Karsten Jorgensen and Peter Gotzsche of the Nordic Cochrane Centre in Copenhagen analyzed breast cancer trends at least seven years before and after government-run screening programs for breast cancer started in parts of Australia, Britain, Canada, Norway and Sweden.
The research was published Friday in the BMJ, formerly known as the British Medical Journal. Jorgensen and Gotzsche did not cite any funding for their study.
Once screening programs began, more cases of breast cancer were inevitably picked up, the study showed. If a screening program is working, there should also be a drop in the number of advanced cancer cases detected in older women, since their cancers should theoretically have been caught earlier when they were screened.
However, Jorgensen and Gotzsche found the national breast cancer screening systems, which usually test women aged between 50 and 69, simply reported thousands more cases than previously identified.
Overall, Jorgensen and Gotzsche found that one third of the women identified as having breast cancer didn't actually need to be treated.
Some cancers never cause symptoms or death, and can grow too slowly to ever affect patients. As it is impossible to distinguish between those and deadly cancers, any identified cancer is treated. But the treatments can have harmful side-effects and be psychologically scarring.
"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."
Jorgensen said that for years, women were urged to undergo breast cancer screening without them being informed of the risks involved, such as having to endure unnecessary treatment if a cancer was identified, even if it might never threaten their health.
Doctors and patients have long debated the merits of prostate cancer screening out of similar concerns that it overdiagnoses patients. A study in the Netherlands found that as many as two out of every five men whose prostate cancer was caught through a screening test had tumors too slow-growing to ever be a threat.
"Mammography is one of medicine's 'close calls,' ... where different people in the same situation might reasonably make different choices," wrote H. Gilbert Welch of VA Outcomes Group and the Dartmouth Institute for Health Policy and Research, in an accompanying editorial in the BMJ. "Mammography undoubtedly helps some women but hurts others."
Experts said overtreatment occurs wherever there is widespread cancer screening, including the U.S.
Britain's national health system recently ditched its pamphlet inviting women to get screened for breast cancer, after critics complained it did not explain the overtreatment problem.
Laura Bell of Cancer Research UK said Britain's breast cancer screening program was partly responsible for the country's reduced breast cancer cases.
"We still urge women to go for screening when invited," she said, though she acknowledged it was crucial for women to be informed of the potential benefits and harms of screening.