WINDSOR, Ont. - The southwestern Ontario hospital where a woman underwent an unnecessary mastectomy said Thursday it was horrified and saddened by the mistake, which it blamed on the doctor who performed surgery.

Dr. Art Kidd, who was chief of staff at Windsor's Hotel-Dieu Grace Hospital last fall when the woman mistakenly had her breast removed, said Dr. Barbara Heartwell admits she misread the results of a needle biopsy that found the woman did not have cancer.

"The error that occurred here was identified by Dr. Heartwell as her own error, she misread the report," Kidd said in an interview.

The hospital said Heartwell then went ahead with the mastectomy.

Heartwell has successfully treated thousands of breast cancer patients in the Windsor area, the hospital said.

Asked if the hospital erred, Kidd said there is nothing to indicate that.

"It would not appear to be. This would appear to be a human error by one individual."

The Hotel-Dieu Grace has begun an internal investigation of what went wrong. The hospital has also contacted both its lawyers and its insurance company, which the insurance company requires and is standard procedure in these cases, said Kidd.

"We always notify our legal representatives," Kidd said. "Our insurers and our legal representatives are linked very closely."

A communications spokeswoman for the hospital said the patient never contacted Hotel-Dieu Grace about the mistake.

"The patient never came to the hospital. We actually heard about this adverse medical event through a local media outlet" after she shared her story with a reporter, said Kim Spirou.

The hospital continues to "reach out" to the woman each day through its patients' advocate. It is offering her counselling and help with any further medical procedures she might need. However, it has not offered any financial compensation.

"Our first concern for her was as a person," said Kidd.

The hospital is reviewing its procedures to confirm they were followed, as well as to look for ways to make sure it doesn't happen again.

Heartwell's pathology reports will be read by another physician until the hospital's investigation is completed, Kidd said.

Heartwell could not be reached for comment, but the hospital says so far it has not received any notice of legal action stemming from the incident.

The Hotel-Dieu Grace will continue to do everything it can to help the patient, said Kidd.

"We are profoundly sorry" that this occurred, he said.

In a press release issued Wednesday, the hospital praised Heartwell as a "well-known and highly respected surgeon."

"Humans are not infallible and mistakes do unfortunately occur," said hospital CEO Warren Chant.