MONDAY, April 2 (HealthDay News) -- As many as one-quarter of
breast cancers identified through routine mammography are
"overdiagnosed," according to a new study that could reignite the
debate about screening guidelines.
Overdiagnosis refers to cancers that are too small to be
detected by means other than a mammogram and would not become
lethal in a woman's lifetime.
"Overdiagnosis and unnecessary treatment of nonfatal cancer creates a substantial ethical and clinical dilemma and may cast doubt on whether mammography screening programs should exist," said lead author Dr. Mette Kalager, a researcher at the Telemark Hospital in Norway and a visiting scientist at Harvard School of Public Health in Boston. "This dilemma can be reduced only when potentially fatal cancer that requires early detection and treatment can be reliably identified."
Until then, Kalager said, "women eligible for screening need to
be comprehensively informed about the risk for overdiagnosis."
But other experts familiar with the study, which is published in
the April 3 issue of
Annals of Internal Medicine, said no one can accurately determine which tumors will and will not progress.
When and how often a woman should have mammography -- an X-ray
of the breast -- was widely debated after the U.S. Preventive
Services Task Force issued new recommendations in 2009. The task
force suggests that women aged 50 to 74 at average risk have a
mammogram every two years. It recommends that women 40 to 49 at
average risk discuss the pros and cons of screening with their
doctors and decide on an individual basis when and if to start in
Other organizations, including the American Cancer Society,
recommend women begin mammogram screenings at 40 and repeat them
The task force reasoned that between ages 40 and 50, the risk of
anxiety-provoking false positive results outweighed the benefits
gained from routine screening.
In the current study, Kalager analyzed data from nearly 40,000
Norwegian women with invasive breast cancer, about 8,000 of whom
were diagnosed after mammography screening was introduced county by
county starting in 1996. The information was gathered through the
Norwegian Breast Cancer Screening Program and included women aged
50 through 69. The researchers compared the number of breast
cancers in women in counties offering the screening with those in
counties not offering it.
The study authors theorized that if mammogram screening helps,
it would lead to a decrease in late-stage breast cancers.
But that was not found. Instead, the investigators found that
from 1996 to 2005, the incidence of invasive breast cancer
increased 18 to 25 percent among the age groups invited to
The researchers estimated that from 15 to 25 percent of the
women were overdiagnosed. The estimates varied depending on the
length of follow-up.
The study didn't include ductal carcinoma in situ, an early form
of breast cancer. Kalager said that would have boosted the percent
of overdiagnosed women higher, as she said the lifetime risk of
progressing from this early stage to invasive cancer is unknown,
but probably is less than 50 percent.
Overdiagnosis probably occurs more often in the United States
than in Norway because U.S. women generally start screening at 40,
whereas 50 is the standard start time for Norwegian women,
according to an accompanying journal editorial.
"For every life you prevent from breast cancer death, you are harming six to 10 women with overdiagnosis," Kalager said. Women who decide to go for screening, she said, have to accept these possible harms.
However, Kalager conceded that some guesswork is involved. "We
cannot distinguish the lethal cancers from the slow or
non-progressive cancers, so we do not know for sure," she
Once a woman is diagnosed, she said, she would recommend
treatment unless the woman is part of a clinical study.
Two experts not involved in the study took issue with the
"It's too early to discuss the concept of overdiagnosis because science can't accurately predict which tumors are harmless from the ones that are more aggressive or deadly," said Dr. Kristin Byrne, chief of breast imaging at Lenox Hill Hospital in New York City.
Byrne's advice to 40-plus women? "Don't stop getting yearly
Judith Malmgren, affiliated professor of epidemiology at the
University of Washington School of Public Health and Community
Medicine in Seattle, also objected to the authors' conclusions.
"I don't like the term overdiagnosis," Malmgren said. "A clinician would be hard-pressed to call a diagnosis of invasive breast cancer overdiagnosis."
Malmgren also finds the study methods flawed. For instance, she
said, comparing screened and unscreened women would have been
better than conducting a county to county comparison.
To learn more about mammograms, visit the
U.S. National Cancer Institute.