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." [...]

Until recently, the prevailing theory among cancer researchers was that most breast cancers have a lead time of three to four years, give or take. (Lead time being the time between when a mammogram can discern a tumor, and when that tumor becomes problematic.) But Lannin's study showed that most aggressive cancers progress to life-threatening within just a year or two. On the other hand, a large proportion of small cancers grow so slowly that they have a lead time of as long as 20 years. Since breast cancer is most frequently diagnosed among women aged 55 to 64, that means that some patients may never actually get cancer. These women could avoid the expense and side effects of chemotherapy, hormone therapies, and mastectomies, all of which remain blunt instruments of disease control.