Technical Methods in X-Ray Therapy 225 



range within which the percentage dose lay, and when one set 

 for each beam was overlapped, the summation isodose curves 

 could be drawn on a superimposed celluloid sheet by considera- 

 tion of the depth of color. 



For a knowledge of the dose distribution throughout a volume, 

 a summation of dose in parallel planes is desirable. Also, if the 

 X-ray beams are not coplanar, isodose curves in planes which 

 do not contain the beam axis are necessary. When the beam 

 has circular symmetry, there are geometric methods by which 

 isodose curves in any plane can be drawn from those in a plane 

 containing the axis. However, Mayneord ^^ has devised an in- 

 strument, the *'dose contour projector," which enables this to be 

 done much more easily. Flanders ^^ has described methods by 

 which sections through isodose surfaces can be made visible by 

 arranging a thin plane sheet of light to cut semitransparent 

 models. The isodose curves in the required section can be 

 sketched in with the aid of a camera obscura, and this method 

 can be used with beams which have not circular symmetry. 



Another instrument devised by Mayneord ^^ is the "dose 

 finder," which aids in the studv of dose distribution in three 

 dimensions. A dummy applicator is adjusted to a shell moulded 

 to the shape of the part of the body under treatment. The shell 

 is then moved 20 centimeters from the applicator, into which 

 is plugged a plane carrying isodose curves (when there is cir- 

 cular symmetry), so that they occupy the correct position in 

 space in relation to the applicator. A rod, with pointers at right 

 angles 20 centimeters apart, is so arranged that when one pointer 

 is adjusted to a chosen point in the shell, the other pointer gives 

 the corresponding position in the region of the isodose curves. 

 The plane carrying these curves is rotated about its axis until 

 the pointer touches it, and the dose is read at the point of con- 

 tact. Rectangular fields can also be studied with a slightly more 

 complicated arrangement.-^- "^^ Light beams have been used in- 

 stead of mechanical pointers. ^^' "*- From a study of each field 

 in turn, the dose distril)ution due to a number of beams can be 

 plotted in a number of parallel planes through the treated region. 

 These can be drawn on glass plates, which also carry anatomical 



