Meesungnoen, J. and Jay-Gerin, J-P. High-LET Ion Radiolysis of Water: Oxygen Production in Tracks. Radiat. Res. 171, 379–386 (2009).
It is known that molecular oxygen is a product of the radiolysis of water with high-linear energy transfer (LET) radiation, a result that is of particular significance in radiobiology and of practical relevance in radiotherapy. In fact, it has been suggested that the radiolytic formation of an oxygenated microenvironment around the tracks of high-LET heavy ions is an important factor in their enhanced biological efficiency in the sense that this may be due to an “oxygen effect” by O2 produced by these ions in situ. Using Monte Carlo track simulations of pure, deaerated water radiolysis by 4.8 MeV 4He2 (LET ∼ 94 keV/μm) and 24 MeV 12C6 (LET ∼ 490 keV/μm) ions, including the mechanism of multiple ionization of water, we have calculated the yields and concentrations of O2 in the tracks of these irradiating ions as a function of time between ∼10−12 and 10−5 s at 25 and 37°C. The track oxygen concentrations obtained compare very well with O2 concentrations estimated from the “effective” amounts of oxygen that are needed to produce the observed reduction in oxygen enhancement ratio (OER) with LET (assuming this decrease is attributable to the sole radiolytic formation of O2 in the tracks). For example, for 24 MeV 12C6 ions, the initial track concentration of O2 is estimated to be more than three orders of magnitude higher than the oxygen levels present in normally oxygenated and hypoxic tumor regions as well as in normal human cells. Such results, which largely plead in favor of the “oxygen in the heavy-ion track” hypothesis, could explain at least in part the greater efficiency of high-LET radiation for cell inactivation (at equal radiation dose).