250 Applied Biophysics 



the emergent X-ray beam is a heterogeneous one composed of 

 wave lengths varying from the shortest, which has a quantum 

 energy equivalent to that of the original electron, to rays which 

 just come through the tube wall. The peak intensity is at about 

 700 kilovolts, and the mean about 450 kilovolts equivalent. Pass- 

 ing such a heterogeneous beam through single or composite 

 metal filters, the long wave lengths are absorbed to a greater 

 degree than the short wave lengths, resulting in a hardening 

 (shortening) of the average wave length of the emergent beam. 

 What is more important, however, is that the very soft (long) 

 wave length rays are completely removed. These cause con- 

 siderable damage to the first few millimeters of tissue and, as 

 they do not penetrate further, they do not contribute to the 

 lesion dose. 



It was found that there was little difference between lead 

 and tin filters, the lead, if anything, being slightly more efficient. 

 Backing of the lead filter by tin and copper was not found 

 necessary, presumably since the 4.2 millimeter steel wall of the 

 tube effectively removed the anomalous lead radiation.^ 



The distribution of dose in the patient is a much more com- 

 plicated problem and, to simplify physical investigations, it is 

 carried out in a medium which has the same electron density 

 as the average of all the body components. Water is one such 

 medium, while there are others of more complicated nature. ^^ 

 The relationship between (a) the dose at any point in the medium 

 to (b) the dose at the surface of the medium at the beam center, 

 when expressed as a percentage, is called the Percentage Depth 

 Dose (%DD), while the chart giving the %DD-distribution in 

 a plane by lines joining points at equal dose levels is called an 

 isodose. The dose at a depth is made up of many components, 

 and for general purposes here they can be divided up into three : 

 direct beam, backscatter, and forward scatter. The direct beam 

 is that part of the dose originating from the ionization produced 

 at the point by absorption of X-rays from the part of the main 

 beam which has penetrated to the depth. Backscatter is the dose 

 originating from secondary X-rays scattered back from the part 

 of the medium beyond the point of measurement, while forward 



