PARENT SESSION

Physico-Chemical Events

(PP012) Do low energy electrons produce lesions characteristic of high-LET or low-LET radiation?

Hill, Mark^*,1, Goodhead, Dudley¹, ¹ Radiation & Genome Stability Unit, Harwell, Oxfordshire, UK

ABSTRACT- Low energy electrons (0.1 - 10 keV) are produced in abundance by all low-LET radiations. Approximately 30% of the absorbed dose from low-LET radiations such as hard X-rays or gamma-rays is deposited by low-energy (0.1 - 5.0 keV) secondary electrons. The dose contribution from these low energy electrons increases for lower energy X-rays, Auger emitters and low-energy beta emitters. Auger electrons have energies ranging from about 10 eV to 10 keV and are commonly emitted from radionuclides or after interactions of radiation with inner shell electrons. The average energy of the spectrum of beta-particles emitted by tritium is 5.7 keV. With decreasing electron energy the mean LET increases, for example it is 12 keV m^-1 for tritium beta-particles and ∼ 40 keV m^-1 for 0.3 keV electrons, compared with ∼ 0.5 keV m^-1 for a more standard low-LET radiation or ∼100 keV m^-1 for 5 MeV alpha-particles. The dependence of biological effect on electron energy, and general characteristics of low energy electrons, can be studied using ultra-soft x-rays. In almost all published studies, the relative biological effectiveness (RBE) has been found to increase with decreasing x-ray energy and therefore with decreasing electron energy and track length and corresponding increase in the local ionisation density. Some common characteristics of low-LET, as opposed to high-LET, radiation are shouldered survival curves, split-dose recovery and substantial sensitivity differences between repair deficient and wild type cells. The data obtained for low energy electrons are compared to those of typical characteristics of low-LET radiation and high-LET alpha-particles. The data suggest that low energy electrons ('track ends') are the most important component of all low LET radiations for most biological effects, so these are the very essence of what is normally regarded as 'low-LET' damage.

Key words: ultrasoft x-rays, electrons, LET