A long-term study casts doubt over the effectiveness of annual mammograms after it found screenings don't reduce breast cancer deaths more than a normal physical.

The study of nearly 90,000 Canadian women over 25 years found mammograms may not only be useless, but harmful.

Not only did mammograms not reduce the death rate among women aged 40 to 59, more than one fifth of the cancers found by the screening were not a threat to the woman’s health. 

"All over-diagnosis results in unnecessary surgery, unnecessary chemotherapy, unnecessary radiotherapy and unnecessary hormonal therapy," said study author Dr. Cornelia Baines, a retired professor at the University of Toronto’s Dalla Lana School of Public Health.

"Twenty-two per cent of women with screen-detected invasive breast cancer got treatment that they really didn't need," she said.

Study authors are calling for a reassessment of the rationale for mammogram screenings.

"This is not an easy task, because governments, research funders, scientists, and medical practitioners may have vested interests in continuing activities that are well-established," said the editorial accompanying the study in the British Medical Journal.

The American College of Radiology called the Canadian National Breast Screening Study “incredibly misleading” and “deeply flawed,” citing poorly trained technicians and “second-hand” mammography machines.

Sources: CBC, Newser