The former head of Eastern Health faced tough questions from the head of the Cameron inquiry Thursday over the use of the phrase "systems problem."

Justice Margaret Cameron interrupted former Eastern Health chief executive officer George Tilley when he used the phrase Thursday even though it's been used by several witnesses, including former health ministers, to describe what caused breast cancer testing mistakes.

The inquiry is examining how the lab in St. John's produced wrong results for more than 300 breast cancer patients between 1997 and 2005.

Cameron told Tilley that the term is so general, it's open to several interpretations.

"One is a way of not answering the question, two is a way of saying, 'It's so complicated we're not going to try to answer the question,' and three is, 'That's the best way of describing what we have found and that is that so many things are contributing to the problem,'" she said.

Tilley chose option three, but he said he doesn't know if that was explained clearly to patients when they asked questions.

"When the patient says to you, 'What caused this, why am I in that situation?' would the response be, 'It was systems problem?'" Cameron asked.

"I wasn't involved in any of those direct contacts, so I really can't say if that was the answer given or not," Tilley responded.

Tilley told the inquiry he never felt comfortable that Eastern Health had ever received a definitive answer on what went wrong.

The issue of what caused the faulty lab tests is paramount to the judicial inquiry, which began hearing evidence in March.

The inquiry has frequently been told that Eastern Health believed that equipment that had been replaced several years ago was at fault. However, reviews by two external experts — which Eastern Health fought but lost in Newfoundland Supreme Court to keep confidential through the inquiry process — said the equipment was not related to the problems.

Instead, they pointed to such things as poor training and high turnover.

Former health minister Tom Osborne told the inquiry that he was frustrated that he could not get a conclusive answer as to what caused the lab problems.