No abstract

A suitable physical parameter of radiation quality must represent not only the mean but also the fluctuation of the energy deposition per particle passage through that microscopic site of tissue in which proximity between radiation-induced lesions is decisive for the action mechanism. For sites of nanometre dimensions restricted LET is considered to have this property because (a) the distribution moments j and j2/j of the number j of ionisations per delta ray within the site are almost independent of the type and energy of the primary particle, and (b) the Poisson fluctuation of the number of primary ionisations within the site is determined by its mean value, which is proportional to restricted LET. Corresponding to this theoretical qualification as a suitable parameter, restricted LET has been experimentally identified as determining the yield factor a of various cellular radiation effects.

Monte Carlo calculations of the deep and shallow dose equivalent index produced by electron beams in the energy region 5-22 MeV have been performed for unidirectional, opposing, rotating and isotropic irradiations of the spherical tissue equivalent phantom. The observed dose maxima are attributed to multiple scattering and range effects and occur in the outer 10 mm shl:ll. The isotropy factor varies from 2.48 at 5 MeV to 1.89 at 22 MeV. The signal of a detector with isotropic response and wall thickness 0.5 g/cm2 is shown to be a suitable measure of dose! equivalent index for fast electrons. The average dose equivalent in the spherical phantom is disc:ussed as an alternative concept.