Considerations on the biological effect of laser induced radiation with high dose rates

Values for the number of DNA damages of different types caused by sparsely ionizing radiation produced by conventional accelerators are well known. Table II shows approximate yields of DNA damages per Gy and cell and in compasion the calculated additional multiple DNA damages due to stochastic independent ionizations. It is obvious, that for dose fractions applied in radiation therapy the number of additional double-strand breaks induced by accidental single hits is negligible small. This is even the case for fraction doses as high as 24 Gy, a dose fraction, which is applied in stereotactic radio surgery for example. In reality it should be even smaller because it was assumed that all ionization events would take place at the same time what is especially due to the temporal spread of the dose pulse when interacting with matter not the case. The temporal spread of a 20MeV electron beam in water in a depth of 6 cm was also calculated using GEANT4 and can be seen in Fig. 6. As this figure shows the radiation pulse needs around 200 ps to reach a depth of 6 cm. At this depth the temporal spread of the pulse has become so broad, that it takes a time of ca 13 ps to deposit 50% of the total dose. During this period of time it could be possible, that recombination takes place and that cracked chemical bonds are stabilized before further ionizations can cause a complex damages. This makes it clear, that the calculated values are upper limits anyway. However this predication is valid for direct radiation effects. It should be noticed that this investigation is restricted to the direct radiation effect. The influence of the indirect radiation effect on the relative biological effectiveness of ultra shortly pulsed radiation was not assessed by these calculations.