32 H. COTTIER, E. P. CRONKITE, E. A. TONNA, AND N. O. NIELSON 



^HTDK/g body weight should be sufficient to rise the incidence of neoplastic 

 disease (Lisco et al., 1961), it is interesting to note that three doses given at 

 weekly intervals and each ten times higher, as used in our exjDeriment has not 

 as yet, up to 250 days after mjection, produced any detectable thymic 

 lymphomas. It must be mentioned however, that we are not dealing with the 

 same strains of mice, and that Lisco and co-workers (1961) mjected the ^HTDR 

 into animals of different ages. In addition to this, these authors followed their 

 mice over a longer period of time than we have to date. It is evident that 

 more data are necessary to clarify the problem. Experiments like these, 

 aimed at detecting possible tumorigenic and/or leukaemogenic effects of 

 tritiated nucleosides, have not only practical implications in determinmg the 

 possible hazards involved in the use of these radioactive compounds but they 

 are also of interest for the understanding of basic phenomena related to 

 radiation leukaemogenesis and carcinogenesis. With '^HTDR, radiation is 

 delivered predominantly within the nuclear material sparing most of the 

 cytoplasm. To some extent the contrary is true for ^HCR, smce autoradio- 

 graphic findings at later stages following isotope injection often reveal a much 

 higher grain count over the cytoplasm. Thus it may be expected that 

 experiments such as those of the present investigation, may provide some 

 information as to the relative importance of nuclear versus cytoplasmic 

 damage in the pathogenesis of radiation-mduced neoplastic growth. 



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