PSA test alone not an ideal predictor of prostate cancer

Recent studies suggest that testing blood for prostate specific antigen (PSA) alone does not produce an ideal predictor of prostate cancer, and emerging data suggest this is especially true for obese men.

A recent study by Oregon Health & Science University Cancer Institute researchers shows that a simple adjustment in how clinical measurements are interpreted in the diagnosis of prostate cancer can accurately predict the presence of prostate cancer in obese men.

"Some studies have shown that obese men are more likely to have deadly prostate cancer, yet diagnostic tools like PSA are less likely to accurately predict the presence of cancer in this population," said Mark Garzotto, M.D., an Oregon Health & Science University Cancer Institute researcher presented the study at the 2006 annual meeting of the American Society of Clinical Oncology.

PSA is a protein made only by prostate cells. Certain prostate conditions, including prostate cancer, are associated with high levels of PSA in the blood. However, obesity or high body mass index (BMI) has been tied to lower PSA levels. BMI measures weight in relation to height.

Compounding this diagnostic difficulty is that increasing BMI leads to increasing prostate size, but, again, lower PSA. Larger prostates produce more PSA in men with normal BMI's, not less.

"This is a perfect storm in the diagnostic world," said Garzotto, who has studied and developed more accurate methods of diagnosing prostate cancer. Today, just one in four men who receive prostate biopsies after positive screening tests for cancer actually are diagnosed with the disease. "We needed a better understanding of how BMI affects prostate cancer diagnosis and what can be done about it," he said.

For this study, Garzotto and his colleagues conducted a retrospective study of clinical data for 647 men with a PSA of 10 or greater and who had an ultrasound-guided prostate biopsy. They analyzed age, BMI, digital rectal exam, PSA, prostate volume, PSA density (PSA divided by prostate volume), hypoechoic lesions found in ultrasound examinations, and cancer on biopsy.

Researchers found that in cases with men whose BMI was 25 or above, a lower PSA density (PSAD) calculation needed to be used to accurately predict who had cancer. "This is important because the tendency for obese men to have lower PSA values and higher prostate volumes lead to a lower overall PSAD," Garzotto said.

"We discovered that one set of rules apply to men with low to normal body mass index and another set of rules apply to men with high body mass index," Garzotto said. "This new risk factor beats all the prior predictors in a race and, importantly, it helps us provide appropriate care for the 66 percent of men in the United States who are overweight or obese."

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