Room: Exhibit Hall | Forum 4
Purpose: A number of published studies suggest that the value of Î² in the LQ cell survival model tends to decrease with increasing particle LET. Other published studies, as well as some biophysical considerations, suggest that Î² may be independent of LET or even an increasing function of LET. The purpose of this study is to test hypothesized trends in Î² with particle LET.
Methods: Bayesian bootstrap sampling is used to construct confidence intervals on estimates of Î± and Î² from a regression analysis of cells irradiated by particles of varying LET under oxic and hypoxic conditions. Two scenarios are considered: (1) Î± and Î²beta are independently adjusted (2 degrees of freedom) and (2) Î± is independently adjusted and Î² is set a priori to Î²(ref) (1 degree of freedom), i.e., Î²/Î²(ref)=1. Alternative hypotheses (fitting scenarios) are tested by comparing estimates of Î± and the measured and predicted cell survival fraction for the two scenarios.
Results: For 200-250kV x-rays (reference radiation), LQ parameters for human kidney T1 cells are Î±(ref)=2.28Ã—10â?»Â¹Gyâ?»Â¹ and Î²(ref)=2.43Ã—10â?»Â²Gyâ?»Â² for oxic cells and alphaREF=7.80Ã—10â?»Â²Gyâ?»Â¹ and Î²(ref)=7.12Ã—10â?»Â³Gyâ?»Â² for hypoxic cells. For oxic and hypoxic conditions, differences in the normalized Î±/Î±(ref) of â?´HeÂ²â?º ions (166keV/Âµm, 110keV/Âµm, 88keV/Âµm, 61keV/Âµm, 26keV/Âµm) are insignificant between the decreasing and constant Î² scenarios. Comparisons of the measured and predicted cell survival fraction with absorbed dose are also insignificant compared to the uncertainties in measured data, dosimetry and LQ parameters.
Conclusion: The presented results provide evidence disputing the widely held believe that Î² must decrease with increasing particle LET in order to explain the results of measurements. Additional work is needed to extend the analysis to other cell lines and higher LET particles.