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Model Introduced to Predict Life-Saving Benefits of
Mammograms at Different Frequencies
Annual screening for senior citizens would reduce
mortality by 37 percent
Feb. 20, 2005 – Researchers today presented a
mathematical tool that predicts the lives saved by various frequencies
of mammograms to detect cancers at early stages. Annual screening from
age 50 to 79 of women with average breast cancer risk would reduce
mortality by 37 percent, they predict.
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With screening guidelines and financial coverage
varying among health systems and insurers – sometimes dramatically – the
model provides quantitative predictions of the mortality benefits, on
average, in populations of women over the course of 40 years, say the
researchers from the Dana-Farber Cancer Institute in Boston.
"We're not advocating any particular interval for
mammography screening," says Sandra Lee, ScD, a biostatistician at
Dana-Farber who developed the model along with Marvin Zelen, PhD, of
Dana-Farber and the Harvard School of Public Health. "This is a
preliminary tool to show policymakers the kind of information they can
draw on to help them make decisions."
Lee will describe the development of the
mathematical model, which made use of data from several past clinical
trials of mammography screening and from cancer databases, in a
presentation at the annual meeting of the American Association for the
Advancement of Science on Sunday, Feb. 20, 8:30 am (Marriott Wardman
Park Hotel, Lobby Level, Maryland Suite C). She also will present that
data at a press briefing later that day at 2 pm (Marriott Wardman Park
Hotel, Mezzanine Level).
The mathematical tool generates comparative
information that's impossible to obtain in the real world, say the
scientists, because clinical trials would require hundreds of thousands
of volunteers following a variety of schedules over many years to
demonstrate small mortality differences – and would be prohibitively
expensive. Moreover, adds Lee, such trials would be ethically
questionable because of the need for unscreened control groups.
At present, American Cancer Society guidelines
recommend that women age 40 and older have a screening mammogram every
year and that they "should continue to do so for as long as they are in
good health."
But payors differ in their coverage for the tests:
in Great Britain, said Zelen, the National Health System pays for
mammograms only at three-year intervals and doesn't cover any screening
whatsoever for women younger than 50, when the incidence of breast
cancer is lower and mammograms are effective.
The model can be helpful to women, he said, by
eliminating unnecessary screening exams when the chance of detecting an
unknown breast cancer is too low to warrant them.
"It's clear that the more mammograms you give, the
more able you are to locate disease that a person didn't know about,"
Zelen says. But, testing with increasing frequency has diminishing
returns, while boosting the odds of "false positives" that can be
traumatic to women and lead to unneeded biopsies that drive up health
costs.
Lee and Zelen, along with Hui Huang, MS, of
Dana-Farber, described the model in 2004 in Statistical Methods in
Medical Research. Among their conclusions:
-
Annual screening from age 50 to 79 of women
with average breast cancer risk would reduce mortality by 37 percent
– compared to 30 percent with screenings every two years, and 26
percent with mammograms every three years.
-
Beginning mammograms at age 40 – when breast
cancer risk is low – rather than at 50, reduces overall risk of
death by five percent because the incidence of cancer in the younger
women is very low. However, Zelen said he believes screening women
between 40 and 50 has merit because their breast cancers are more
aggressive.
-
If women underwent mammograms every two years
beginning at 40 and then annually starting at 50, there would still
be a 33 percent reduction in mortality.
-
Because breast cancer risk increases with age,
an alternative screening schedule derived from the model calls for
fewer mammograms at early ages, but increasingly often later on.
This "threshold" method provides for 18 screenings between 40 and
79, and predicts a mortality reduction of 26 percent.
Women who have a higher breast cancer risk because
of their family history are advised to begin mammography at an early
age. Using the model, say the researchers, health care providers can
determine when to schedule mammograms depending on the amount of a
woman's extra risk.
The model also provides estimates of the relative
costs incurred by screening populations of women at greater or lesser
intervals – an important issue for health policymakers.
Dana-Farber Cancer Institute (www.danafarber.org)
is a principal teaching affiliate of the Harvard Medical School and is
among the leading cancer research and care centers in the United States.
It is a founding member of the Dana-Farber/Harvard Cancer Center (DF/HCC),
designated a comprehensive cancer center by the National Cancer
Institute.
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