Technical Methods in X-Ray Therapy 231 



The present author has measured the transmission of radiation 

 of quaHty 0.9 milHmeter copper HVL passed anteroposterior^ 

 through the midregion of a patient's kmg. A dosemeter sand- 

 wiched between the appHcator and chest wall measured a 

 backscatter factor of 1.33, compared with the water-phantom 

 value 1.31. The dosemeter was then arranged at the beam's 

 exit point on the posterior surface 17 centimeters from the 

 applicator, and scatter-bags were packed around it to give 

 a measurement comparable with that at a depth of 17 centimeters 

 in a water-phantom. The depth dose was 20.5% compared with 

 1 1 % in water. The fact that the backscatter factor was unaltered 

 suggests that the diminution of scatter from any particular part 

 of the lung is compensated by the less absorption of this scattered 

 radiation on its way to the point considered. Accordingly, it is 

 assumed that any point in the lung will receive the same amount 

 of scattered radiation as the corresponding point in water, but 

 the primary beam will he less absorbed. If the primary beam 

 has passed through a distance d centimeters of lung tissue of 

 density o grams per cubic centimeter this is equivalent in ab- 

 sorption to only od centimeters of water. The radiation which 

 reaches any point in the water-phantom can be divided into 

 primary and scattered radiation by the method of Meredith and 

 Neary.^^ At 17 centimeters deep in water, a surface dose of 131 

 provides a primary beam dose of 2.20 and a scattered radiation 

 dose of 12.8. The absorption coefficient in water of the primary 

 beam is 0.19 centimeter"^, and if we assume there is a 12 centi- 

 meter path in lung tissue of density about 0.3 this is equivalent 

 to 3.6 centimeters of water. Therefore the primary beam value 

 2.20 must be increased by a factor (? + oi9x8.4 __ 5 44^ [^^ it l3e_ 



comes 12.0. The total dose should therefore be 24.8, and the 

 corresponding depth dose 19%. This agrees reasonably with 

 the measured value 20.5%, and suggests that this method could 

 be used to deduce doses in lung tissue. 



Conclusion 



It has been the purpose of this paper to survey what seem 

 to the physicist the best technical methods in X-ray therapy. 



