Late one afternoon this summer, Dr. Laura J. Esserman, a breast cancer surgeon at the University of California, San Francisco, sat in a darkened room scrutinizing a breast M.R.I. With a clutch of other clinicians at her side, she quickly homed in on a spot smaller than a pencil eraser.

She heard the words “six-millimeter mass.” Her response was swift:

“No.”

Meaning no biopsy.

Most doctors, including the radiologist seated next to her, would have said yes. But Dr. Esserman, who has dedicated much of her professional life to trying to get the medical establishment to think differently about breast cancer, foresaw only unnecessary anxiety for the patient, who had had several biopsies in the past — all benign.

Dr. Esserman, 58, is one of the most vocal proponents of the idea that breast cancer screening brings with it overdiagnosis and overtreatment. Her philosophy is controversial, to say the least. For decades, the specter of women dying for lack of intervention has made aggressive treatment a given.

But last month, her approach was given a boost by a long-term study published in the journal JAMA Oncology. The analysis of 20 years of patient data made the case for a less aggressive approach to treating a condition known as ductal carcinoma in situ, or D.C.I.S., for which the current practice is nearly always surgery, and often radiation. The results suggest that the form of treatment may make no difference in outcomes.