228 Applied Biophysics 



The radium compo or wax not only helps in the correct fitting 

 of the jig to the patient, but also fills up air spaces with tissue- 

 like material, so that the standard isodose charts give the correct 

 dose distribution. 



It is also necessary that the correct quantity of dose should 

 be given to each field. Frequently, this is done by making a 

 daily measurement of the X-ray output of the tube, and then 

 controlling the doses by stop watch and adjustment of the tube 

 milliamperes and kilovoltage. The latter are often difficult to 

 keep in correct adjustment, especially when radiographers must 

 watch more than one tube, and the switching on and ofif of tubes 

 aflfects the line voltage. The aggregate error in a dose may be 

 considerable, and can be avoided by the use of an integrating 

 dosemeter with an ionization chamber built into the master cone 

 of the tube on which the various applicators fit. Such a dose- 

 meter has been developed by Farmer.^ 



Theory and Practice 



It is evident from the above discussion that much effort can 

 be spent on the study of dose distributions based on sets of 

 isodose curves. It is, therefore, well to consider to what extent 

 the actual dose distributions obtained in the human body may 

 differ from the charts. The latter are usually based on measure- 

 ments made in water, so that one step is to consider what dif- 

 ferences are to be expected in the body. However, although in 

 the ideal, water-phantom measurements should be made for each 

 individual tube and applicator, in practice this is too time con- 

 suming, and usually a radiotherapy center assumes that pub- 

 lished charts of depth-dose values for the same quality of radia- 

 tion, focus-skin distance, and field area, will apply. Tables of 

 depth-dose values based on a survey of published values have 

 been compiled by Mayneord and Lamerton,-^ and by Quimby.-*^ 



There are considerable differences between British and Amer- 

 ican values. This may be due to the use of different phantom 

 materials — pressedwoods, wax, and rice flour, in addition to 

 water; to different types of ionization chambers — the thimble 



