Million-Volt Therapy 



251 



scatter is from secondary scatter from the part of the medium 

 above the point of measurement. 



Investigation into the variation of %DD with filtration of the 

 X-ray beam indicated that not only was the lead filter the most 

 efficient in increasing the ''^ DD, but also that the maximum effi- 

 ciency was between and 1 millimeter of added filter. With 

 heavier filters, the improvement was linear but less noticeable. 

 ( See figure 2, curve 4. ) It will be noticed that the criterion of 

 efficiency is the improvement of %pD in a 10 X 10 centimeter 



% DD 



♦•os.ofo 



T — I r— T 1 — I — I — I 1 — r 



XOO 90 ao 70 60 50 40 30 20 10 



FIG. 2. Percentage of Beam Intensity Left 



Abscissae = % intensity. Ordinates = % depth dose at 10 cm depth 



Curve 4. Percentage of depth dose with"" increasing lead filtration, filter thickness 

 marked on the curve, plotted against residual beam intensity. 10 X 10 cm, 100 cm 

 FSD, 1,000 kv. 



Curve 6. Percentage depth dose with FSD, FSD, marked on the curve, plotted 

 against residual beam intensity. 10 X 10 cm, 2 mm lead added filter, 1,000 kv. 



field at 100 centimeters FSD plotted against percentage of the 

 beam intensity left. The improvement of the %DD with FSD 

 is given in figure 2, curve 6, which shows that the maximum 

 effi^ciency of improvement is produced between 60 and 80 centi- 



