ON TECHNICAL METHODS IN X-RAY THERAPY 



J. READ, B.Sc, Ph.D. 

 Physicist, Radiotherapy Department of the London Hospital 



General Survey 



X-RAY therapy is often roughly divided into various 

 classes — contact therapy, superficial X-ray therapy, deep 

 X-ray therapy, supervoltage therapy — yet these classes, and 

 the various methods within each, all have certain physical prin- 

 ciples in common. Firstly, it is desired to produce a chosen 

 distribution of X-ray dose through a patient's tissues by combin- 

 ing the necessary number and arrangement of fields. It mav be 

 considered adequate to produce more than a certain minimum 

 dose throughout a region, such as a tumor, with as httle as 

 possible elsewhere, without caring what the maximum in this 

 region may be. A more stringent requirement is that the dose 

 be uniform throughout the region. Ungar *^' has shown that 

 under certain conditions, the total radiation absorbed by the 

 body is a minimum, for a given tumor dose, when that dose is 

 uniform throughout the tumor. A general requirement is that 

 the dose at the skin, where each beam enters, shall not exceed a 

 certain value, account being taken of all contributions from 

 other beams. There may also be other regions where it is 

 particularly necessary to keep the dose small. 



Secondly, it is desired to keep the radiation dose absorbed by 

 all the healthy tissues as small as possible in relation to that 

 absorbed in the treated volume. This recjuirement not only in- 

 fluences the manner in which the X-ray fields are arranged to 

 give the desired dose distribution ; it also largely determines the 

 class of therapy chosen. If a lesion is near the surface of the 

 body, or accessible through a body cavity, or with the aid of 



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