Hundreds of skin cancer patients received the wrong dose of radiation over a three-year period due to a calculation error programmed into one of the radiation units at the Ottawa Hospital Cancer Centre.

The 326 patients received a dose of as much as 17 per cent less than the prescribed dose while being treated between November 2004 and November 2007 for basal and squamous cell carcinomas, the Ottawa Hospital said in a news release Monday.

Those types of skin cancers typically don't spread and tumours usually can be successfully removed or treated with radiation. Patients with more aggressive melanoma skin cancer were not affected.

However, the normal acceptable variation in radiation dose is five to seven per cent, the release said.

"We regret what happened and don't want to create a lot of anxiety and worry," said Dr. Chris Carruthers, chief of the medical staff at the Ottawa Hospital. "However, the patients … should be monitored and we should watch them to see if the underdosing really had any effect over time."

He added that patients are seen by doctors on a regular basis following radiation treatment as part of the usual protocol.

The affected patients' oncologists are currently reviewing the their files, and the patients will be contacted soon after the review is complete on Tuesday, the release said.

Meanwhile, the hospital has ordered an independent review of the issue.

The calculation error was programmed into a radiation unit in November 2004, when it was moved from the general campus to the civic campus of the Ottawa hospital, being assembled and disassembled in the process.

The mistake arose because the radiation is measured for a particular size of radiation beam, and calculated for the other beam sizes using a formula that contained an incorrect number.

The error was discovered in November 2007 by measuring the radiation at other beam sizes using a dosimeter, Carruthers said.

It was immediately corrected, the hospital said. Other radiation units were also checked.

Despite the error, 294 people got the right radiation dose because they were treated with the beam size used to calibrate the machine.

Review blamed for delay

In addition, the hospital undertook an internal review and hired a Toronto-based medical reviewer to determine what effect the wrong dosage may have had on patients.

Carruthers said the reason why the hospital waited so long to disclose the error was because it took awhile for the facility to find that reviewer, and they only received his report last week.

"Ideally, if we could have expedited it, it would have been better, but these are generally very slow-growing cancers," he said.

Who was affected? Patients who: Received radiation treatment for basal cell or squamous cell carcinomas. Were treated from November 2004 to November 2007. Were treated at the civic campus of the Ottawa hospital. Concerned patients can reach a special information line by calling 613-739-6800 or toll free at 866-253-2603.