Nationwide evaluation of 60Co teletherapy calibrations

As a result of a reported incident which resulted in the unintended overexposure of several hundred patients, teletherapy installations throughout the United States were evaluated to verify the accuracy of teletherapy equipment calibrations. Selected licensees received TLDs for exposure under specified conditions. Site visits were made to each licensee showing an apparent discrepancy greater than +/- 5% and intercomparison measurements made. This paper describes results of a sampling of 373 teletherapy licensees and the success of site visits in reducing final discrepancies to less than +/- 5%. TLD and site visit measurement techniques, results, comparison with data from other studies, and possible sources of uncertainty in the management of a large-scale screening program are discussed.