"It's a very worrisome situation, obviously most worrisome for the patients and their families involved, and we will work to find out how this happened," said Wynne.
"I don't know exactly how this happened, but we obviously need to find out how it happened."
Five hospitals — four in Ontario and one in New Brunswick — were contacting patients who received the too-low chemo doses to tell them to immediately get in touch with their oncologist to discuss treatment plans.
There was too much saline added to the bags containing the chemotherapy medications, in effect watering down the drugs by up to 20 per cent.
The hospitals are conducting their own investigations, as is Cancer Care Ontario, and the Ontario College of Pharmacists was on scene at Marchese Hospital Solutions, which prepared the drugs for the hospitals, said Health Minister Deb Matthews.
"I think it's really important to let those people doing the investigations to do their work," said Matthews.
"We all want the highest quality of care, and that includes the appropriate mixing and labelling of drugs."
Questions were raised about the impact the lower than intended doses of the chemotherapy drugs might have had on the treatment for the cancer patients and whether or not they could have lived longer with proper doses.
"What is the implication of that dilution? Does it mean it would affect the outcomes of people's treatment, their longevity," asked Ontario NDP Leader Andrea Horwath.
The provincial Health Ministry and Cancer Care Ontario have a lot of questions to answer, added Horwath.
"I think this rocks people's trust in the health-care system and in the distribution of drugs in Ontario, and that's worrisome," she said.
However, Matthews said the experts assured her the health risks from the watered down cancer drugs were minimal.
"Oncologists tell me the likelihood of this having a serious impact on the health of patients is slight," she said.
Dr. Eshwar Kumar, the CEO of the New Brunswick Cancer Network, said he doubts the diluted drug would have much impact on patients.
"My gut instinct as an oncologist and speaking very generally, it is probably of very minimal impact, but every patient would have to be evaluated individually," Kumar said. "I don't see a situation where a patient would be called back to be given more chemotherapy."
Kumar said the hospital stopped using the drug supply as soon as it learned of the problem last Thursday, and is now mixing the drug themselves.
Oncologists sometimes adjust a patient's dose by up to 20 per cent for a number of reasons such as weight changes and side effects, said Dr. Wylam Faught, who heads the cancer program at The Ottawa Hospital.
"We would anticipate the same chance for a cure or long-term cancer control as we would if we hadn't adjusted the dose," he said in a phone interview.
In this case, he said it's likely "the worry and anxiety is more than the actual negative effect of the cancer treatment."
The chief of oncology at Windsor Regional Hospital said it's important doctors not dismiss their patients' concerns, even if the risk is low.
"I think we just have to be sensitive," Dr. Ken Schneider said.
Marchese Hospital Solutions said it was "deeply concerned" by the questions raised about the quality of its work and is addressing those issues.
"Our preliminary investigation of this issue leads us to be confident that we have met the quality specifications of the contract," the company said on its website.
A total of 990 patients treated or being treated at London Health Sciences Centre, Windsor Regional Hospital, Lakeridge Health in Oshawa and Peterborough Regional Health Centre are affected.
Windsor Regional said 100 of its 290 cancer patients affected had already been contacted by midday Wednesday and given an appointment with their oncologist.
The Ontario patients received lower than intended doses of the drugs cyclophosphamide and gemcitabine, while the Saint John Regional Hospital in New Brunswick said 186 patients received watered down doses of cyclophosphamide.
The drugs were being used as far back as February of last year.
Cancer Care Ontario said all four Ontario hospitals immediately removed the medications received from the drug manufacturer when the problem was discovered late last month, and informed Marchese of the error.
Each hospital has secured new supplies of the medications and Cancer Care Ontario said patients' treatment cycles will not be interrupted.
"It's important to note that chemotherapy preparation and delivery is a complex process and as a result of this complexity, there are sources for potential error," Dr. Carol Sawka, Cancer Care Ontario's vice-president, said in a statement.
The underdosing affected 665 patients at London Health Sciences Centre, 290 patients at Windsor Regional Hospital, 34 at Lakeridge Health, and one patient at Peterborough Regional Health Centre.
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