NBS (National Bureau of Standards) Measurement Services: Calibration of Gamma-Ray-Emitting Brachytherapy Sources

摘要

The calibration of small radioactive sources used for interstitial radiation therapy, short distance therapy (brachytherapy), is performed in terms of the physical quantities exposure or air kerma. (60)Co and (137)Cs sources are calibrated by comparison with NBS working standard sources of the same type, while (192)Ir and (125)I sources are calibrated by measurement in a reentrant ionization chamber that was calibrated using NBS working standard sources of the same type. The working standard sources were calibrated using the NBS graphite cavity ionization chambers except for (125)I, for which the NBS measurement standard was a free-air chamber. The working standard sources of the two long-lived sources have been measured a number of times over the years; the reliability of the reentrant chamber for the two short-lived sources is assured by use of sealed radium sources as a constancy check. The overall uncertainty (considered to have the approximate significance of a 95% confidence limit) is given as 2% for all the sources except for (125)I seeds, for which it is given as 5%, 6%, and 7%, depending on the type of seed. The stated uncertainty for (125)I seeds does not include possible errors due to low-energy X rays not recognized at the time the standards were established.