Total Energy Absorption in Radiotherapy 



195 



of scatter is maximal. If all the radiation scattered outside the 

 geometrical beam were confined to it then, for any size of field, 

 the energy absorbed at any point of the beam would be the same, 

 at the same depth, as for the saturated pencil. Thus, assuming 

 that all the scattered radiation is absorbed and that none 

 escapes from the body, the volume dose is estimated by the 

 product of the area of the field on the skin, the dosage in rontgens 

 (corrected to allow for the *'unsaturation" of the field) and a 

 graph reading. The graph (figure 1 ) is obtained by integrating 

 the area under the depth-dose curve for the saturated axial 

 pencil of a very large field. The correction for unsaturation is 

 the ratio of the dosage rate with maximum scatter to the 

 measured axial skin dose of the field concerned. 



FSD. 



2 4 6 8 10 12 14 16 IB 20 22 24 26 28 50 

 Thickness of Tissue in centimetres. 



FIG. 1. Graph Relating \'olume Dose per Rontgen at Skin Surface to 

 Thickness of Tissue through Which the Beam Passes. (From Ellis."*) 



Thus, comparing a field of 400 square centimeters and one of 

 50 square centimeters, we have the following factors (200 kilo- 

 volts, constant potential 1 millimeter Cu 1 millimeter Al 40 

 centimeters FSD *) : 



* FSD ^ Focus-skin distance. 



