Total Energy Absorption in Radiotherapy 



209 



gamma radiation respectively are in the ratio of 15 : 35 : 40, for 

 a wide beam enclosing the body, and a very large FSD — i.e., the 

 conditions under which radiation is received by medical workers. 

 In other words, for a given dose in "rontgens" to the skin — which 

 is the present method of estimating tolerance dose — the energy 

 absorbed by the body may vary considerably from one type of 

 radiation to another. Since the biological effect considered in 

 the internationally accepted figure of 10"^ r per second is a gen- 

 eral effect rather than a local one, it would seem more accurate 

 to aim at a volume dose estimation rather than a surface dose. 

 It is interesting to note that the international figure for diagnostic 

 X-rays (10'^ r per second) is three times that for gamma rays, 

 and that this ratio, decided by experience, is of the order of the 

 ratio of the volume doses of 40 kilovolt X-rays and gamma rays. 

 The following table (IV) shows the influence of technique on 

 the volume dose in treating cancer of various sites. 



TABLE IV 



Technique and Total Absorption or Volume Dose. HVL-0.15 mm Cu FSD-40 cm 



Dose 



Region 1,000 r 



Tonsil 4.5 



Fauces 4.0 



Larynx 5.0 



Brain 4.0^ 



Bladder 5.6 



Pelvis 3.0 



(supplement to radium) 3.0 



Esophagus 6.0 



Lung 4.0 



Lung 5,5 



Fields 

 No. cm^ 



2 X 10/8 

 2x 6/4 



2 X 10/15 

 2x 6/4 



2x 6/8 



1 X 6/4 



2 X 10/8 

 IX 6/8 



8x 8/10 



2 X 10/15 

 2 X 10/15 



8 X 15/4 



4 X 10/15 



5X 6/8 



Total 



absorption 



r cm3 



777 X 106 



11.26 X 106 



4.53 X 106 



11.97 X 106 

 17.24 X 106 

 25.97 X 106 



31.1 X 106 

 30.3 X 106 

 19 X 106 



