28 H. COTTIER, E. P. CEONKITE, E. A. TONNA, AND N. O. NIELSON 



a function of the distance from a point-source lias been computed (Robertson 

 and Hughes, 1959; Eobertson et al., 1959), but appHcation of these data to 

 in vivo dosimetry (e.g. autoradiography) is not readily feasible since it 

 involves such largely unsolved problems as distribution and availability of 

 the precirrsor in the tissues, magnitude and rate of its uptake by various cell 

 types synthesizing DNA, proHferative scheme of aU cell lines, geometry and 

 coincidence error in autoradiography, unequal intranuclear distribution of the 

 label and its possible re-utilization when the labelled ceUs disintegrate. 

 Tentative estimations of the radiation dose delivered, based on grain counts 

 in autoradiograjihs therefore depend on number of debatable assumptions 

 (Lajtha and Oliver, 1959; Cronkite et al., 1961). 



In contrast to the substantial amount of information available on acute 

 radiation damage caused by application of ^HTDR very little is known about 

 late effects of this radioactive compomid. One of the important questions is, 

 whether and at what dose level ^HTDR may exert a tumorigenic effect. 

 Since theoretical prediction in this respect is not practicable on the basis of 

 dosimetry and comparisons with the tumour-promoting effect of whole- 

 body X- or y- irradiation, long-term experiments with animals injected with 

 tritiated nucleosides are essential. Lisco and co-workers (1961) reported that 

 in CAF-mice as little as 1 ^c ^HTDR (specific activity 360 mc/m mole) per g 

 body weight, given in a single injection, may stimulate tumorigenesis. 

 However, the data presented by these authors does not at present conclusively 

 prove the carcinogenic effect of ^HTDR. 



This preliminary report deals with the time-course of development of 

 tli}Tiiic lymphomas in female C57BL/6J mice given fractionated whole-body 

 ^°Co y-irradiation as compared to animals treated with large doses of ^HTDR 

 and/or ^HCR and untreated controls, 



MATERIALS AND METHODS 



Two hundred and twenty-four female, 6-week-old C56BL/6 J mice (Roscoe 

 B. Jackson Memorial Laboratory, Bar Harbor, Maine) weighing 16 to 18-5 g 

 (average 17-3 g) were assigned by random selection to seven groups of 32. 

 They were kept in cages containmg 8 animals each and had ad libitum access 

 to water and food (Purma Laboratory Chow for mice, Ralston Purina 

 Company, St. Louis, Missouri). 



Groups I-III (32 animals in each) were given fractionated whole-body 

 ^''Co-y-irradiation (total dose 480 r, given in three fractions of 160 r at 7-day 

 intervals; dose-rate 22 r/min; mice kept in air during irradiation). 



Group IV (32 animals) was given three single subcutaneous injections of 

 ^H-thymidine at weekly intervals (3 x 10jU,c/g body weight, based on 

 average weight of each cage. Specific activity 1-9 c/m mole (Schwarz Bio- 



