Studies have shown in the past that identifying women with low risk breast cancer just with clinical factors alone have not been particularly helpful but increasingly we understand the molecular biology of breast cancer. So our study was really focussed on using clinical pathological factors but also molecular factors such as the luminal A subtype which we identified just using protein markers – so ER, PR, HER2 and, in particular, Ki67.

We did a prospective cohort study where we enrolled 500 women who had low risk breast cancer based on clinical factors. So they had to be greater than 55 years of ago or 55 years of age and they had to have a T1 cancer or N0, grades 1 and 2. Then we did routine ER, PR and HER2 testing in their own hospital laboratory and we did Ki67 in central laboratories, three laboratories across Canada. If they had a low Ki67, less than 13.25%, they were determined to be luminal A subtype and they were enrolled in the trial and followed for five years. 

What we found was a very, very low rate of local recurrence if they were just treated with breast conserving surgery and endocrine therapy alone without RT. So at five years the rate was 2.3% and the confidence interval, particularly the upper confidence interval, was 3.8%, well below the 5% cut-off that we chose a priori. So we felt pretty confident that this rate was low and that these patients didn’t require radiotherapy.

Of interest, the rate in the contralateral untreated breast cancer was 1.8% so very similar to the rate of 2.3% that we observed in the treated breast. Overall recurrences were very low as well, 2.7%.

So the implication is that if you have a woman that fits these clinical pathological criteria and also the luminal A subtype as we measured it that the risk is so low that we believe that they don’t require radiotherapy after lumpectomy as long as they are receiving endocrine therapy for five years.