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The Ottawa Hospital Cancer Centre Orthovoltage Calculation Error. External Report Findings

Discovery of Calculation Error

In April 2008, The Ottawa Hospital Cancer Centre reported a calculation error with one of its Civic Campus-based radiation machines. The machine delivers low energy radiation therapy and is known as an orthovoltage treatment unit. It is a type of radiation therapy that is strong enough to kill cancer cells but does not penetrate more than a few millimeters beyond the surface of the skin.

Orthovoltage treatments are given for very superficial, small tumors such as skin cancers. The calculation error affected the treatment of 326 patients treated for Basal Cell and Squamous Cell carcinomas with orthovoltage. Patients affected were treated between November 2004 and November 2007 and in some cases, patients received radiation up to 17% less than the prescribed dose.

The calculation error took place in November 2004 when the radiation unit was moved from the General Campus to the Civic Campus. When the unit was reconstructed in its new location and recalibration took place, there was an error in treatment dose calculation. At the time the error was discovered, all other radiation therapy units at The Ottawa Hospital Cancer Centre were verified and found to be operating safely. The calculation was immediately corrected and no further treatments were affected.

Affected Patients

The medical charts of all 326 patients affected were reviewed by their respective treating physicians and were contacted for an immediate follow up appointment.

The error did not involve patients with melanoma skin cancer and the patients concerned were limited to those who:

• received radiation treatment for a Basal Cell and Squamous Cell carcinomas;
• were treated between November 2004-November 2007;
• were treated at the Civic Campus.

Patients who have received radiation therapy for any other type of cancer are not affected.

External Review Requested

At the time the error was discovered, the Cancer Centre immediately undertook an internal review and subsequently engaged an expert medical reviewer to determine the clinical impact of this under dosage. In addition, the hospital asked Cancer Care Ontario to conduct an independent review of the issue to determine the cause of the error. The external review was important to allow the cancer system to learn from this incident and to make recommendations to prevent similar occurrences at TOH or any other cancer centres.

The basis of this review an analysis which focused on the following questions:

1. Why were the incorrect output tables prepared during recommissioning?
2. Why was an independent second check not done prior to release of the output tables?
3. Why was the error not detected for 3 years?

Click here to see the Report of the Panel of Experts convened by Cancer Care Ontario. (Please note that the report is available in English only).