THRESHOLD FOR RADIATION-RELATED SEVERE MENTAL-RETARDATION IN PRENATALLY EXPOSED A-BOMB SURVIVORS - A REANALYSIS

Significant effects on the developing human brain of exposure to ioniz ing radiation are seen among individuals exposed in the 8th-25th week after ovulation. These effects, particularly in the highly vulnerable period of 8-15 weeks after ovulation, manifest themselves most dramati cally as an increased frequency of severe mental retardation. However, the distribution df cases of severe mental retardation suggests a thr eshold in the low-dose region. The 95% lower bound of the threshold in those survivors exposed 8-15 weeks after ovulation was zero for the i ndividual data based on the simple linear model, and 0 . 15 Gy based o n the exponential linear model used in our previous report (1987), but the 95% lower bound of the threshold based on all of the data includi ng 21 additional cases with known doses appears to be 0 . 05 Gy using the maximum likelihood estimates derived from an exponential-linear mo del. The latter model was selected because it provides the best fit fr om the standpoint of the stableness and reasonableness of the estimate s among the five models applied to the data. When two probably non-rad iation-related cases of Down's syndrome are excluded from the 19 menta lly retarded cases exposed 8-15 weeks post ovulation, the 95% lower bo und of the threshold is in the range of 0 . 15-0 . 25 Gy based on the exponential-linear model used in 1987, but is in the range of 0 . 06-0 . 31 Gy when the more reasonable and better model applied here is use d. For exposure in the 16-25-week period based on the same model, the 95% lower bound of the threshold changed from 0 . 25 to 0 . 28 Gy, bot h with and without inclusion of the two probable non-radiation-related mentally retarded cases; one of these cases was probably familial in origin since there was a retarded sibling, and the other due to infect ion, since the individual had Japanese B encephalitis at age 4 years.