152 



BENSTED, BLACKETT, COURTENAY, AND LAMERTON 



injection and not subsequently moving up would receive an accumulated 

 dose of 9,000 r for the repeated injections and 10,000 r for the single injection. 

 There are still other factors which may be of importance in determining 

 the difference in response between single and repeated injections, for instance 

 the change in radiation dose-rate and the change in type and number of cells 

 irradiated due to the changing cell population in the bone. Because of the 

 considerable difference in dosimetry between repeated and single injections 

 it is bound to be difficult to pin-point the factor or factors responsible for the 

 difference in tumour response. 



239Pu EXPERIMENTS 



A long lived bone-seeking a-emitting isotope such as 239p^ -^^rjj] gjyg g^ yg^y 

 different pattern of radiation dose distribution in bone from that given by 

 22P (Taylor et al., 1961). It is of interest to study the effect of fractionation of 

 the injection and Fig. 2 shows a comparison of mortality and bone-tumour 



Mortality with time of appearance of bone tumours 



239p^ 



3-0/zc/kg 



20 



4-> 



O 



10 



(L) 



E 



Z3 



! 



l-0+4x0-5;xc/kg at 

 2 -week intervals 



x}Bo 



ne tumour 



-I : I I I — 1 1 1 I I 1 \ ] 1 1 I ^ ' ■ ' ■ O . . . 



10 15 



Months 



20 



25 



Fig. 2. Comparison of bone-tumour incidence with single (3-0 /ic/kg) and repeated (1-0 -j- 

 4 X 0-5 ^ic/kg) injections of ^^^Pu. 



incidence for a single dose of 2-9 jLtc/kg and for an injection of 1 juc/kg followed 

 by four two- weekly injections of 0-5 ju-c/kg. About twenty animals were used 

 in each experiment and it can be seen that the final incidence of bone tumours 

 was high in both groups. With a single injection, 17 out of 22 animals 

 developed bone tumours between twelve and twenty-two months. With 

 repeated injections 11 out of 20 animals developed tumours between ten and 

 seventeen months. However, the greater mortality from other causes in the 

 repeated injection group prevents one from drawing firm conclusions about a 



