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.