Evidence for a significant timer error on a linear accelerator: consequences for a special therapy application

A significant timer error has been found on a linear accelerator used for intraoperative radiation therapy. Since typical treatment doses range up to 20 Gy, calibrations that do not account for this effect can incur a large dosimetry error. The effect is dose rate dependent, but energy independent. For a dose rate of 900 cGy min(-1), the error in dose calibration varies between 2 and 3% for an average of 2.5%. At dose rates of 600 and 300 cGy min(-1), the error was 1.7% and 0.8% respectively. The average timer error at 900 cGy min(-1) was found to be 1.2 +/- 0.1 monitor units. It is argued that even a small timer error has serious dosimetry consequences for the implementation of intensity modulated radiation therapy.