It’s hard to wrap your head around the notion that routine mammography may, in the grand scheme of things, do more harm than good. But a study published Monday in the Annals of Internal Medicine adds to a growing body of evidence that the potential risks of routine breast-cancer screening via mammography might in fact outweigh such screening’s benefits.

A woman undergoes a mammogram. (Enrique Castro-Mendivil/Reuters)

The study included nearly 40,000 women with invasive breast cancer, nearly 8,000 of them diagnosed after routine screening was instituted. Through complicated calculations, the researchers determined that between 15 percent and 25 percent of those diagnoses fell into the category of overdiagnosis — the detection of tumors that would have done no harm had they gone undetected.

The case for routine mammography is grounded in the twin beliefs that screening can catch breast cancer at its earliest stages and that cancer detected early can be more successfully treated than cancer found later. But recent science has suggested it’s not as simple as that. Some breast cancers, in some patients, may grow so slowly that they can be left untreated or monitored through watchful waiting, for instance. When treatment for such a cancer is triggered by a mammogram finding, that treatment counts as unnecessary and therefore as a negative outcome. The authors note that given differences between the way mammograms are interpreted here versus in Norway, the percentage of cases representing overdiagnosis is likely even higher in the United States.

The problem is that nobody yet knows how to predict which cancers can be left untreated and which will prove fatal if untreated. So for now the only viable approach is to regard all breast cancers as potentially fatal and treat them with surgery, radiation, chemotherapy or a combination of approaches, none of them pleasant options and all with potential to do damage of their own.

The study’s authors conclude that “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. This dilemma can be reduced only when potentially fatal cancer that requires early detection and treatment can be reliably identified. Until then, women eligible for screening need to be comprehensively informed about the risk for overdiagnosis.”

An editorial commenting on the research suggests that, “Instead of focusing on the exact extent of overdiagnosis, it is time to agree that any amount of overdiagnosis is serious and to start dealing with this issue now. Ultimately, better tools are needed to reliably identify which breast cancer will be fatal without treatment and which can be safely observed over time without intervention, but we cannot wait for these tools to be developed.” The editorial urges that women faced with the prospect of mammography be fully informed as to both the potential risks and the likely benefits of such screening.

Recommendations for breast-cancer screening in the United States vary: The U.S. Preventive Services Task Force recommends women ages 50 and up have mammograms every other year, while the American Cancer Society calls for annual mammograms starting at age 40.