ULTRAVIOLET RADIATION AND CANCER 



549 



Table 14-4. Effect of Discontinuing Schedule of Doses 



(From Blum, 1950b.) 

 For all pxperimonts, data arc for 50 per cent tumor incidence; £) = 1.8; i = 1.4. This 

 is the experiment described in Fig. 14-8. All the series were run simultaneously. 



* Period without exposure. 



14-4, observed values for td are compared with values calculated on the 

 basis of the equations describing progressive acceleration of tumor growth. 

 It is seen that the observed values of td are greater than the calculated, 

 indicating that the rate of tumor growth is slower at some point than is 



2 1 2 2 2.3 2.4 



TUMOR DEVELOPMENT TIME (td), lOg days 



Fig. 14-9. Effect on development time of the interruption of doses. In experiments 

 DA, DC, DE, DG, and DI the exposures were discontinued for 30 days and then 

 resumed. The arrangement of the periods of respite are indicated in Table 14-5. 

 The control, DK, consisted of a smaller number of animals; the curve drawn is based 

 on pooled data. {From Blum, 1950.) 



predicted by the equation. The discrepancy between the observed and 

 calculated values of td increases systematically with increase in td- 



Another type of experiment is illustrated in Fig. 14-9.'- In this, an 



'2 Similar experiments have been described by Rusch and Kline (1946), but the data 

 are not applicable to the present analysis and therefore direct comparison cannot be 

 made. Their interpretation does not take growth rates into accovmt and is subject to 

 general criticisms already made in this chapter. 



