Technical Methods in X-Ray Therapy 



221 



Skill in arrangement of multiple beams is acquired by a study 

 of existing dose distributions, of model isodose surfaces,-^ and 

 by trial arrangement of isodose charts and modifications of these 

 arrangements. A few examples are given below. When the 

 beam axes are coplanar, the case is simpler. Wilson ^^ has shown 

 an arrangement of three fields to give a good dose distribution 

 for treatment of a larynx (figure 3). A case in which it is 

 desired to keep the X-ray dose low over a region is in the treat- 

 ment of the cervix uteri by combined X-ray and radium. Intra- 

 uterine and vaginal radium applicators, which give an adequate 

 local dose, give too little to the more distant parts of the pelvis, 

 which must therefore be dealt with by X-rays. The beams are 

 directed to give maximum efi^ect at the lateral wall of the pelvis, 

 but be limited where the gamma rays are efifective, the two 

 together giving a uniform distribution. Reference should be 

 made to papers by Walker,"*^^ and Sandler ^- for diagrams which 

 give the dose distribution throughout the pelvis. 



FIG. 3. Fields Prearranged Using Dose Contours 



Larynx uniformly irradiated with a dose equal to 140% of maximum skin dose 



Arrangement of three fields with coplanar axes to give a relatively uniform dose 

 distribution through the larynx 1.4 times that of the maximum skin dose (Wilson ■'i} 



