Reduction in Oxygen Enhancement Ratio with Increase in LET: Tests of Two Hypotheses

Summary

Measurements of o.e.r. and of the ‘oxygen constant’ K may be used to calculate effective concentrations of oxygen engendered in the tracks of ionizing particles, if it is correct to invoke that phenomenon to account for decreasing o.e.r. with increasing LET. Effective O₂ concentrations calculated from results with Shigella flexneri and Chlamydomonas reinhardii are in good agreement, although the radiobiological responses of those organisms are widely different in several respects. The relevant oxygen concentrations so calculated account plausibly also for variations of o.e.r. with radiation quality in mammalian cells. Tests have also been formulated to examine the validity of the ‘interacting-radicals’ hypothesis of Alper (1956) and Howard-Flanders (1958) by the use of data on changes in o.e.r. and biological effectiveness with radiation quality. These tests have failed to support the latter hypothesis.