| Abstract: | Abstract Evaluation of data on the frequency of severe mental retardation among prenatally exposed survivors of the atomic bombing of Hiroshima and Nagasaki, and the performance of such survivors on intelligence tests and in school has shown the most striking effects on the developing brain of exposure to ionizing radiation to occur among those individuals exposed in the 8th through the 15th, and the 16th through the 25th week after fertilization. This is true under the earlier T65DR and the newer DS86 systems of dosimetry. Seizure data have also revealed a significant effect of radiation in the 8-15 week period after fertilization. Although the observed frequency of severe mental retardation in the 8-15 week period can be satisfactorily described by a linear no-threshold model with both dosimetries, there is some suggestion of a threshold in the low dose region with the DS86 uterine absorbed doses but not with the T65DR fetal absorbed doses. However, the location and even the existence of a threshold in these weeks after fertilization is difficult to demonstrate statistically with the DS86 uterine absorbed doses. If the threshold is estimated using all 19 cases of mental retardation within these weeks, the lower bound of the confidence interval of the estimate includes zero, and thus a threshold cannot be convincingly established. However, when we exclude two probable nonradiation-related cases of Down's syndrome, the results suggest a threshold with a lower bound in the 0.10-0.20 Gy region. Both dosimetries indicate a threshold in the dose-response function for mental retardation in the 16-25 week period, probably in the 0.23-0.70 Gy interval. The seizure data provide no persuasive evidence of a threshold in the 8-15 week period after fertilization; the 95% lower bound of the estimate of the threshold includes zero. Finally, although the mean IQ scores, and the mean school performances in the low dose region are similar to the values in the control group, particularly so with doses under 0.10 Gy, evidence for a threshold is not compelling. |
