MONDAY, April 2 (HealthDay News) -- As many as one-quarter of breast cancers identified through routine mammography are "overdiagnosed," according to a new study that could reignite the debate about screening guidelines.

Overdiagnosis refers to cancers that are too small to be detected by means other than a mammogram and would not become lethal in a woman's lifetime.

"Overdiagnosis and unnecessary treatment of nonfatal cancer creates a substantial ethical and clinical dilemma and may cast doubt on whether mammography screening programs should exist," said lead author Dr. Mette Kalager, a researcher at the Telemark Hospital in Norway and a visiting scientist at Harvard School of Public Health in Boston. "This dilemma can be reduced only when potentially fatal cancer that requires early detection and treatment can be reliably identified."

Until then, Kalager said, "women eligible for screening need to be comprehensively informed about the risk for overdiagnosis."

But other experts familiar with the study, which is published in the April 3 issue of Annals of Internal Medicine, said no one can accurately determine which tumors will and will not progress.

When and how often a woman should have mammography -- an X-ray of the breast -- was widely debated after the U.S. Preventive Services Task Force issued new recommendations in 2009. The task force suggests that women aged 50 to 74 at average risk have a mammogram every two years. It recommends that women 40 to 49 at average risk discuss the pros and cons of screening with their doctors and decide on an individual basis when and if to start in their 40s.

Other organizations, including the American Cancer Society, recommend women begin mammogram screenings at 40 and repeat them annually.

The task force reasoned that between ages 40 and 50, the risk of anxiety-provoking false positive results outweighed the benefits gained from routine screening.

In the current study, Kalager analyzed data from nearly 40,000 Norwegian women with invasive breast cancer, about 8,000 of whom were diagnosed after mammography screening was introduced county by county starting in 1996. The information was gathered through the Norwegian Breast Cancer Screening Program and included women aged 50 through 69. The researchers compared the number of breast cancers in women in counties offering the screening with those in counties not offering it.

The study authors theorized that if mammogram screening helps, it would lead to a decrease in late-stage breast cancers.

But that was not found. Instead, the investigators found that from 1996 to 2005, the incidence of invasive breast cancer increased 18 to 25 percent among the age groups invited to screenings.

The researchers estimated that from 15 to 25 percent of the women were overdiagnosed. The estimates varied depending on the length of follow-up.

The study didn't include ductal carcinoma in situ, an early form of breast cancer. Kalager said that would have boosted the percent of overdiagnosed women higher, as she said the lifetime risk of progressing from this early stage to invasive cancer is unknown, but probably is less than 50 percent.

Overdiagnosis probably occurs more often in the United States than in Norway because U.S. women generally start screening at 40, whereas 50 is the standard start time for Norwegian women, according to an accompanying journal editorial.

"For every life you prevent from breast cancer death, you are harming six to 10 women with overdiagnosis," Kalager said. Women who decide to go for screening, she said, have to accept these possible harms.

However, Kalager conceded that some guesswork is involved. "We cannot distinguish the lethal cancers from the slow or non-progressive cancers, so we do not know for sure," she noted.

Once a woman is diagnosed, she said, she would recommend treatment unless the woman is part of a clinical study.

Two experts not involved in the study took issue with the findings.

"It's too early to discuss the concept of overdiagnosis because science can't accurately predict which tumors are harmless from the ones that are more aggressive or deadly," said Dr. Kristin Byrne, chief of breast imaging at Lenox Hill Hospital in New York City.

Byrne's advice to 40-plus women? "Don't stop getting yearly mammograms."

Judith Malmgren, affiliated professor of epidemiology at the University of Washington School of Public Health and Community Medicine in Seattle, also objected to the authors' conclusions.

"I don't like the term overdiagnosis," Malmgren said. "A clinician would be hard-pressed to call a diagnosis of invasive breast cancer overdiagnosis."

Malmgren also finds the study methods flawed. For instance, she said, comparing screened and unscreened women would have been better than conducting a county to county comparison.

More information

To learn more about mammograms, visit the U.S. National Cancer Institute.