Data obtained by the CBC showed the Quebec Association of Pathologists had 15 breast cancer tissue samples analyzed and then sent the samples to other medical labs across the province for a retest.
The tests for different proteins guide doctors as to which form of chemotherapy is bested suited to the patient. Of the samples sent for second tests, 30 percent of them were wrong, the association found.
Association President Dr. Louis Gaboury told the broadcaster, based on the error rate, there were fears hundreds of women could be receiving the wrong treatment.
"If we think that there's about 10 (percent) to 30 percent variation, then it's a significant number of people out there with a wrong diagnosis," he said.
The province of Newfoundland and Labrador discovered similar problems with breast cancer screening this year. Over an 8-year period, an investigation found almost 400 women were inaccurately diagnosed, and more than 100 of them died, the report said.