Technical Methods in X-Ray Therapy 



217 



surgery, it is generally better to use a beam of less penetration 

 and small focus-skin distance, so that the dose in the healthy 

 tissues beyond the lesion diminishes rapidly with the depth in 

 the tissues. A rough measure of this total body dose is obtained 

 by summing the product of dose and volume throughout the 

 body, though it is evident that this is only a rough guide, as the 

 susceptibility to radiation of each element of volume as well 

 as the dose there determines the aggregate effect.^ 



I 



I 



FIG. la. IsoDOSE Curves for a Contact-Therapy Field 



Field 1 cm circle, focus-skin distance 2.2 cm, radiation generated by 45 kv and filtered 

 by 2.5 mm aluminum, HVL 1.6 mm aluminum (Mayneord ^) 



The possible ways of combining X-ray fields to produce a 

 desired distribution are studied with the aid of isodose charts. 

 Typical charts are shown in figure la (for a low-voltage contact- 

 therapy tube), and figure lb (for a deep-therapy tube).^^ The 

 dose distribution in a plane through the body due to a certain 

 field is described by curves, which each join points of the same 

 dose rate expressed as percentages of that at the center point of 

 the field on the skin. Strictly speaking, these charts are not 

 obtained by measurements in the human body, but in a "phantom" 

 constructed of material the absorption and scattering of X-rays 



