270 Applied Biophysics 



anterior surface. For X-rays excited at 200 kilovolts (Philips' 

 therapy tube with 1.1 millimeters Cu added; HVL of 1.35 milli- 

 meters Cu), the value is about 46,000 gram-rontgens per surface 

 rontgen. Again, for 1,050 kilovolt X-rays (Metropolitan- 

 Vickers' tube with filtration of 4.22 millimeters steel -j- 2.0 milli- 

 meters Pb + 2.0 millimeters Al ; HVL of 10.4 millimeters Cu), 

 the integral dose is 51,000 gram-rontgens per surface rontgen, 

 while for radium gamma rays (filter equivalent to 1.3 millimeters 

 Pt ; HVL of 16 millimeters Cu), the value is 59,000. This varia- 

 tion of the integral dose indicates that it may, in future, be 

 necessary to express the tolerance dose of X- or gamma radia- 

 tion in terms of the integral dose, measured in gram-rontgens, 

 rather than in terms of the surface dose, measured in rontgens. 

 Alternatively, since in practice it will be the surface dose which 

 is likely to be measured, it may be necessary to adopt different 

 values of the tolerance dose, expressed in rontgens, for various 

 qualities of radiation. 



Genetic Eflfects 



At this stage, it would be well to consider briefly the effects 

 of ionizing radiations on genes and chromosomes and the influ- 

 ence which this knowledge may have in fixing limits to the 

 amount of radiation which a person should be given. It is known 

 that all types of ionizing radiations produce mutations, either 

 of the individual genes or of the chromosomes, the rate of muta- 

 tion being linearlv proportional to the amount of radiation 

 received. That is to say, no matter how small the given dose, 

 there is a chance that a mutation may occur, although that 

 chance will be very small. There is, therefore, no such thing 

 as a tolerance dose for genetic effects, if one interprets the phrase 

 "tolerance dose" in its ordinary sense, namely, that the human 

 body suffers no ill effects from such a dose. The genetic effects 

 of radiation are accumulative and irreversible since, apparently, 

 the mutation of a stable gene leads to another gene which is 

 equally stable. 



As the majority of hereditable changes are recessive in char- 



