Mammography screening results in overdiagnosis of breast cancers that would have been harmless, say researchers who want women to be well informed.

It was hoped that mammography would detect breast cancer at an earlier stage and save lives. Investigators in Norway tested the assumption that mammography leads to a decrease in late-stage, invasive breast cancers.

Mammography did not pan out.

"After 10 years of biennial mammography screening, for every 2,500 women invited, six to 10 women have been overdiagnosed, 20 women are diagnosed with breast cancer who are not overdiagnosed and one death from breast cancer has been prevented," Dr. Mette Kalager of Telemark Hospital in Norway and her co-authors concluded in Tuesday's issue of the Annals of Internal Medicine.

In overdiagnosis, a woman is diagnosed and treated with no possible survival benefit, they said.

Overdiagnosis subjects the patient to treatments such as lumpectomy, radiation and chemotherapy and the fear of breast cancer returning and burdens the health-care system.

Overall, Kalager and her colleagues estimated that 15 per cent to 25 per cent of cases diagnosed by screening were overdiagnosed based on all cases of cancer in the country.

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In overdiagnosis, a woman is diagnosed and treated with no survival benefit. (Eric Gaillard/Reuters)

It opens a huge ethical dilemma that can't be resolved until researchers find a way to tell potentially fatal cancers more reliably.

"In order to find the cancers that do kill we have to remove some of the ones that really we could have left alone," said Martin Yaffe, a senior scientist in imaging research at Sunnybrook Health Sciences Centre in Toronto.

"We can do tremendous good for the few women who actually have cancer that's found early through the screening process."

The alternative to overdiagnosis from screening is waiting until symptoms occur — something that looks or feels different and leads a woman to go to her doctor, said Cornelia Baines, professor emerita of the Dalla Lana School of Public Health at the University of Toronto.

"We are seeing overwhelming evidence, even in the absence of screening women are having a reduced death rate from breast cancer 37 per cent  in 40- to 49-year-olds in Europe," said Baines. "They're not eligible for screening, but they've had a 37 per cent decline in breast cancer deaths. That is amazing. That is due to improved therapy."

Screening in context

The Norwegian study looked at data from 39,888 patients with invasive breast cancer from 1986 to  2005. Of these, almost 8,000 were diagnosed with routine screening.

In the study, attendance at the screening program was high, 77 per cent.

But it was not a randomized trial, and women in different regions of the country may have differed in some way that could have influenced the result, a journal editorial accompanying the research said in pointing out the limitations.

The investigators said the length of the screening wasn't long enough in some areas of Norway but overall they could estimate the degree of overdiagnosis.

The study did not include women diagnosed with ductal carcinoma in situ, a different type of breast cancer. The authors did take use of menopausal hormone therapy into consideration in the analysis.

A recent Nordic Cochrane review and U.S. Preventive Services Task Force panel came to similar conclusions on overdiagnosing breast cancer, Baines said. Last year's new Canadian guidelines also found routine mammography, self-examinations and MRIs had no significant benefit for women aged 40 to 49 at average risk.

Watchful waiting

In their journal editorial, titled "Overdiagnosis in breast cancer screening: time to tackle an underappreciated harm," Dr. Joann Elmore of the University of Washington School of Medicine and Dr. Suzanne Fletcher of Harvard Medical School say better tools are needed to reliably tell which breast cancer will kill without treatment and which ones can be safely watched.

Until then, Elmore and Fletcher suggested evaluating strategies to observe lesions over time instead of recommending an immediate biopsy, a watchful waiting approach that they acknowledged is a tough sell.

Jaqueline McManus of Halifax had a close call when a mammogram showed a suspicious spot that turned out to be fibrous tissue on nerve endings that caused her pain.

"When they get back to you and they want to do more research it is a horrible feeling," McManus recalled. "I gave myself a favour and said I don't need to be under that pressure any more," from mammography screenings.

 The study was funded by the Norwegian Research Council and Frontier Science.

With files from CBC's Kelly Crowe and Pauline Dakin