On the use of LET for a revised quality factor for photons

Linear energy transfer, L, is currently used as reference parameter for the quality factor in radiation protection, and this practice is likely to be continued in an impending revision of the quality factor. But the numerical convention is not, at present, actually applied to photon or electron fields; their quality factor is, instead, summarily equated to unity. The current trend of tightened dose limits in radiation protection may create the need for precise computations of quality factors even for photon and electron radiations. Such computations can, however, not be performed in terms of unrestricted linear energy transfer. The reasons for this difficulty are explained, and the formulae for the quality factor as an integral over electron fluence and restricted linear energy transfer are given. A correct energy balance in this and in a variety of linked dosimetric relations requires a modification of the definition of restricted linear energy transfer. For large energy cut-off values the correction is of minor importance, but for small cut-off values - such as those invoked in biophysical considerations - the modification is essential. (author)