Technical Methods in X-Ray Therapy 229 



chamber and the extrapolation chamber ; "^ or even, perhaps, to 

 the prevalence of a different type of tube in the two countries. 

 Oil-immersed tubes, where the beam emerges through a layer 

 of oil, seem to give a more rapid diminution of dose rate with 

 distance, near the tube, as the oil, by scattering, acts as a sec- 

 ondary source nearer than the focus. Spiers ^^ has compared 

 the behavior of a number of materials with water, as phantom 

 materials. Paraffin wax and rice flour differed in the 200 kilo- 

 volt range, and pressedwoods in the 100 kilovolt range. ^ The 

 most suitable substitute for water (suitable also for the filling 

 of scatter-bags) for the 200 kilovolt range was a mixture by 

 weight of about 60% rice flour and 40% sodium bicarbonate. 



When jigs are fitted to the body with wax molds, it is im- 

 portant that the wax should behave towards the X-rays in the 

 manner of water. Some of the dental waxes are much too 

 absorbent, being loaded with elements of relatively high atomic 

 number. If a dosemeter is immersed in a water-phantom, and 

 a piece of wax, etc., is interposed between the dosemeter and 

 the X-ray source, the change in dosemeter reading is an index 

 of the difference of the wax from water. Slabs 3 centimeters 

 thick gave the following diminution of dose rate: parabar (gum 

 kauri, stearine, and magnesium silicate), 12%; perspex, 4%; 

 radium compo (gum kauri, stearine, and charcoal powder), 1.7%. 



If it is desired to use isodose charts in the study of treatment 

 of parts of the body of smaller dimensions than the phantom, 

 e.g., the neck, then the body must be built up with scatter-bags 

 approximately to the full size. Reinhard and Goltz ^^ have 

 studied the changes produced by the lack of an adequate thick- 

 ness. With radiation of 0.9 millimeter copper HVL, about 5 

 centimeters of material beyond appoint of measurement is neces- 

 sary to give adequate backscatter there ; differences could be 

 observed 8 to 10 centimeters preceding the exit surface. The exit 

 doses were less than those in a deep phantom by 20% for a 

 10 centimeter thickness, 29% for a 20 centimeter thickness, and 

 16% for a 30 centimeter thickness. 



Sometimes a better dose distribution can be obtained by dis- 

 carding scatter-bags. Reinhard and Goltz ^^ have shown how 



