Purpose: It is known that for different charged particle beams with the same linear energy transfer (LET), the oxygen enhancement ratio (OER) can be different, with the lighter ion having a lower OER at a given LET. This study aims to explore if another beam quality factor is a more suitable qualifier of which the OER is a function.
Methods: Cell survival data from Furusawa et al were used, where V79 Chinese hamster ovary cells and human salivary gland (HSG) cells were irradiated with helium-3, carbon-12, and neon-20 beams. The OER was defined to be the ratio of doses for 10% survival between hypoxic and aerobic conditions. We plotted the OER for the two cell lines separately as a function of (dose-averaged) LET and Z*Â²/Î²Â² respectively, the latter being the square of the quotient of the effective charge divided by the speed of the ion. We then fitted the datasets with an Alper-Howard-Flanders-like formula. Differences in the fits and the fitting parameters were then compared.
Results: The fits were reasonable for both LET and Z*Â²/Î²Â² plots and for both cell lines. The notable exception is the fit on the LET data points for V79 cells irradiated with helium-3 where the flex point of the fit occurred at a spuriously low LET, possibly due to statistical variation in the data points. The fits on the Z*Â²/Î²Â² plots appeared visually to have smaller variations than those on the LET plots. The coefficients of variation in the fitting parameter Î³ for the Z*Â²/Î²Â² data points were also smaller: 0.27% (1.39%) compared with 33.81% (7.03%) for the LET fits on the V79 (HSG) data, respectively.
Conclusion: Z*Â²/Î²Â² may be a more suitable qualifier than LET when describing OER of a charged ion beam. More radiobiological data may be needed for a reduction of uncertainties.