approximation (sH in Figure 10-1) at a particular high dose DH is α + βDH, the slope of the low-dose linear approximation (sL in Figure 10-1) is α, and the DDREF corresponding to DH is their ratio, 1 + (β / α)DH (UNSCEAR 1993). A natural numerical quantity for curvature characterization, therefore, is β / α, which is not tied to any particular high dose. This ratio is referred to here as the LQ “curvature” and is represented by the symbol θ (i.e., the reciprocal of the so-called crossover dose).
If the correct curvature, θ, is known, then an LSS DDREF may be defined through the following steps: an LQ model for ERR or EAR is estimated from the LSS data in such a way that the curvature is constrained to be θ, that is, by fitting the relative risk model αLQ(Dose + θDose2) for fixed θ and with unknown linear component αLQ. A separate linear model is estimated from the same data: αLDose, with linear component αL. The LSS DDREF is the estimate of the ratio of the two linear components, αL / αLQ. The resulting DDREF can be used to convert a risk estimate based on the linear model projection to one based on the linear component of an estimated LQ model with curvature determined by a given choice of the value of θ. Figure 10-2 illustrates the definition for two possible choices of this value.
The two definitions of DDREF as a function of LQ curvature must be clearly distinguished: the fixed high-dose DDREF (or UNSCEAR definition), DDREF = 1 + θ × high dose, and the LSS DDREF defined by the estimation process in the preceding paragraph. The first is a function of θ and some particular high dose. The second is a function of θ and the LSS data. Their relationship, as illustrated in Table 10-1, indicates that the LSS DDREF based on A-bomb survivors with doses of 1.5 Sv or less is roughly equivalent to the fixed high-dose DDREF at an effective high dose of about 1 Sv. In other words, in terms of the familiar UNSCEAR single high-dose definition, one can act as if the nonzero LSS doses were concentrated at a dose of 1 Sv.
Table 10-1 may be used as an aid in interpreting radiobiological evidence for curvature. If, for example, radiobiology data indicate that a DDREF of 2 is appropriate for adjusting risks based on a linear model derived at the single high dose of 2 Sv, then the implicit curvature is 0.5 Sv−1 and the corresponding LSS DDREF is 1.5.
The committee estimates LSS DDREF in this report by combining radiobiological and LSS evidence concerning curvature via a Bayesian statistical analysis and applying the definition of LSS DDREF to the result. As detailed in Annex 10B, the