TecJinical Methods in X-Ray TJierapy 219 



a desired dose distribution on paper, means must be found to 

 direct the X-ray beams sufficiently accurately to give this distri- 

 bution in practice. If the absorption of the radiation in the 

 healthy tissues is to be a minimum, beams no wider than neces- 

 sary must be used. This makes accurate aiming very important. 

 Rarely is more than one tube used at a time ; usually a single 

 tube is directed successively in the desired ways. This may be 

 done by adjustment of the tube applicator to skin markings, with 

 orientation of the tube to calculated angles. To assist in this, 

 numerous beam-direction devices have been developed. Alterna- 

 tively, jigs can be made, which are attached to the patient in 

 fixed positions, and aid in the correct adjustment of the tube. 

 Finally, methods must be mentioned in which there is a relative 

 rotation of X-ray tube and patient, so that the axis of rotation 

 and the X-ray beam pass through the tumor roughly at right 

 angles to each other. 



The desirability of beams being no wider than necessary was 

 mentioned earlier. A broad beam provides a greater depth dose 

 than a narrow beam, as the dose is enhanced by the scattering 

 from a greater block of tissue. Beams broader than the tumor 

 cross section have been used to give an adequate tumor dose at 

 a depth, but it is preferable to use more beams with a cross-fire 

 technique, or use a more penetrating radiation, so that the min- 

 imum beam width will suffice. 



Illustrative Dose Distributions 



a. Single fields. These are suitable for treatments where the 

 maximum dose must be given to the surface. In this case, it is 

 desirable that the dose rate should decline rapidly, and an easily 

 absorbed X-ray quality, i.e., one generated by a relatively small 

 kilovoltage, is therefore chosen — the so-called Chaoul or contact 

 therapy. Meredith -^' -^ has shown that the dose received by 

 the first millimeter or so of tissue is appreciably altered by 

 secondary radiation from the applicator and metal parts in the 

 tube, and can be reduced in relation to the dose at 5 millimeter 

 depth by spraying the applicator with aluminum paint and cov- 

 ering the tube window with aluminum foil. 



