A key witness told the Cameron inquiry on Wednesday that he didn't hear many concerns about the quality of laboratory samples at the time when mistakes were made on hundreds of breast cancer treatment tests.

Dr. Donald Cook, a pathologist who was clinical chief at Eastern Health in 2005 and 2006, testified he remembered only a small number of problem cases.

Dr. Brendan Mullen, a pathologist at Mount Sinai Hospital in Toronto who was in charge of retesting almost 1,000 of hormone receptor tests in 2005, has testified he wrote to Cook in early 2006 over concerns he had about the quality of the tissue samples being sent from Newfoundland and Labrador.

Mullen said he never received a response to that letter.

Cook told the inquiry Wednesday that his main concern in the lab was the amount of time it was taking technicians to prepare the tissue samples on slides to be analyzed by pathologists.

He said the slides should have been prepared within 24 hours, but the work sometimes took up to four days.

There are dozens of steps to preparing a sample for testing, the inquiry has heard, beginning when a piece of human tissue is taken from a patient in a biopsy. The specimen must be properly refrigerated and preserved before a rigorous preparation to stain it with chemicals so certain characteristics are highlighted.

The stained sample is preserved in a block of wax that's sliced into minute portions, which are then sealed on a glass slide so they can be viewed under a microscope.

The inquiry has heard the process involves using precise amounts of chemicals and performing certain steps within a specific time.

Cook is currently on leave from Eastern Health.

The inquiry, headed by Justice Margaret Cameron, began hearing testimony in March.