252 Applied Biophysics 



meters FSD ; improvements at distances greater than this being 

 slower but appreciable. If %DD improvement were the main 

 object, the optimum condition would be about 0.5 millimeter 

 added lead filter and as long a FSD as possible, since the effi- 

 ciency of improvement is greater by FSD than by filtration at 

 this voltage. 



In our case, however, where the main investigation was 

 whether there was an increase in radiosensitivity of lesions with 

 reduced wave lengths, a harder beam obtained with a 2 milli- 

 meter lead filter was decided on, with a FSD of 100 centimeters, 

 giving an X-ray output of 40 rontgens per minute, comparable 

 with the output of 200 kilovolt equipment. 



Physical Advantages of the High-VoUage Beam 



Since previous experience had been confined to 200 kilovolt 

 X-rays, the main interest physically lay in a comparison between 

 the behavior of the beams in a phantom, and an attempt has 

 been made to formulate reasons for the differences. The main 

 improvement with reduction in wave length is the increased 

 penetration, but the %DD is a complicated feature in which 

 variation of back- and forward scatter, FSD, depth, absorption- 

 coefficient, and field area all play a part, and an attempt was 

 made to sort out these effects bv measurement and calculation. 

 Figure 3 gives the proportions of direct, back- and forward 

 scatter obtained as a percentage of the depth dose on the beam- 

 center-line for 10 X 10 centimeters beams at 40 centimeters FSD 

 200 kilovolts and 100 centimeters FSD 1 million volts. At the 

 surface at 200 kilovolts, the dose is 71% direct, 29% backscatter, 

 while at 1,000 kilovolts, it is 93^/o direct and 7% backscatter. 

 As we progress through the phantom at 200 kilovolts, the direct- 

 beam component decreases more rapidly not only relatively, but 

 also absolutely, while at 10 centimeters depth it becomes even 

 less than the backscatter component. At 1,000 kilovolts, the 

 backscatter component remains only a small portion of the dose. 

 The forward scatter in both cases increases rapidly and is of the 

 same order. 



