News Service
September 21, 2005
"Initially, we envision this new test could be used as a supplement to PSA" (prostate specific antigen test), said Dr. Arul Chinnaiyan, a professor of pathology at the University of Michigan Medical School and lead author of a report on the test published Thursday in The New England Journal of Medicine. "In the future, I think this could replace PSA."
More than 230,000 American men are diagnosed with prostate cancer each year, but the current screening test that measures levels of prostate specific antigen does not always accurately warn that cancer is present. Elevated PSA levels can be present even when a prostate problem is benign.
A cancer patient's immune system recognizes a tumor as an invader and produces antibodies that attempt to destroy it. The new test relies on a panel of 22 immune-system antibodies that are generated against proteins that play a role in this form of cancer.
"There is no detection instrument that rivals the sensitivity and specificity of the immune system," said Olivera Finn, an immunologist at the University of Pittsburgh's School of Medicine, in a journal editorial.
Working with blood samples taken from 331 prostate-cancer patients and 159 men with no history of cancer, the researchers narrowed 2,300 antibody candidates down to 186, and then tested all of them further against blood samples from the cancer patients and controls.
Using the 22 biological markers, only two of 70 blood samples from the controls incorrectly tested positive for prostate cancer, while seven of 59 prostate-cancer samples gave false negatives.
Next, the researchers validated the test with the remaining blood samples and found that 81.6 percent of the time, samples that were cancerous were classified properly, and 88 percent of the time, samples that were not cancerous were correctly identified.
"These 22 biomarkers appear to be the right number. If you used too many or too few, the accuracy went down a bit," Chinnaiyan said.
The results are considerably more reliable than those of the PSA test, which gives false positives about 80 percent of the time, requiring unnecessary prostate biopsies. Many cancer specialists now think that a high PSA level may be less significant than a sharp rise in PSA levels.
The new screening test was reliable at identifying prostate cancer in samples where the PSA levels were in an intermediate range that doesn't always indicate cancer, and where doctors might hesitate to recommend biopsy to a patient.
Chinnaiyan noted that the new test requires only a routine blood sample, and that most blood-processing labs could easily be equipped to scan for the 22 markers, which the team is continuing to study with a larger group of patients.
Olivera noted that eventually scientists may be able to design diagnostic tests using some antibodies that reveal that a tumor is developing, and other antibodies indicating that the tumor has been or is likely to be destroyed.
Testing for cancer based on immune response offers another advantage, Olivera wrote. "Perhaps in the future, a set of cancer proteins will be arrayed on a diagnostic chip to be used for early cancer detection. The same set of proteins will also be formulated into a cancer vaccine and delivered to the doctor's office, ready to boost the patient's natural immunity against the cancer."
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