Curvature in the cancer mortality dose response in Japanese atomic bomb survivors: absence of evidence of threshold

Purposes: To investigate the evidence for a threshold in the cancer dose-response curve. Materials and methods: Japanese atomic bomb survivor cancer mortality data, based on follow-up to 1990, was used, laking account of random errors in DS86 dose estimates. Results: For all solid cancers analysed together, there is a significant positive dose response (two-sided P<0.05) if all survivors who received <0.5 Sv are considered, but the significance vanishes if doses of <0.2 Sv are considered; the same is also true for leukaemia. For solid cancer mortality there is no indication of curvilinearity in the dose response: no statistically significant improvement in fit to a linear relative risk model is provided by addition of quadratic or threshold dose terms. If a relative risk model with a threshold (the dose response is assumed linear above the threshold) is fitted to solid cancer mortality data, the best estimate of the threshold is <0.00 Sv (95% CI <0.00-0.13). If a linear-quadratic-threshold model is used the best estimate of the threshold is <0.00 Sv (95% CI <0.00-0.15). For leukaemia mortality there is highly statistically significant upward curvature in the dose response. In particular, ifa relative risk model with a threshold (the dose response is assumed linear above the threshold) is fitted to the leukaemia data, the best estimate of the threshold is 0.16 Sv (95% CI 0.05-0.40) (two-sided P=0.001 for test of departure of threshold from 0). However, there is no evidence for a threshold effect (two-sided p=0.16)when a quadratic term is included in the dose response: the best estimate of threshold in this case is 0.09 Sv (95% CI <0.00-0.29). Moreover, addition of a quadratic term improves the fit of a linear-threshold model at borderline levels of statistical significance (two-sided P=0.07). Therefore, the most parsimonious description of the leukaemia dose response is provided by a linear-quadratic function of dose. Conclusions. There is no evidence of threshold-type departures from the linear-quadratic dose response either for solid rumours or for leukaemia in the Japanese atomic bomb survivor mortality data.