From the start, Ms. Long said; “There was no question in my mind. I was going to have a mastectomy on both sides. I wanted to maximize my survivability, and I didn’t want to ever think that I hadn’t done everything that I possibly could to prevent this from coming back.”

Image Darcy Long, who had cancer in one breast but chose a double mastectomy, said, I wanted to maximize my survivability. Credit... Ben Garvin for The New York Times

The study’s lead author, Dr. Todd M. Tuttle, chief of surgical oncology at the University of Minnesota Medical School, started the study because so many patients were requesting the procedure. Still, Dr. Tuttle said, he was surprised by the increase in contralateral prophylactic mastectomies, an upward trend that shows no sign of leveling off and is occurring even as the practice of breast-conserving surgery expands.

“The comment patients make is, ‘I just want to be done with it,’” he said. “They never want to have another mammogram again; they never want to have another biopsy again.”

He and other experts emphasized that though the removal of the healthy breast sharply reduced the risk of a new cancer developing in that breast — the risk is not zero because some tissue can remain — it was the cancer that had already been diagnosed that posed the greatest threat to a patient’s life.

“For the vast majority of our patients, this does not impact the chances of dying of breast cancer, and that’s the key thing here,” said Dr. Julie R. Gralow, the chairwoman of the communications committee of the American Society of Clinical Oncology and an associate professor of medical oncology at the University of Washington and the Fred Hutchinson Cancer Research Center in Seattle.