Million-Volt Therapy 253 



The direct component of the beam can be represented by 

 Id and is 



Id — lu^ 



where I^ is the air dose at the surface — FSD ; F ; ^i, the absorp- 

 tion-coefficient and Id the dose at depth d, due to direct beam. 

 Both the backscatter and forward scatter components will in- 

 crease with field area up to a maximum, beyond which any 

 further added beam area will not contribute to the central dose, 

 since it will be beyond the range of the secondary scatter. 



From the curves and the above, certain forecasts can be 

 made. (1) Since the penetration, i.e., the direct beam, is higher 

 at 1,000 kilovolts, and the greater portion of the dose at all 

 depths is due to direct beam, the depth doses will be greater 

 than those met with at 200 kilovolts (the forward scatters being 

 nearlv the same). Not onlv will this be the case but, since at 

 1 ,000 kilovolts so little of the dose at a depth depends on scatter, 

 there should be little change in %DD with field area, quite 

 contrary to 200 kilovolt experience, where the %DD is governed 

 to a greater extent by the backscatter and hence by field area. 

 Further, the improvement with 1,000 kilovolts will be the 

 greater, the greater the depth. At 200 kilovolts, there is little 

 change in %DD with FSD beyond 50 centimeters FSD, and this 

 can be understood by examining the information in figure 3. 

 Since the direct-beam contribution is a small portion of the dose 

 at the depth, any variation in its value due to alteration in F 

 (in the formula) will be masked in the %DD by the small part 

 it takes in the whole. At 1,000 kilovolts, on the other hand, the 

 direct contribution even at 20 centimeters depth is over 50% 

 of the dose, so increases in the direct component by increase in 

 the FSD will be appreciable in the %DD. 



Figure 2, curve 6, indicates that the last deduction holds, 

 while figure 4 indicates that the forecasts about relative %DD 

 at 1,000 kilovolts and 200 kilovolts are along the lines indicated. 

 The gain in small field sizes is particularly noticeable, being as 



