Identification of biophysical mechanisms of damage by ionising radiation

New analyses of published data for the inactivation of enzymes, viruses, and higher cells by accelerated ions and X and gamma rays indicate that the dominant mechanism for reproductive death in mammalian cells is determined by the ''matching'' of the mean free path between ionisations to the strand separation in double stranded DNA. Electrons have greatly reduced intrinsic efficiencies of action when compared with heavier particles because they can reach the requisite specific primary ionisation only at very low energies when they undergo large range straggling. In all of the systems tested the action of electrons seems to be intra-track rather than inter-track. Additional evidence in support of the proposed mechanism is found in the occurrence of the RBE maxima at the same critical specific primary ionisation of 0.55 per nanometre and independently of radiation type. In view of the foregoing findings microdosemeters measure the wrong quantity, but information of practical value can be obtained by applying saturation corrections which are dependent on radiation type. In radiotherapy best results should be achieved by selection of the appropriate specific primary ionisation rather than the maximum LET for the radiation concerned. (author)