Dr Teo said that in hindsight he would do differently two surgeries that are now the focus of disciplinary hearings of the Health Care Complaints Commission (HCCC) in Sydney.
In the case of another female patient who was left in an essentially vegetative state, Dr Teo told the hearing it was plain from the outcome he removed too much of her brain.
"I did the wrong thing. I obviously did the wrong thing by the patient," he said.
"Clearly I have taken out a part of the right frontal lobe that has caused a deficit.
"Did I intend to hurt her? Absolutely not."
When asked what he would do differently Dr Teo replied, "pull my punches. Try to leave a little more tumour behind. Maybe a different approach".
"That's the $6 million question. 'What did you do wrong? What can you learn from it?," he said.
Throughout the hearing process the 65-year-old has maintained he was acting in what he believed at the time to be the patient's best interests and denied any wrongdoing or being negligent.
"The word negligent, I find that offensive," Dr Teo said.
"It wasn't negligent. Maybe ignorant on my behalf, but it wasn't negligence."
The high-profile star was greeted at Monday's hearing by a large group of supporters who said Dr Teo had saved or improved their lives through surgery for conditions that in some cases had been deemed by others as untreatable.
Dr Teo was at odds with expert opinion given during the hearing regarding both surgeries, over his decisions to operate when other neurosurgeons likely would not have.
Differences stemmed from, among other things, the difficulty of discerning from brain scans what is a tumour and what is cerebral edema, or swelling of the brain.
"There are just two different ways to look at it," Dr Teo said.
The hearing has run into overtime and will resume with a cross examination of Dr Teo in March.
During that time he is continuing to operate on patients under additional oversight imposed by the HCCC, including surgery on another malignant brain stem glioma next week.
"I'm doing one next week. It's the same story and I hope I'm going to get a good outcome by learning from all the past outcomes," he said.
During Monday's hearing, Dr Teo was accused of lying under oath about the risks involved with the surgeries and how he conveyed that to the patients.
Lawyer for the commission Kate Richardson SC put it to Dr Teo that he changed his evidence to protect himself after proceeding with the high-risk surgery.
Dr Teo firmly denied he had deliberately altered his evidence in order to mislead the committee.
"I want to suggest to you that you deliberately gave untruthful evidence to the committee for that reason. Do you accept that?" she asked.
"No," Dr Teo replied.
Dr Teo told the committee the husband of one of the patients was "genuinely destroyed" by the outcome and had initially blamed himself, before deciding to make an official complaint.
"I think he has been hoodwinked into thinking my intentions were not honourable," Dr Teo said.
"We had a good relationship and that seems to have soured. I made the assumption that he had been 'got to' by my enemies.
"He's been suggested to that it's Dr Teo's fault, nobody else's fault, not the tumour."
In August 2021, the NSW Medical Council restrained Dr Teo from operating without the approval of another doctor after an investigation by the HCCC.
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