Mammography screening leads to a "substantial amount of over diagnosis" of nonfatal breast cancer, researchers reported.
A retrospective analysis in Norway found 6 to 10 cases of over diagnosis for every 2,500 women invited to have mammographic screening, according to Mette Kalager, MD, of Harvard School of Public Health in Boston, and colleagues.
In other words, the screening found a relatively large number of cancers that would not have become clinically significant during the woman's lifetime, Kalager and colleagues concluded in the April 3 issue of Annals of Internal Medicine.
Mammograms increase the incidence of cancer first by detecting tumors earlier than they would have been otherwise found, and second by picking up tumors that would never have been clinically apparent in the woman's life, the researchers noted.
In the second case, "the woman would be diagnosed and treated with no possible survival benefit."
But gauging the extent of over diagnosis is difficult and estimates have ranged from 0% to 54%, they noted.
Among the difficulties are disentangling other factors that might affect breast cancer incidence and accounting for lead time, or the amount of time that screening advances a diagnosis, compared with clinical methods.
To help clarify the issue, they took advantage of a nationwide breast screening program in Norway that was rolled out on a regional basis over about nine years, starting in 1996. They compared invasive breast cancer incidence with and without screening.
By 2005, all women in Norway, ages 50 to 69, were invited to have a mammogram every two years and, overall, 77% of eligible women have taken part, the researchers reported.
Using data from the public health system and the government statistics branch, Kalager and colleagues examined figures for invasive breast cancer from Jan. 1, 1986 to Dec. 31, 2005, enabling them to compare periods before and after the screening program started.
In that time, 39,888 women were diagnosed with a first case of invasive breast cancer, including 28.6% diagnosed after introduction of the screening program.
They used two different analytic approaches to minimize the effects of confounders, such as lead time.
In the first approach, attempting to control for temporal trends, they estimated overdiagnosis to be 18% in the region where the screening had been in place the longest. Over the whole country, including some regions with short follow up, they estimated over-diagnosis to be 25%
In the second approach, attempting to control for lead time, the estimated over diagnosis ranged from 15% to 20%, Kalager and colleagues reported.
They cautioned that the study was retrospective and therefore could not eliminate all potential confounding factors. As well, unlike most studies of the type, it excluded ductal carcinoma in situ (DCIS). Over diagnosis is an important issue for patients with DCIS, but there are fundamental methodological differences in the approach to estimate over diagnosis of invasive cancer versus DCIS, the authors said.
Nonetheless, they concluded, the findings suggest that for every 2,500 women invited to take part in biennial mammography over a 10-year period, 6 to 10 were over diagnosed.
On the other hand, they estimated, 20 women were diagnosed with breast cancer but not overdiagnosed, and one death from breast cancer was prevented.
However, focusing on the exact extent of over diagnosis is missing the point, according to Joann Elmore, MD, of the University of Washington in Seattle, and Suzanne Fletcher, MD, of Harvard Medical School in Boston.
"It is time to agree that any amount of over diagnosis is serious and to start dealing with this issue now," they argued in an accompanying editorial.
In the long run, that will need better tools to distinguish between cancers that need intervention and those that can be safely observed over time, they argued, but "we cannot wait for these tools to be developed."
Mammographers should consider changing the threshold at which an observed feature is called abnormal. "Evaluating strategies for observing change in some lesions over time instead of recommending an immediate biopsy has been suggested," they pointed out.
While that may be a "tough sell" to women, they concluded, that "unless serious efforts are made to reduce the frequency of over diagnosis, the problem will probably increase."
There is also an ethical obligation, they argued, to alert women to the issue and explain it clearly.
The study had support from the Norwegian Research Council and Frontier Science. Kalager and colleagues did not report any conflicts.
The editorial authors did not report any financial links with industry.
Primary source: Annals of Internal Medicine
Source reference:
Kalager M, et al "Over diagnosis of invasive breast cancer due to mammography screening: results from the Norwegian Screening Program" Ann Intern Med 2012; 156: 491-499.
Additional source: Annals of Internal Medicine
Source reference:
Elmore JG, Fletcher SW "Over diagnosis in breast cancer screening: time to tackle an under appreciated harm" Ann Intern Med 2012; 156: 536-537.
This article is By Michael Smith, North American Correspondent, MedPage Today
Published: April 02, 2012
Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, RN, BC-ADM, Nurse Planner
