¢t ‘time, since the pathological response is saturated in this experiment, it is inappropriate to draw any inference about the magnitude of the response at smaller burdens. The smallest burden (at death) in a dog showing lung cancer was 0.2 uCi. Presumably this would correspond to a particle burden of about 107 particles. Burdens which are’ smaller by orders of magnitude may still induce a substantial incidence of cancer. Indeed, P,P the cancer risk may, as for skin and soft tissues, correspond to a risk per particle in the neighborhood of 1/1000 to 1/10,000. VI, Critical Particle Activity Not all particles would be expected to result in these high cancer probabilities. As the particle size or specific activity per particle is reduced so is the dosage to the surrounding tissue. Indeed, at sufficiently small particle — size or specific activity, one would expect the radiation - insult to behave similar to uniform irradiation. The study of Albert on induction of cancer in rat skin indicates a precipitous change in the dose response curve as the dosace | exceeds 1,000 rem>>. (See Figure 2). This suggests that a particular level of tissue damage must occur before this unique carcinogenic response occurs. 55/ Albert, R.E. ' ect al, —— Radiation The experiments of “OS =O, . On. cit. — t pr. Figure 7; reproduced in Geesaman, UCRL-12357 Addendum, On. -p. 2.