Dr. Bernard Fisher, a University of Pittsburgh surgeon whose research transformed the way breast cancer is treated, bringing an end to the routine use of the debilitating radical mastectomy, died on Wednesday in Pittsburgh . He was 101.

The university announced his death on Saturday.

Dr. Fisher’s research, which began in the late 1950s and spanned four decades, showed that early-stage cancers could be treated with simpler surgeries, and that treatment with chemotherapy or hormonal drugs could extend patients’ lives. In a radical mastectomy, the breast, the lymph nodes under the armpit, the chest muscle and, in some cases, ribs are removed.

Dogged and ambitious, Dr. Fisher prevailed against fierce resistance from academic surgeons, who believed more surgery was always better for patients, while withstanding allegations of scientific misconduct that nearly derailed his career.

Through dozens of clinical trials involving thousands of patients, Dr. Fisher brought the scientific process to bear on medical decision making, which, he said, had too long relied on anecdotes, opinions and untested theories passed down through generations of physicians. “In God we trust,” he once told a reporter. “All others [must] have data.”