  In this work, the induction and repair of radiation-induced 53BP1 foci were
studied in human umbilical vein endothelial cells irradiated at the PTB
microbeam with protons and {\alpha}-particles of different energies. The data
were analyzed in terms of the mean number of 53BP1 foci induced by the
different ion beams. The number of 53BP1 foci found at different times
post-irradiation suggests that the disappearance of foci follows first order
kinetics. The mean number of initially produced foci shows the expected
increase with LET. The most interesting finding of this work is that the
absolute number of persistent foci increases with LET but not their fraction.
Furthermore, protons seem to produce more persistent foci as compared to
{\alpha}-particles of even higher LET. This may be seen as experimental
evidence that protons may be more effective in producing severe DNA lesions, as
was already shown in other work, and that LET may not be the best suited
parameter to characterize radiation quality
