According to a report in the British Medical Journal (BMJ), one in three women diagnosed with breast cancer in public screening programs are treated unnecessarily as their tumor will not be life-threatening. 

The study by Karsten Jorgensen and Peter Gotzsche of the Nordic Cochrane Centre in Copenhagen analyzed breast cancer trends and highlighted the dilemma that faced doctors when faced with detecting and treating the disease.

Nicknamed "silent killer" of women, for the way it can stealthily claim lives, not all breast cancers kill and in some cases the cancer grows so slowly that the patient will die of other causes before it produces symptoms, or it may remain dormant over the years or even shrink.

Doctors often don’t know if the cancer will be lethal or lie dormant and therefore they err on the side of safety and treat all patients diagnosed with a tumor.

The researchers examined data from screening programs in Australia, Britain, Canada, Norway and Sweden and looked at trends seven years before the programs were implemented, and seven years after.

The study showed that once screening processes were introduced more cases of breast cancer were detected. The researchers said if a screening process is working then there should be a decrease in the number of advance cases that are detected in older women as they cancers should have been detected earlier when they were screened.

The researchers found it was the opposite and the national breast cancer screening systems, which usually test women aged between 50 and 69, simply reported thousands more cases than previously identified.

"One in three breast cancers detected in a population offered organized screening is over-diagnosed," the researchers said. Jorgensen said women were urged to undergo breast cancer screening without them being informed of the risks involved. 

"This information needs to get to women so they can make an informed choice," Jorgensen said. "There is a significant harm in making women cancer patients without good reason."

In an accompanying editorial in the BMJ, H. Gilbert Welch of VA Outcomes Group and the Dartmouth Institute for Health Policy and Research wrote, "Mammography is one of medicine's 'close calls,' ... where different people in the same situation might reasonably make different choices. Mammography undoubtedly helps some women but hurts others."

He added that one study has suggested that one death is avoided for every two women who are "over-diagnosed," while another puts the ratio far higher, at one death avoided for every 10 cases of unnecessary treatment.

Laura Bell of Cancer Research UK said the reason for the country's reduced breast cancer cases partly were Britain's breast cancer screening program.

"We still urge women to go for screening when invited," she said, acknowledging the importance for women to be informed of the potential benefits and harms of screening.