Evaluation of the need for radiotherapy calibrations in the United States of America

The custody, maintenance, and development of the United States of America's national standards of measurement, together with the provision of calibration services related to these standards, has been assigned to the National Bureau of Standards (NBS) of the US Department of Commerce. As with all of the national standards, including those for radiation, the Bureau's principal emphasis is on calibrations and other tests requiring an accuracy that can be obtained only by direct comparison with the standards of NBS. For many years, NBS has been the only organization which was generally available for the calibration of instruments needed for radiation therapy in US hospitals and clinics. In recent years, three regional calibration laboratories, located in New York, Houston and Cleveland, have been accredited by the American Association of Physicists in Medicine (AAPM). The calibration chain between the primary standards of NBS and the field instruments used in the hospitals and clinics of the entire country may be direct, or indirect through the regional calibration laboratories by use of secondary standard or transfer instruments. It may be that this system of promulgation of radiation standards is no longer adequate for the USA. Ionizing radiation is being used increasingly for the treatment of cancer patients. At present, approximately 325000 new cancer patients per year are given radiation treatment at over 1500 centres. With this growing use of ionizing radiations, an increased awareness of the importance of accurate and precise dosimetry has developed. To determine the adequacy of these dosimetric needs, the AAPM has initiated a national study which is being conducted by means of a mailed questionnaire, and by a limited number of site visits to individuals who are in charge of radiation therapy centres as well as to those who do the actual calibrations. To date, the questionnaire has been developed, and a mechanism for reaching virtually all therapy centres in the USA has been devised. (author)