NEW YORK - For years, women have been urged to get screened for breast cancer because the earlier it's found, the better. Now researchers are reporting more evidence suggesting that's not always the case.

A study in Norway estimates that between 15 and 25 percent of breast cancers found by mammograms wouldn't have caused any problems during a woman's lifetime, but these tumors were being treated anyway. Once detected, early tumors are surgically removed and sometimes treated with radiation or chemotherapy because there's no certain way to figure out which ones may be dangerous and which are harmless.

"When you look for cancer early and you look really hard, you find forms that are ultimately never going to bother the patient," said Dr. H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, who was not part of the research. "It's a side effect of early diagnosis."

The study is the latest to explore overdiagnosis from routine mammograms - finding tumors that grow so slowly or not at all and that would not have caused symptoms or death. Previous estimates of the problem have varied.

The researchers took advantage of the staggered decade-long introduction of a screening program in Norway, starting in 1996. That allowed them to compare the number of breast cancers in counties where screening was offered with those in areas that didn't yet have the program. Their analysis also included a decade before mammograms were offered.

They estimated that for every 2,500 women offered screening, one death from breast cancer will be prevented but six to 10 women will be overdiagnosed and treated.

Study leader Dr. Mette Kalager and other experts said women need to be better informed about the possibility that mammograms can pick up cancers that will never be life-threatening when they consider getting screened. The dilemma is that doctors don't have a good way of telling which won't be dangerous.

"Once you've decided to undergo mammography screening, you also have to deal with the consequences that you might be overdiagnosed," said Kalager, a breast surgeon at Norway's Telemark Hospital and a visiting scientist at Harvard School of Public Health. "By then, I think, it's too late. You have to get treated."

Kalager and her colleagues looked only at invasive breast cancer. The study did not include DCIS, or ductal carcinoma in situ - an earlier stage cancer confined to a milk duct.

Under the Norway program, screening was offered every two years to women ages 50 to 69.

Researchers analyzed nearly 40,000 breast cancer cases, including 7,793 that were detected after routine screening began. They estimated that between 1,169 and 1,948 of those women were overdiagnosed and got treatment they didn't need.

Their findings appear in Tuesday's Annals of Internal Medicine.

The problem of overdiagnosis has been long recognized with prostate cancer. Darthmouth's Welch said it's also a problem in thyroid and lung cancer, a childhood tumor called neuroblastoma and even melanoma. He considers breast cancer screening a close call.

"The truth is that we've exaggerated the benefits of screening and we've ignored the harms," he said. "I think we're headed to a place where we realize we need to give women a more balanced message: Mammography helps some people but it leads others to be treated unnecessarily."

An editorial published with the study said overdiagnosis probably occurs more often in the United States because American women often start annual screening at an earlier age and radiologists in the U.S. are more likely to report suspicious findings than those in Europe.

Radiologists could help by raising the threshold for noting abnormalities, wrote Dr. Joann Elmore of the University of Washington School of Medicine and Dr. Suzanne Fletcher of Harvard Medical School.

A "watch-and-wait" approach has been suggested instead of an immediate biopsy, but the editorial writers acknowledge that could be a "tough sell" for some women and radiologists alike.

They said most women aren't aware of the possibility of overdiagnosis.

"We have an ethical responsibility to alert women to this phenomenon," they wrote.