TOTAL ENERGY ABSORPTION IN RADIOTHERAPY 



FRANK ELLIS, M.Sc, M.D., F.F.R. 

 Medical Director, Radiotherapy Department, London Hospital 



Introduction 



THE dose of radiation absorbed at a point affecting indi- 

 vidual structures, such as chromosomes, determines the 

 local effect on these structures, and is the effect which is 

 desired by the radiotherapist in the neighborhood of the 

 malignant tumor. To enhance this effect by variations in quality, 

 dose, dosage rate, fractionation, and total time is one of the 

 chief aims of the radiotherapist. At the same time, however, 

 general effects are produced by the radiation and manifest them- 

 selves in organs which have not been irradiated. These effects 

 are troublesome and difficult to avoid and, in attempting to 

 correlate them with dose, I perceived the necessity for estimates 

 of the total energy absorption by the body. I, therefore, asked 

 Dr. Happey to investigate the problem so as to provide an 

 estimate of the volume dose in ^'rontgen cubic centimeters" 

 (rcm.^). Mayneord, however, was also engaged in a similar 

 investigation on different lines, and had coined the terms "in- 

 tegral dose" and "megagram-rontgen." The latter is a more 

 convenient unit and a more euphonious term, and so is to be 

 l)referred to the term "rontgen cubic centimeter." It is intended 

 in this short paper to discuss briefly the physical approaches, 

 attempts at correlation with biological effects, and then the 

 practical value of the conception of volume dose. 



Physical Estimates 



Happey '•*• ^*^* points out that the energy absorbed in the axial 

 pencil of a very large field is maximal because the proportion 



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