756 ADVENTURES IN RADIOISOTOPE RESEARCH 



that this does not hold good in the subsequent period in which the ante- 

 rior sarcoma grows twice as quickly as the posterior sarcoma. This observ- 

 ation, however, cannot affect the results of the previous section in this 

 paper, since we have always alternately shielded and irradiated first 

 the anterior and then the posterior sarcoma in the irradiation experi- 

 ments. 



DISCUSSION 



Comparison of the nucleic acid metabolism in the irradiated sarcoma 

 with that in a sarcoma protected against the effect of radiation has been 

 found during the first 2 hr after irradiation to be in both cases about 

 one-half of that in the sarcomas of unirradiated rats. The shielding was 

 effectively shown by the control measurements, since the protected 

 sarcoma received a dose of only 13 r at an irradiation level of 2000 r. 

 The inhibition of the nucleic acid metabolism in the shielded sarcoma 

 arises from either circulation of products, formed by the action of 

 X-rays which inhibit the formation of nucleic acid, into the shielded 

 sarcoma or the effect of the radiation on the central regulating mechanism 

 of the rat. 



A decrease in the formation of nucleic acid should be accompanied 

 by a lower rate of growth of the sarcoma. The growth of the sarcoma is 

 therefore inhibited, and this also holds good for the shielded sarcoma. 

 From the observation of approximately equal formation of nucleic acid 

 the conclusion can be drawn regarding equality of volume increase. 

 Comparative volume measurements on irradiated and shielded sarcomas 

 in the same rat showed that, during the irradiation period of 4 to 6 days 

 in which a dose of from three to four times 500 r was delivered, the 

 volume increases did not much differ (the shielded sarcomas grow at 

 about 1.3 times the rate for the irradiated ones). In the two periods 

 following, of 8 and 6 days, the irradiated sarcoma showed a substantially 

 lower (one-third) increase than the shielded sarcoma. The formation of 

 nucleic acid in the shielded sarcoma must, therefore, be considerably grea- 

 ter in these periods than that occurring in the irradiated sarcoma. The 

 inhibition of nucleic acid formation which arises in the shielded sarcoma 

 as a result of the action of X-rays is, therefore, much more easily com- 

 pensated by regeneration processes than that caused by direct irradiation. 

 An indirect effect of X-rays on the growth of the tumours is consider- 

 ably less permanent than the effect of direct irradiation. 



