RADIATION-INDUCED BONE TUMOURS 



151 



111 growing bone, the epiphyseal plate moves away from the deposit of 

 32p as the bone lengthens, with the result that the isotope deposit and con- 

 comitant bone damage appears to move down through the metaphysis and 

 eventually becomes a part of the diaphysis. With repeated injections the 

 continued growth of bone leads to a new band of maximum uptake in the 

 newly-formed bone just below the epiphyseal plate, after each injection. The 

 volume of bone irradiated is therefore greater than with a single injection. 



Mortality with time of appearance of bone tumours 



32r 



3-0/^c/g 



l-0+3)t0-6/ic/g at 

 2-week intervals 

 = 2-8/xc/g 







10 15 



Months 



20 



Fig. 1. Comparison of bone-tumour incidence with single (3-0 fj.c/g) and repeated (1-0 -f 

 3 X 0-6 jLtc/g) injections of ^^P. 



Histological studies show that, with repeated injections, obvious damage 

 does extend over a greater area and is of a somewhat different character. In 

 particular, there is a greater degree of peritrabecular fibrosis, which in the past 

 has been suggested by some workers as an important factor in bone-tumour 

 production. 



Another factor which might increase the relative carcinogenic efficiency 

 of the repeated injections is that the mean radiation dose to the cells in the 

 irradiated volume is less than with a single injection, permitting a greater 

 survival of potentially mahgnant cells. On the other hand there may be some 

 groups of cells which receive a higher radiation dose with repeated injections. 

 In support of this suggestion there is evidence of a migration of cells up the 

 trabeculae, following the retreating epiphyseal plate (Kember, 1960), and 

 such cells could be in the region of maximum radiation dose for each of the 

 repeated injections. For 3-0 juc/g total injection, radiation dose measurements 

 in bone (Blackett, 1962) show that such cells would receive an accumulated 

 dose of about 11,000 r with repeated injections, compared with 6,000 r for a 

 single injection. On the other hand cells just below the plate at the first 



