Total Energy Absorption in Radiotherapy 



199 



megagram-rontgens, neglecting the absorption by the filters 

 in which the radium is packed. 



Measurements of Volume Dose 



Measurements of volume dose have been attempted by 

 Boag/ using the model constructed by Grimmett/- ^ This model 

 consists of spaced plates 6 millimeters thick, of density 0.985, 

 graphited and spaced 2 millimeters apart by thick washers of 



c a 



FIG. 3. Curve relating 



a2 _ c2 a + c 

 F(= 2a -1 log, ) 



to — where c is the distance of a point source of radium from 

 a 

 the center of a sphere of radius a. (From Mayneord.'*) 



the same material (cellulose acetate) as the plates. Alternate 

 plates are connected together, thus forming two groups of plates, 

 each of which is connected to opposite poles of a battery with a 

 sensitive galvanometer in circuit, to measure the total ioniza- 

 tion current collected from all the air-gaps, i.e., from the whole 

 body under radiotherapeutic conditions. Under these conditions, 

 guard rings were found to be necessary to prevent insulation 

 leakage, and allowance had to be made for their effect. More- 

 over, the absorption conditions for a wide range of wave lengths 

 and various angles of incidence of the X-ray beam had to be 

 similar to those for the human body. These points were all 



