More than half of the women who have annual mammograms starting at age 40 will be called back for more testing because of false positives, a new U.S. study suggests.

The study in Monday's online issue of the Annals of Internal Medicine looked at 169,456 women who had their first mammogram in their 40s or 50s and 4,492 women diagnosed with invasive breast cancer. The researchers said they hope that if women know what to expect with screening, they'll feel less anxiety if they are called back for more tests. Andreas Rentz/Getty

The goal of mammography screening is to detect breast cancer when it is localized and curable to prevent advanced disease and breast cancer deaths.

After a decade of annual screening, a majority of women will receive at least one false-positive result. Another seven to nine per cent will be advised to get a breast biopsy but will ultimately not be diagnosed with breast cancer — a false positive biopsy recommendation that can lead to unnecessary pain and scarring, Karla Kerlikowske, a professor of medicine at the University of California, San Francisco and her co-authors said.

The researchers said that screening every other year would likely lessen the probability of false-positive results but could also delay cancer diagnosis.

Those screened every two years did not appear much more likely to be diagnosed with late-stage cancer compared with those screened annually.

"We conducted this study to help women know what to expect when they get regular screening mammograms over the course of many years," said study leader Rebecca Hubbard, an investigator at Group Health Research Institute of Seattle.

"We hope that if women know what to expect with screening, they'll feel less anxiety if — or when — they are called back for more testing. In the vast majority of cases, this does not mean they have cancer," she added in a statement.

The researchers also stressed the importance of radiologists being able to review a patient's previous mammograms because it may halve the odds of a false-positive recall.

Digital mammograms comparison

Screening every year instead of every two years may lead to greater detection of small, non-life-threatening cases of cancer, Dr. Philippe Autier of International Prevention Research Institute in Lyon, France, said in a journal editorial accompanying the study.

"In summary, current evidence indicates that mammography screening every year is less efficient than screening every two years or more," Autier said.

All provincially organized screening programs in Canada encourage women aged 50 to 69 with no symptoms to have regular screening mammograms. The frequency depends on a woman's individual risk factors, such as family history.

Dr. Robert Smith, director of cancer screening for the American Cancer Society, said the study should have more precisely defined intervals. The study's authors called annual screening an interval of nine to 18 months, and biennial screening, 19 to 30 months.

"A false positive is commonly discussed as if it were a catastrophic event. For the large majority of women, it isn't," and surveys say women will accept the risk in return for finding cancer early, Smith said.

A second study in the same issue compared traditional and digital mammography for nearly 330,000 women in the U.S. between the ages of 40 and 79.

For every 10,000 women aged 40 to 49 who are given digital mammograms, two more cases of cancer will be found, researchers said. There would also be another 170 false-positive results, they said.

Digital mammography was more sensitive for detecting breast cancer in pre- or perimenopausal women, mainly in women whose breasts are extremely dense, and for detecting estrogen receptor-negative cancer, Autier said.

The key question is whether digital mammography is better at predicting advanced disease than film mammography, he said.

"Given that cancer rates were similar for film-based and digital mammography, these data suggest that digital technology is probably no better than film-based mammography for preventing advanced disease, and thus for reducing the risk for death from breast cancer," Autier concluded.

With files from The Associated Press