Mammography, the boob-smooshing imaging technique used to detect breast cancer, has an overdiagnosis problem. Doctors have long known that some portion of the tumors revealed by the scans might never become life-threatening—but they haven't been able to discern harmless growths from those that grow and spread. Finally, though, researchers have learned which cancers account for the majority of problematic diagnoses—and their work suggests mammograms are better at catching innocuous tumors than deadly ones.

In a paper published Wednesday in The New England Journal of Medicine, Yale University scientists analyzed invasive tumor data from hundreds of thousands of breast cancer patients nationwide. The researchers divided the tumors according biological features—how closely they resembled normal breast cells and whether they had certain hormone receptors. Turns out those features could predict whether a small tumor would grow into a big one. Most don't. And those that do become problematic grow so quickly that mammograms rarely identified them before patients could feel a lump.

"For 100 years we thought that small cancers had a better prognosis because we caught them earlier," says surgeon and study co-author Donald Lannin. "But it turns out small cancers have better outcomes because they're fundamentally different in their composition."