The percentage of Canadian women who have survived breast cancer has doubled in the past 15 years, according to researchers at the University of Toronto, a reflection of the dramatic advances in the treatment of the disease in recent years.

The study published in the Journal of the National Comprehensive Cancer Network found that about 371,000 patients were diagnosed with breast cancer between 2007 and 2022. That's 2.1 per cent of the total number of Canadian adults in 2022. Eighty-six per cent of them were still alive that year.

That would mean there are 2.5 times as many survivors today as there were at the time of the last survey in 2007, researcher Amy Kirkham said in a press release.

"This improvement is due in part to the development of personalized medicine," said Professor Alain Nepveu, who is a researcher at the Rosalind and Morris Goodman Cancer Institute of McGill University.

"In the past, we treated everyone the same way," he said. "You take the tumour out and then the standard treatment is radiation and chemotherapy. But with the advent of what is called gene expression, we can determine the gene expression profiles in breast tumours. We can predict who is at risk and who is not."

This kind of approach prevents unnecessary treatment for at least 70 per cent of patients, Nepveu said, which is "a huge step forward" because it allows these patients to avoid undesirable side effects of treatments, such as cardiomyopathy.

The Toronto analysis found that 2 per cent of women diagnosed with breast cancer between 2007 and 2021 would likely be hospitalized for heart failure, at an enormous and potentially unnecessary cost to the system beyond the impact on their health.

Nepvu would add to the list of advances in breast cancer treatment, the refinement of radiation therapy that allows the tumour to be targeted with unprecedented precision, neoadjuvant treatments that shrink the tumour before surgery, and immunotherapy that harnesses the immune system so that it will attack and destroy the disease.

"When I started 35 years ago, you never heard of side effects," he said. "Today, we have a population of survivors who complain about long-term side effects. And that's progress."

Even in cases where a complete cure is unlikely, he said, going from a three-year life expectancy to 15 years, for example, is a huge improvement.

Breast cancer survivors account for 1 per cent of Canadian women aged 20 to 64, and 5.4 per cent of Canadian women aged 65 and older, the study said.

This report by The Canadian Press was first published in French on Nov. 3, 2022.