Obesity Affects Prostate Cancer Test
For Big Men, Smaller PSA Scores May Mean Prostate Cancer
Nov. 20, 2007 -- Obese men may
have prostate cancer despite relatively low values on a
prostate cancer screening test called
the PSA test, a new study suggests.
The PSA test detects PSA --
prostate-specific antigen -- in the blood. Only prostate cells give off PSA.
PSA levels can rise as prostate tumors grow.
Earlier studies showed that obese
men have slightly lower PSA concentrations than lean men do. It's not clear why
this is so. New findings from Duke University researcher Stephen J. Freedland,
MD, and colleagues now suggest an answer.
"Being a big guy, you have
enormous blood volume, so PSA is diluted," Freedland tells WebMD.
"Obese men having these lower PSA values due to the dilution factor means
we are going to miss some of them early on."
The researchers reviewed medical
records from some 14,000 men with prostate cancer who had their prostate glands
removed. Based on the men's body mass index, and adjusting for differences in the
men's clinical conditions and prostate pathology, the researchers used
pre-operative PSA test values to calculate the actual amount of PSA in the
They found that obese men had
lower PSA values than did non-obese men even though they had equal or even
higher amounts of PSA in their blood.
"What this suggests is if we
use the same PSA threshold for obese and normal-weight men, we may be missing
some cancers," Freedland says. "For example, a PSA score of 4.1 in an
obese man would be diluted down to a 3.3."
Alan R. Kristal, DrPH, one of the
researchers investigating the link between obesity and prostate cancer, strongly disagrees with
Freedland's calculation. Kristal is associate head of the cancer prevention
program at Seattle's Fred Hutchinson Cancer Research Center.
"This is an
over-interpretation of the data. Most studies show the difference is much
smaller," Kristal tells WebMD. "Doctors will look not just at the PSA
level but the speed of PSA increase over time, the PSA relative to other men
the same age, findings from a digital rectal exam, and other information before
they make a decision on whether to do biopsy. These teeny-tiny differences due
to obesity have no impact."
It's an important dispute to
resolve. As Freedland points out, there soon will be other blood tests for
other kinds of cancer.
"This issue is something we
need to keep in mind as we move forward with blood-based cancer screening,"
Freedland and colleagues report
their findings in the Nov. 21 issue of The Journal of the American Medical