London ? One in three breast cancer patients identified in public screening programs may be treated unnecessarily, an analysis of cancer trends in five countries suggests.

The researchers said it means women need to understand that mammograms can lead to treatment of cancers that may do no harm. However, another expert pointed to several “gold-standard” studies that show lives can be saved by breast cancer screening.

The research by Karsten Jorgensen and Peter Gotzsche of the Nordic Cochrane Centre in Copenhagen was published today in the BMJ, formerly known as the British Medical Journal. They 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.

Once screening programs began, more cases of breast cancer were inevitably picked up, the study showed. If cancer screening is effective, there should be a drop in the number of advanced cancer cases found in older women, since their cancers should theoretically have been caught earlier when they were screened.

However, the study found that national breast cancer screening systems, which usually test women between 50 and 69, simply reported thousands more cases than previously identified.

Overall, the study 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 being told of the risks involved, such as having to endure unnecessary treatment if a cancer was identified, even if it might never threaten their health.

However, Dr. Otis Brawley, chief medical officer for the American Cancer Society, wrote in an e-mail that there is evidence that lives are saved through breast cancer screening.

“We have eight, high quality, randomized prospective clinical trials — the gold standard in screening methodology — that consistently show that screening using mammography and clinical breast exams reduces the risk a woman will die of breast cancer by up to 35 percent,” he said.

He called the methodology of the new analysis “unusual, but even if we accept the estimated rate of over-diagnosis in this study, that rate is probably worth the reduction in the risk of death.”

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 screening 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 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.