Shape
of the doseresponse relationship and its impact on leukemia risk
A
number of scientists have pointed out that official dose calculations often use
inadequate propagation models and very low dose coefficients for internal
emitters which, together, could account for a possible underestimation of doses
by a factor 10 to 100. But we need to explain a difference of a factor of 1000.
What did we overlook? A
common assumption is that a 100% increase in effect requires a 100% increase in
dose. This, however, is not true if the doseresponse is nonlinear. In addition,
residents
near NPPs are exposed to widely fluctuating dose rates over the year and not to
a constant low dose rate. With a curvilinear doseresponse, the effect will be
determined not by the average dose but by the dose from emission peaks. This
means close examination should be made of emission peaks, eg those resulting
from the opening of reactors during refuelling. To date, we do not know the shape of the doseresponse curve for prenatal leukaemia induction. But after the Chernobyl accident, a significant increase of perinatal mortality was found in Germany and the risk depended on the dose to the power of 3.5, ie there was a curvilinear, not linear, doseresponse relationship [1]. Assuming that the doseresponse for in utero leukemia induction is curvilinear with also a power of dose of 3.5, a 10% increase of dose, eg an increase from 1 mSv per year to 1.1 mSv per year, would yield a 40% increase in leukemia risk. And if the emissions are not constant but characterised by peaks, the resulting effect will be even greater. If the whole annual dose is delivered during the month of refuelling, a 10% increase in annual dose will translate into a 120% increase in leukaemia risk. This will be shown in the following. With a background dose rate of 1 mSv per year (ie 0.083 mSv per month) the total dose rate during the month of refuelling will be 0.1 + 0.083 = 0.183 mSv per month. Expressed per year, this is a rate of 2.2 mSv per year. For the rest of the year, the dose rate would remain unchanged at 1 mSv per year. Assuming a doserisk relationship where risk is proportional to dose to the power of 3.5, the radiation risk would be 2.2^3.5 = 15.8 times greater during the refuelling month, than the risk during the rest of the year. Therefore the increase in radiation risk over the whole year would be a factor of (15.8 x 0.083) + (1 x 0.917) = 2.23. A
curvilinear doseresponse would be a consequence of the reasonable assumptions
that individual doses and radiosensitivities are randomly distributed in a
population. As can be shown, the mathematical form of the resulting
doseresponse relationship is a cumulative lognormal distribution function which,
at low dose values, has a strongly curvilinear shape [2]. Such a nonlinear shape,
together with corrected dose estimates, have the potential to explain the
doubling of leukaemia rates observed near German NPPs.
2
Körblein A. Einfluss der Form der DosisWirkungsbeziehung auf das Leukämierisiko
(in German).
