260 



Applied Biophysics 



bad case came to light in an investigation into distribution, in 

 the course of postoperative radiation in carcinoma of the breast, 

 where, at 200 kilovolts, the measured dose was one-third of that 

 calculated, mainly due to the fact that the angles of the beams, 

 at that particular point, were the same as the ribs. 



FIG. 7. Device for Measuring the Angle of a Line Joining Two Points 



At a million volts, discrepancies have been small and rarely 

 more than 10%. This may be partly due to the fact that so little 

 of the dose at a depth depends on scatter, and the surrounding 

 conditions do not therefore aflfect the dose to any appreciable 

 degree. 



Because of these discrepancies, there is sometimes a tendency 

 to feel that the complicated and sometimes laborious calculation 

 of the theoretical distribution of radiation is unnecessary. It 

 must be pointed out that the cases quoted are the worst en- 

 countered, and that unless investigations commence from some 

 mathematical basis, particularly when analyzing a group of sim- 



