Seemingly overnight, treatment of men with early-stage prostate cancer has undergone a sea change. Five years ago, nearly all opted for surgery or radiation; now, nearly half are choosing no treatment at all.

The approach is called active surveillance. It means their cancers are left alone but regularly monitored to be sure they are not growing. Just 10 percent to 15 percent of early-stage prostate cancer patients were being treated by active surveillance several years ago. Now, national data from three independent sources consistently finds that 40 percent to 50 percent of them are making that choice.

In recent years, major research organizations have begun to recommend active surveillance, which for years had been promoted mostly by academic urologists in major medical centers, but not by urologists in private practice, who treat most men. In 2011, the National Institutes of Health held a consensus conference that concluded that it should be the preferred course for men with small and innocuous-looking tumors. Last year, the American Society of Clinical Oncology issued guidelines with the same advice.

The data includes a large new national registry established by the American Urological Association involving 15,000 men nearly all treated by urologists in private practice through 2015; a national registry of 45 mostly private urology practices; and a Michigan registry of mostly private urology practices. In addition, preliminary 2016 data from the urology association indicates that the numbers are growing, with even more than 50 percent of patients choosing active surveillance.