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Screening Tests May Miss Prostate Cancer in Obese Patients

Posted on: Wednesday, 21 November 2007, 06:00 CST

By Rita Rubin

Higher blood volumes probably cause lower concentrations of prostate-specific antigen, or PSA, in obese prostate cancer patients, reports a study today, leading the authors to speculate that screening with PSA tests might miss some cancers in obese men.

Most U.S. prostate cancers are diagnosed by a biopsy prompted by a high PSA, the researchers write in The Journal of the American Medical Association.

"The ability to accurately detect prostate cancer can be compromised by any factor that decreases PSA concentrations," they write.

The study of more than 13,000 men who had undergone prostate cancer surgery found that patients with a body mass index, or BMI, of 35 or greater had PSA concentrations that were 11% to 21% lower than normal-weight patients. A 5-foot-8 man who weighs 230 pounds has a BMI of 35, which is considered moderately obese. A BMI of less than 25 is considered normal.

An alternative explanation for obese men's lower PSA concentrations is their lower testosterone levels. If this were true, the researchers write, obese men would be expected to have lower total amounts of PSA, which is excreted by prostate tumors. However, obese patients in the study had similar or higher total amounts of PSA than normal-weight patients.

Obese prostate cancer patients have a higher risk of dying than normal-weight patients, says senior author Stephen Freedland, an assistant professor of urology and pathology at Duke University. One reason could be that screening is missing some of their early cancers, he says.

Although the link between obesity and lower PSA concentrations remains unproven in men who have not been diagnosed with prostate cancer, Freedland says, he has begun to use a lower PSA cutoff when screening obese patients. While the test's manufacturer recommends a cutoff of 4, Freedland says he is lowering it by 20% -- roughly the difference found in his study -- in men whose BMI is 35 or higher. So instead of 4, he's using 3.2 or 3.3.

Freedland's study "just drives one more nail in the coffin of the concept that one size fits all for PSA," says Bruce Roth, a professor of medicine and urology at the Vanderbilt-Ingram Cancer Center in Nashville.

"There wasn't anything overly scientific about choosing 4.0" as a PSA screening cutoff, says Roth, a board member of the American Society of Clinical Oncology. PSA levels increase as prostates enlarge with age, Roth says, and "there are likely racial differences."

Still, Roth says, it's too soon to start using a lower PSA cutoff when screening obese men. "It's hard to say what you should do in a screening population based on data only in diagnosed patients," he says. If an obese man has no symptoms and a normal-sized prostate, lowering the PSA threshold for performing a biopsy "is stretching it a little bit." (c) Copyright 2005 USA TODAY, a division of Gannett Co. Inc.

Source: USA TODAY

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