The new U.S. measurement standard for the air-kerma strength from low-energy photon-emitting brachytherapy seed sources is formally described in detail. This instrument-based standard was implemented on 1 January 1999, with its salient features and the implications of differences with the previous standard given only through a series of informal communications. The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years. For the two 125I seed models that have been subject to both the old and new standards, the new standard reduces the air-kerma strength by 10.3 %. This change is mainly due to the removal of the influence on the measurement of the Ti K x rays produced in the source encapsulation, a component with no clinical significance.

XBSTRACT. Traditional methods of carrying out internal-dose calculations are not adequate for the detailed models now in existence, nor for the many types of radionuclides presently in use. A formalism for absorbed-dose calculation is presented which is applicable to all radionuclides and all models. It is based on equations which are derived in general terms, independent of the properties of any single type of radiation. The development of the formalism starts with the concept of absorbed fraction, but a new concept, speci$c absorbed fraction (i.e., absorbed fraction per gramme), is shown to be more widely applicable. The proposed formalism will increase the complexity of dose calculation only under circumstances where traditional methods oversimplify the concepts and distort the result.

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