The risk coefficient, E, for different age groups, computed from total
dose resulting from, internal plys external exposure for Marshall Islanders,
ranged from 1.5xl0 to 7.4x10° per person-rad-years at risk, assuming a 10year latent period, and L.1x107° to 5.8xl0 ", asssuming a S-year latent
period.
These data are indicated in Tables 5 and 6,
respectively.
The total
risk coefficient, E, was used in Eq. (1) to determine the internal risk coefficient, B. For external risk coefficjents and 10-year latent period, we
chose 3.3x10°° for age <20 and 1.0xl0™" for age >29 based on the Working Grou
study (8); for S-year latent period we chose 2.5x10°” for age <18 and 1.3x107
for age >18, based on NCRP Report 80 (2). The results for internal risk coefficients are in Table 7. Finally, as we explained in the Methods, we chose a
special value for the <lO-year age group, since it was baged on a large group
of children exposed to x rays (1). This value was 4.9x10 cancers per personrad~-years at risk, and the estimate for the internal risk coefficlent was
1.3x10°°, virtually the same as the value given in Table 7 for the 10-year
latent period.
A tabulation of risk coefficient versus internal thyroid dose is given in
Table 8. These internal dose groupings resulted in Little variation in external dose as a function of age. These groupings were made to examine the
affect of dose on the value for internal risk coefficient.
Table 8
Average Dose Versus
Internal and
External Risk Coefficients, lLO-Year Latent Period
Average
Average
Internal
Internal
External
External
Total
Group
Thyroid
Risk
Thyroid
Risk
Risk
Age 1954
Dose, rad
Coefficient?
Dose, rad
Coefficient?
Coefficient
<10
1400
1.4x1076
63
3.3x1076
1.5x1076
>18
400
6 .Lx107®
66
1.0x107°
5.4x107°
10-18
560
8.0x107°
78
3.3x107°
7.4x107°
“This study.
DReference 8.
A sensitivity analysis, of the parameters in Eq. (1}, shows that the
value for the total risk coefficient, £, impacts greatly on the estimate of
the internal risk coefficient, 3,
in this specific Marshall Islands study.
This is because of the wide difference between internal thyroid dose, A, and
external thyroid dose, C. Thus, our estimate of internal risk coefficient
depends largely on the observed incidence of thyroid cancer because the total
risk coefficient, E, is very sensitive to the small number of spontaneous and
excess thyroid cancers observed.
27