LONDON: The
research, published Tuesday in the British Medical Journal, is the
latest in a series of studies that question the value of annual breast
X-rays for pre-menopausal women and whether too many women are being
"overdiagnosed" by the popular test.
"We found
absolutely no benefit in terms of reduction of deaths from the use of
mammography," said study leader Dr. Anthony Miller, an epidemiologist at
the University of Toronto's Dalla Lana School of Public Health.
The
controversial finding is unlikely to trigger an immediate change in
national screening policies, although it will enliven an already heated
debate over screening. Experts have been arguing the merits of breast
X-rays since 2009, when a government panel recommended that most women
under 50 could safely skip the test. The U.S. Preventive Services Task
Force determined that the chances a 40-year-old woman would be diagnosed
with invasive breast cancer in the next 10 years was 1.44% and that her
odds of dying from it were just 0.19%.
However, the
breast cancers that strike women in their 40s are often more aggressive,
and they account for about 17% of deaths from the disease, according to
the American Cancer Society.
The ACS and the American
College of Obstetricians and Gynecologists recommend annual mammograms
for women beginning at age 40, and the National Cancer Institute advises
women in their 40s to have the test once every year or two. The
Canadian Task Force on Preventive Health Care advises women to have
mammograms every two to three years between the ages of 50 and 74.
The
British Medical Journal report, based on data from the Canadian
National Breast Screening Study, argues that mammography all too often
finds small cancers that would never become dangerous if left alone.
Roughly half of all cancers found by mammography — yet undetected
through physical examination — fell into this category, the study
authors wrote.
The researchers examined the medical
records of 89,835 women in six Canadian provinces between the ages of 40
and 59. All of the trial participants received annual physical breast
examinations, while half of them also had yearly mammogram screenings
for five years, beginning in 1980.
Over the next 25 years,
3,250 of the 44,925 women in the mammography arm of the study were
diagnosed with breast cancer, along with 3,133 of the 44,910 women in
the control group. In addition, 500 patients in the mammography group
died of breast cancer, as did 505 women in the control group.
The
researchers found that women who got mammograms were more likely to be
diagnosed with breast cancer, but that the test did not reduce their
risk of dying from the disease.
The research team
calculated that 22% of the cancers found on mammograms were
overdiagnosed. That means that for every 424 women who were screened,
one received unnecessary cancer treatment.
The study did not address the use of mammography as a diagnostic tool, which most experts agree is valuable.
In
light of their findings, Miller and his colleagues concluded, "The
rationale for screening by mammography should be urgently reassessed by
policymakers."
The American College of Radiology, one of
the leading critics of the task force recommendations, was quick to
denounce the study's conclusions. The group said in a statement that the
Canadian National Breast Screening Study was "deeply flawed" and
"incredibly misleading." Among other problems, the study relied on
"second-hand" mammography equipment that was operated by poorly trained
technicians, the group said.
"It would be an outrage for
women if access to screening was curtailed because of the poor results
in the Canadian National Breast Screening Study," said Dr. Daniel
Kopans, a senior breast imager at Massachusetts General Hospital in
Boston. "It has been known for years that the trial was compromised from
the start."
The study authors said they stood by their
conclusions and challenged the critics to produce data showing that
mammograms reduced deaths. Other recent studies have found that advances
in breast cancer treatment have eroded some of the benefits of early
detection.
"Modern treatment is so much more effective now
that the lead time gained by mammography has little impact on the
outcome," Miller said.
Dr. H. Gilbert Welch, an
epidemiologist and biostatistics professor at Dartmouth College's Geisel
School of Medicine, said the study offered the highest-quality evidence
yet on the prevalence of overdiagnosis.
"I think there's
growing realization that all is not well with mammography," said Welch,
who co-wrote the book "Overdiagnosed: Making People Sick in the Pursuit
of Health." "People in the cancer community and the cancer surgery
community are aware of the problem of overdiagnosis. They're aware that
mammography was oversold, that its benefits were exaggerated and its
harms were kind of downplayed."
In an editorial that
accompanied the study, three breast cancer experts from the University
of Oslo who have studied the effects of screening in Europe said Miller
and his colleagues made a convincing case that current policies should
be reconsidered.
"This is not an easy task, because
governments, research funders, scientists and medical practitioners may
have vested interests in continuing activities that are well
established," they wrote.
Similar studies on mammography
screening have been conducted in Europe, and critics have said that they
don't apply to American women. The Canadian researchers said that their
results are highly pertinent to the United States, and that their study
is probably the largest we will ever see.
"Many people
believe you do not adopt policy on the result of one trial, and yet
there's not likely to be another trial like this," Miller said. "It
takes too long."
