EVIDENCE AGAINST THE OXYGEN-IN-THE-TRACK HYPOTHESIS AS AN EXPLANATIONFOR THE RADIOBIOLOGICAL LOW-OXYGEN ENHANCEMENT RATIO AT HIGH LINEAR-ENERGY-TRANSFER RADIATION

Oxygen sensitizes cells toward the effect of ionizing radiation. This sensitization, quantified by the oxygen enhancement ratio (OER), decre ases with increasing ionization density or linear energy transfer (LET ) of the radiation applied. One explanation for the decreased OER at h igh LET offers the ''oxygen-in-the-track'' hypothesis. It claims that oxygen is produced in the track of densely ionizing particles providin g an oxic microenvironment around the relevant cellular target molecul es, even if cells are exposed under anoxic atmospheric conditions. Exp erimental evidence is presented against this hypothesis. It is based o n the different kinetic pattern of DNA double-strand-break rejoining o bserved in yeast cells exposed under oxic or anoxic conditions to 3.5 MeV alpha-particles.