In 2015, the American Cancer Society revised its breast cancer screening guidelines to recommend women start yearly mammograms at age 45, instead of 40. After age 55, a mammogram every other year is OK for most women, the organization states. The changes revived the controversy that’s been swirling for some time about the benefits and concerns of mammography.

Study results published recently in The New England Journal of Medicine add to the conversation. The researchers looked at records of breast cancer diagnoses from 1975 through 2012. The data showed a significant jump in the number of small cancerous tumors detected via mammography. But over 30-plus years, no sizable reduction in the number of large cancerous tumors was detected.

As the researchers point out, if regular mammograms were useful for preventing growth of large cancerous tumors, as experts said they would be, then the data would show that. Accompanying the increase in small tumors found, there should be a notable drop in the number of large tumors. This would happen because tumors would be caught earlier, and treated before they grew large.

Absent such a trend, the study authors say most of the small tumors identified via mammography would likely never have grown large or caused any symptoms. The researchers referred to these cases as “overdiagnosed.”

Overdiagnoses cause much distress for patients and their loved ones, and also can lead to unnecessary treatment including radiation, chemotherapy and surgery, all of which can have long-lasting negative effects.

More study is needed, but for now, the debate over the best use of this important diagnostic tool is likely to continue.