Energy dependence of dose and dose-rate effectiveness factor for low-let radiations: Potential importance to estimation of cancer risks and relationship to biological effectiveness

A dose and dose-rate effectiveness factor (DDREF) for low-linear energy transfer (LET) radiations (photons and electrons) is used in cancer risk assessments to represent an assumption that risks at low doses and low dose rates may be less than estimates that are based mainly on linear extrapolations of observed risks at higher acute doses. DDREF generally is assumed to be independent of energy. However, a variety of radiobiological data reviewed in this paper suggest that DDREF may decrease with decreasing energy. This effect, which parallels increases in biological effectiveness with decreasing energy of photons and electrons that have been observed in many radiobiological studies, has received little attention. The importance of an overestimation of DDREF at low energies of photons and electrons is that cancer risks at low doses and low dose rates could be underestimated. This paper also discusses (1) the link between DDREF and the usual assumption of a linear-quadratic dose-response relationship for low-LET radiations and (2) concerns about the validity of estimates of DDREF and biological effectiveness used in cancer risk assessments that are raised by results of recent studies that cast doubt on whether the underlying radiobiological data can be represented by a simple linear-quadratic model.