Accident prevention through calibration of radiotherapy equipment
Major cases of accidental exposures due to equipment calibration
» Incorrect decay data (USA)
» Miscalibration of beam (Costa Rica)
A cobalt unit for teletherapy at a hospital in Ohio was initially calibrated correctly. During the period 1974-1976 the physicist failed to perform regular measurements (calibrations and QA), but relied on estimations of the decay of the source to predict the dose rate and calculate the treatment time. Rather than calculating the decay, the physicist plotted the dose rate on a graph paper and extrapolated the decaying dose rate over time. Decay was determined from a straight-line plot on a semi-log graph paper with a calendar ordinate. However, the physicist continued the plot on a page that had linear scales on both axes.
This created two problems:
- the linear Y-axis did not correspond to the original log Y-axis, so straight line extrapolation resulted in ever-more incorrect output values;
- the linear X-axis did not correspond to the original calendar axis, so extrapolation led to incorrect date values.
These errors in predicting the dose-rate were made by the physicist in the time period 1974-1976. The errors resulted in the dose-rate being under-estimated by 10% to 45%, which translates to the patients receiving corresponding overdoses of 10% to 55% (where the magnitude of error increased almost linearly with time).
The incident came to light because patients started exhibiting symptoms of overexposure. When the accident was investigated, the physicist produced ten falsified calibration documents showing the correct machine output.. The output of the cobalt unit had, in fact, not been checked for 22 months. 426 patients received significant overdoses as a result.
Lessons learned for health professionals:
- Provide independent check of physicist's safety critical work;
- Institute formal procedures for calibrating a treatment unit on a regular schedule;
- Ensure that records accurately document performance of accepted QA procedures.
Read more:
- COHEN, L., SCHULTHEISS, T.E., KENNAUGH, R.C., A radiation overdose incident: initial data, Int. J. Radiat. Oncol. Biol. Phys. 33 (1995)217-224.
» Miscalibration of beam (Costa Rica)
A cobalt source was exchanged for a new one in 1996 in a hospital in Costa Rica. At the subsequent calibration, the medical physicist incorrectly interpreted 0.3 minutes as being 30 seconds (instead of the correct interpretation of 18 seconds). As a consequence, the absorbed dose rate of the new source was underestimated, resulting in treatment times being overestimated by 66%.
A radiation oncologist from another hospital, whose patients were treated in the hospital where the event occurred, noticed some unusually severe reactions in some of the patients treated with the cobalt unit. These reactions involved the skin and lower intestinal track, e.g., diarrhoea and abdominal pain. When another physicist crosschecked the dose rate, the error was found. 115 patients were affected, and two years after the event at least 17 patients had died from the overexposure.
Lessons learned for health professionals:
- Ensure that staff is properly trained and competent;
- Instruct staff to be aware of inconsistencies, for example, it is unexpected for a new teletherapy source to require longer treatment times;
- Ensure there are written procedures for calibration of beams and for independent verification of safety critical tasks before clinical implementation;
- Thoroughly investigate any high incidence of patients experiencing severe radiation reactions.
Read more: