Dr. Schrag noted that it required “unbelievably courageous patients” and doctors who were confident that the study was ethical.

“You live with this on your conscience,” Dr. Schrag said.

Radiation has long been used as a way to prevent the recurrence of rectal cancer. Chemotherapy and surgery often controlled the disease, but all too often, cancer emerged again in the pelvis. Horrific effects could follow — tumors that eroded the bladder, the uterus, the vagina.

The addition of radiation addressed recurrence in the pelvis but caused its own set of problems.

As years went by, some researchers began to wonder if radiation was still necessary. Chemotherapy, surgery and medical imaging had improved, and patients were being diagnosed earlier, before their cancer was as advanced.

Dr. Schrag and her colleagues decided to test the idea of eliminating radiation with a pilot study with what she called “30 courageous patients.” The results were encouraging enough to make the case for a broader study.

Dr. Venook said the study was a triumph in more ways than one.

“In rectal cancer, there are schools of thought,” he said. “People think they know what the right answer is.”

So, for the study to succeed, he added, “surgeons, oncologists and radiation oncologists all have to buy into the protocol.”

And so, of course, did patients like Awilda Peña, 43, who lives in Boston. She found out she had rectal cancer when she was 38.