140 K. SUNDARAM 



From the above it is seen that the skeletal burden of ^^'Sr is a function of 

 age of the animal at the beginning of the experiment. The animals in the 

 youngest group retain thirty-three times more ^°Sr as compared to the oldest 

 group. This is in conformity with the earlier findings of Finkel et at. (1960), In 

 addition, the specific activity and the site of deposition of ^°Sr are dependent 

 upon the metabolic state of the skeleton. 



It is thus justifiable to assume a significant difference in both the dose- 

 rate and the total dose received by the organ systems under consideration. 

 These differences would be of the same order of magnitude as the skeletal 

 burdens in the four age groups. 



The histological changes in the haematopoietic tissue contamed in the 

 femur of these animals further substantiate these assumptions. The bone- 

 marrow cells in the youngest group of animals with a skeletal burden of 33 ju,c 

 showed marked atrophy (Fig. 1). In animals belonging to group C with a 

 skeletal burden of 11 /xc, the damage was moderate (Fig. 2). In the oldest 

 group of animals with skeletal burdens of 2 or 1 /zc of ^°Sr, the cellularity of 

 the bone-marrow did not show any significant alteration. These variations in 

 the degree of damage are reflected in the frequency of leukaemia, or osteo- 

 genic sarcoma, in the four groups. Table II gives the incidence of leukaemia 

 and osteogenic sarcoma in these various groups. 



Table II 



The lowest skeletal burden of ^^Sr to cause leukaemia, is observed in 

 group B, suggesting that the dose-rate and the total dose received by the 

 bone-marrow in this group may be regarded as the minimum effective dose 

 for the induction of leukaemia in these experiments. In the group A with 

 1 /xc of ^''Sr as the skeletal burden, the absence of leukaemia during the entire 

 life-span of the animals, which incidentally was the same as the controls, 

 would indicate that the potential hazard is insignificant if the organism is 

 sufficiently old at the beginnmg of exposure. In group C with a higher skeletal 

 burden (11 /^c) and a moderate reduction in the life-span, the incidence of 

 leukaemia per rat week of observation shows an approximately 2-5 times 

 increase over that of group B. With the highest skeletal burden (33 iic) in 

 group D and with marked reduction in life-span, no leukaemia was observed. 



