The estate of a Windsor, Ont., woman is suing a local pathologist, alleging she made diagnostic errors and seeking damages of $30 million on behalf of herself and other people hurt by the doctor's mistakes.

The lawsuit, which seeks class action status, was prompted by the case of the late Margaret Musgrave. Musgrave, who was 80 at the time, was rushed to the Hôtel-Dieu Grace Hospital in Windsor with a ruptured appendix in early 2007.

Dr. Olive Williams was given three slides of Musgrave's appendix tissue to examine. She only looked at two and declared there was no cancer present.

A year later, Musgrave was diagnosed with cancer that appeared to have started in her appendix. She died in May 2008.

A subsequent investigation by the Ontario College of Physicians and Surgeons led to an admission from Williams that she had overlooked the third slide.

"I am at a loss to explain how it is that I did not examine the third slide," she wrote to the investigator.

The college later issued her a written caution for her mistake.

Williams subsequently had her privileges at Hôtel-Dieu suspended in January after the discovery of several other serious mistakes in her work.

An internal report by the Erie St. Clair local health integration unit found that the hospital uncovered five mistakes attributed to Williams, one of which led at an unnecessary colostomy from a wrong cancer diagnosis.

A sixth case — which led to an unnecessary lumpectomy — later came to light.

15,000 cases under review

Williams worked on 15,000 cases between 2003 and 2010. In addition to working at Hôtel-Dieu, she provided pathology services to Windsor Regional Hospital and Leamington District Memorial Hospital.

An internal report obtained by the Ontario Conservative Party says the hospital suggests that Williams's cataracts, which can seriously impair vision, may have played a role in her diagnostic errors.

Investigators looking into pathology tests in the Windsor area are expected to deliver their findings to the Ontario Minister of Health at the end of the week.

After that, the report will be publicly released. That could happen next week.

With files from the CBC's Pat Jeflyn