For our considerations of the 1920-1960 breast-irradiation-doses, we wish to include only doses up to 1960 from weapons testing. For 1960 and beyond, such irradiation will be considered in further research on this topic. While we shall see in this discussion that weapons-test fallout contributes very little to the 1920-1960 breast-doses, the much larger fraction of the fallout in the years beyond 1960 implies that a larger contribution from weapons-test fallout can be anticipated in the post-1960 period.
Since there were no doses of any consequence before 1945, we have 62% of the 1920-1960 period without any contribution of fallout to dose. The period 1945-1960, or 38% of the total years, needs an estimate of dose contribution. Whatever that total dose-contribution is, it will be divided by 40 for the total years, 1920-1960.
Two Major Classes of Radionuclides
There are two major classes of radionuclides, those of very short half-lives, and those of relatively long half-lives. For those of relatively long half-lives (such as strontium-90 and cesium-137), the radiation dose per year is a small fraction of the ultimate dose-commitment. Since we are dealing with yearly radiation doses in this analysis of breast-cancer, we need to consider nuclear test fallout doses on a per-year basis. For those radionuclides of very short half-lives, material injected into the stratosphere largely decays before returning to earth, and hence much of the potential dose does not occur.
The Time-Distribution of Fallout Deposition
UNSCEAR 1977 (Annex C) Table 2 (at p.122) provides some of the requisite information. We have there the annual deposition and cumulative deposition of strontium-90 up through January 1976. We shall, of course, use the data for the Northern Hemisphere, since pre-1960 fallout was largely there.
Strontium-90 Deposition Year Deposition in Mega-Curies Pre-1958 1.80 1958 0.63 1959 1.05 1960 to Jan. 1976. 8.65 _________________________________________ Grand total 12.13
Therefore, we have the approximation that before 1960, 3.48 mega-Curies out of a total of 12.13 mega-Curies, or about 28.7 % of the total, fell out. As a good approximation, we shall state that the cesium-137, the other prominent long-lived radionuclide, fell out quantitatively, before and after 1960, as did strontium-90.
The Short-Lived Radionuclides
The combination of Ce-144, Ru-106, Zr-95, Ru-103, Ce-141, and Ba-140 (plus their short-lived daughters) provide the major share of worldwide exposure in the very early years post-test.
At Table 26, p.153 of UNSCEAR 1977, external radiation from short-lived nuclides is given as 48 milli-rads. As a reasonable approximation for the pre-1960 period, we multiply this value by the same fraction, 0.287, as used for strontium-90. External dose from short-lived nuclides before 1960 is (0.287 x 48), or 13.8 milli-rads.
While we believe UNSCEAR has overestimated the correction for body shielding of the breast, we shall, as part of our conservative approach, utilize their values for organ doses directly, and we shall accept 13.8 milli-rads as dose to the breasts for the short-lived gamma-emitters during the 1945-1960 period.
The Long-Lived Radionuclides: Cesium-137 and Strontium-90
At Table 26, the dose contribution from external Cs-137 is given as 62 milli-rads (lifetime commitment). Using our 0.287 factor, we have 0.287 x 62, or 17.8 milli-rads. Gofman (1990, Ch.36, p.29) provides the datum that cesium-137 external dose during first 10 years is 20% of the all-time total. And since the pre-1960 period means less than 10 years since deposition, on the average, we can say external Cs-137 dose is less than 20% of total, or less than 20% of 17.8 milli-rads, or 3.6 milli-rads. Let us assign one-half of this value, or 1.8 milli-rads, for the shortness of the exposure period.
Total dose in the early period (before 1960) = 13.8 + 1.8, or 15.6 milli-rads delivered to breast from all external gamma-emitting sources.
For internal dose from Cs-137, it is estimated that 95% of the total effect is attained in a few years after deposition. The general rule is that internal / external dose, for Cs-137, is 3 / 7. But since the internal commitment is nearly over a few years after deposition, we must use the total external committed Cs-137 dose, which is 17.8 milli-rads, to be multiplied by (3/7), and this gives a value of 7.6 milli-rads for internal breast dose from Cs-137.
So, for the early period, total external dose to breast = 15.6 milli-rads. And for the same period, total internal dose to breast = 7.6 milli-rads. Combined total, internal plus external = 23.2 milli-rads to breast.
What about dose from strontium-90? Up to 1960, we can neglect it. This nuclide, having no gamma ray, delivers no breast-dose when it is external to the body. And the internally deposited strontium would add very little to the breast-dose during this short period of time (pre-1960). However, by also ignoring the external and internal dose from cesium-134 (radiological half-life of 2.06 years), we do underestimate dose here.
The Total Breast-Dose up to 1960
The 23.2 milli-rads must be divided by 40 to obtain the average year's contribution to dose in the 1920 to 1960 period.
Therefore, final entry, for all ages, in the Master Table, for fallout from weapons testing is (23.2 / 40), or 0.58 milli-rads per year of high-energy radiation. And since all our entries in the Master Table are in medical rads, we first convert to rads, and then divide by 2 to correct to medical rads. The final result in medical rads to transfer to the Master Table, Column "O," is 0.00029 medical rads per year for the average year of 1920-1960 period.
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