According to a group of Canadian researchers, radiotherapy may be overutilized and unnecessary for treating patients with a particular, low-risk subtype of early-stage breast cancer.

In a new study, scientists at McMaster University, BC Cancer, Hamilton Health Sciences, and the University of British Columbia said women, 55 and up, who have luminal A, a stage I subtype that represents 60% of breast cancers diagnosed annually, may just need breast-conserving surgery and endocrine therapy, sparing them from the side effects of radiotherapy while freeing up space and reducing waiting times for patients who require it urgently.

The researchers assessed 500 women across Canada for five years who underwent surgery for tumors that were less than two centimeters and did not have cancer in their arm lymph nodes. They used a made-in-B.C. version of a molecular test called Ki67, an antibody that produces a simple chemical reaction to identify how fast tumors are growing. Ki67 quantitatively identified luminal A subtype tumors with a low reading based on the growth rate. The presence of estrogen and progesterone receptors also helped determine these tumors.

"If freed-up resources are not used, there could be direct cost savings to providers who normally would have treated these patients with RT," Dr. Tim Whelan, lead author of the study, professor of oncology at McMaster University, and a radiation oncologist at Hamilton Health Sciences, told HCB News.

The Ki67 test uses immunohistochemistry to replace more costly and complicated testing methods. The version used in their study was developed by co-author Torsten Nielsen, clinician-scientist at BC Cancer and professor of pathology and laboratory Medicine at the University of British Columbia.

Those with low readings were enrolled in the trial and received standard endocrine therapy. The researchers then performed regular clinical exams and annual mammograms and successfully predicted that at the five-year post-surgery mark, the risk of breast cancer recurrence would be very low. The risk was 2.3% without radiotherapy, comparable roughly to a 1.9% risk of developing breast cancer in the other, untreated breast.

Ki67 is open-sourced and can be accessible to hospital labs worldwide, according to Nielsen. It was previously used in a 2015 translational study in Canada to also identify low-risk luminal A breast cancer patients.

The findings were published in The New England Journal of Medicine.

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