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Researchers are again questioning what was once unassailable thinking about breast cancer – that mammograms save lives. They've discovered that the benefits of the screening are modest.

In a study published Wednesday in the New England Journal of Medicine, researchers report the results of a study that followed more than 40,000 Norwegian women between 50 and 69 with breast cancer.

Because Norway introduced mammogram-based screening programs to its 19 counties gradually, some of the women were diagnosed before their county had regular screening, while others were diagnosed after.

The researchers found that women who were screened with a mammogram had a 28 per cent lower risk of death from breast cancer, only a third of that reduction was due to screening itself.

Among women in the screening group, the breast cancer death rate declined by 7.2 deaths per 100,000 people compared with women in the decade before the screening program started. The death rate in the non-screening group fell by 4.8 deaths per 100,000 people compared with its historical counterpart.

That means that mammography reduced mortality by only 2.4 deaths per 100,000 people -- a third of the total risk of death.

"They expected the mortality rate to be reduced by 30 per cent after 10 years. So after 10 years we wanted to see if that was the case. And finding a 10 per cent reduction, instead of 30 per cent reduction was a surprise to us," study co- author Dr. Mette Kalager of the Harvard School of Public Health told Â鶹ӰÊÓ.

Kalager's team found that based on the data they gathered, they estimate that 2,500 women over the age of 50 would need regular mammograms over a decade, to prevent just one woman from dying of breast cancer.

At the same time though, thousands of other women would have received false positives and forced to undergo further invasive testing and biopsy procedures.

"It will help one or two women in a group of 2,500 but it leaves thousands to have unnecessary testing and anxiety," said Dr. Gilbert Welch of Dartmouth Institute for Health Policy, who wrote an accompanying NEJM commentary.

"I think the number needed to screen is high, and the mortality reduction is low," says Kalager.

(It is important to note that the study looked only at screening programs, not at diagnostic mammography, which is used to investigate a breast lump or other abnormality.)

It's a result that some researchers call "uncomfortable," because it questions the value of something promoted as a "health home run," and suggests that mammography may not be as powerful a lifesaving tool as once thought. The World Health Organization estimates that mammograms reduce the breast cancer death rate by 25 per cent in women over 50. Other groups put the figure at 15 to 23 per cent.

Other studies have questioned whether looking for breast cancer in otherwise healthy older women, may turn up small cancers that may not need aggressive treatment.

Kalager believes that the breast cancer landscape has changed in the last 20 years, so that women are more aware of breast cancer and finding breast lumps and small tumours on their own. Moreover, better breast cancer treatment is the most important factor for what's responsible for the drop in deaths.

"Only 10 per cent of one-third of the reduction we see can be attributed to that screening program. The rest is heightened awareness in the public, among doctors and then improved treatment," she said.

Dr. Welch appears to agree, writing: "It is quite plausible that screening mammography was more effective in the past than it is now," he writes. "The increased awareness about the importance of promptly seeking care for overt breast abnormalities… and the widespread use of adjuvant therapy have probably combined to make screening now less important."

Dr. Cornelia Baines with the University of Toronto's Department of Public Health Sciences says while it's good news that overall, breast cancer deaths are declining, this study is yet more evidence that "breast screening programs for women are not doing as much for women as they think they are."

She says women undergoing mammograms need to be better informed about how effective mammograms are, and about the "significant ill effects that may occur" because of a false positive.

The Canadian Cancer Society recommends that all women aged 50 to 69 have a breast exam by a doctor at least every two years and a screening mammogram every two years.

It says women aged 40 to 49 should undergo a clinical breast exam, but should also be able to have mammograms if they are referred by a doctor or a nurse practitioner.

In the U.S., a fierce debate was ignited last year when an influential panel recommended scaling back screening programs to begin at age 50 instead of 40 – putting the guidelines in line with those in Canada and Europe. The advice was rejected by the American Cancer Society and other experts, who worried the change would lead to unnecessary deaths.

Dr. Welch notes that the current advice given to women is unlikely to change overnight and that debate continues.

"Mammograms remain a critical tool for finding out if a breast lump is actually cancer. But its role to find small growths is increasingly coming under scientific scrutiny," he said.

"There's no right answer. It's something all women have to recognize this is a close call and different women can make different decisions."

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip