254 



Applied Biophysics 



high as 50% increase at 10 centimeters depth for a field 20 

 square centimeters. This opens up many new avenues in treat- 

 ment design, which will be indicated later. Even for large beams, 

 the improvement, though small (about 12%), is of importance 



T — T 1 1 1 1 r 



6 10 12 14 16 18 20 



FIG. 3. 



Abscissae = depth in cm 



Ordinates = components as % of % of depth dose 



1. 1,000 kv direct beam component as a % of the % of DD 



2. 1,000 kv forward scatter as a % of the % of DD 



3. 1,000 kv backscatter as a % of the % of DD 



4. 200 kv direct beam component as a % of the % of DD 



5. 200 kv forward scatter as a % of the % of DD 



6. 200 kv backscatter as a % of the % of DD 



in many cases of opposed-field technique. Further, with the re- 

 duction in backscatter, and also since the forward scatter is, 

 with the higher voltages, more in the forward direction, the 

 high-voltage X-ray beams show a much sharper delimitation on 

 the geometric edge of the beam and a flattening of the isodose 

 contours. 



