That was why researchers took another look at the standard treatment for rectal cancer, which affects 47,500 people per year in the United States (although the class of the disease in the study affects about 25,000 Americans annually).

For decades, it was typical to use pelvic radiation. But the radiation puts women into immediate menopause and damages sexual function in men and women. It also can injure the bowel, causing issues like chronic diarrhea. Patients risk pelvic fractures, and the radiation can cause additional cancers.

Yet radiation treatment, the study found, did not improve outcomes. After a median follow-up of five years, there was no difference in key measures — the length of survival with no signs that the cancer has returned, and overall survival — between the group that had received the treatment and the group that had not. And, after 18 months, there was no difference between the two groups in quality of life.

For colon and rectal cancer specialists, the results can transform their patients’ lives, said Dr. Kimmie Ng, a co-director of the colon and rectal cancer center at the Dana-Farber Cancer Institute, who was not an author of the study.

“Now, especially, with patients skewing younger and younger, do they actually need radiation?” she asked. “Can we choose which patients can get away without this extremely toxic treatment that can lead to lifelong consequences, such as infertility and sexual dysfunction?”