BASIC DIFFICULTIES IN TRACER METHODOLOGY 



67 



sible are as follows: (a) The P^- becomes incorporated in the nucleic acids 

 and nucleoproteins of the chromosome, and the probabilities are high that 

 single and multiple "hits" on chromosomes will result from the beta par- 

 ticles emitted, (b) The recoiling atom nucleus will probably have 

 enough energy to break a chemical bond, (c) The P^'^ is converted to 

 S^', so that there is some molecular rearrangement at the point of decay. 

 There may also be effects in tissue from activity located nearby, but these 

 will usually be of secondary importance. 



It is obvious that each radioisotope, having characteristic radiation 

 properties and specific behavior in the biological system, must be consid- 

 ered as an individual case. Thus direct evidence on internal radiation 

 effects should be sought. But this type of experimentation is usually 

 uninspiring, and very few data are available. 



Let us then examine the literature primarily from the standpoint of the 

 lowest levels of radioisotope dosage at which there have been indications 

 of pathology or dysfunction. For practical purposes, the information 

 will be considered on the basis of the amounts of radioisotope used in the 

 system rather than the radiation dosage delivered to the tissue. Amounts 

 of radioisotope are expressed in microcuries (^c), where 1 /xc corresponds 

 to an amount of radioactivity undergoing 3.7 X 10'' dis/sec (see Chap. 3 

 for a definition of the curie unit). 



An extensive compilation by Bloom (17) has included histologic obser- 

 vations in laboratory animals after administration of various levels of 

 certain radioisotopes. Although these studies were aimed primarily at 

 the LDr,n range, nevertheless they serve to establish a starting point. A 

 liberal interpretation of some of the data is presented in Table 2-1. It is 



Table 2-1. Summary of Administered Levels of Radioisotopes at Which 

 Injury or No Injury Was Observed in Mice and Rats" 



LDs^o . 

 Bone . . 



Bone marrow . 



Spleen 



Lymph nodes. 

 Testis 



Effect 



No effect 

 Injury 

 No effect 

 Injury 

 No effect 

 Injury 

 No effect 

 Injury 

 No effect 

 Injury 



Sr89 



7 



0.5 



0.86 



0.2 



0.5 



0.2 



0.5 



. 80 



2.0 



2.9 



Radium 



1 



0.005 



0.02 



0.005 



0.02 



0.02 



0.06 



0.02 



. 06 



. 005 



02 



p32 



4 



2.5 

 9.2 

 0.6 

 2.5 

 2.5 



2.5 



2.5 

 9.2 



" All values in microcuries per gram body weight administered parenterail}\ 

 (Compiled from William Bloom, ed., " Histopathology of Irradiation," .McGraw- 

 Hill Book Company, Inc., New York, 1948.) 



