226 Applied Biophysics 



drawings, and stacked in correct relation to each other, so that 

 a three-dimensional representation of the dose distribution and 

 anatomical features is obtained. ^^ 



Means of Realizing a Desired Dose Distribution 



If the paper plan of fields to produce a chosen distribution 

 of dose is to be successful, the fields must be applied to the patient 

 accurately. Various appliances have been devised to make this 

 easier and quicker. First, the center of the region it is desired 

 to treat must be located radiographically — by relating it to bone 

 or soft-tissue shadows, by insertion of an inactive gold seed, 

 skin clip, small balloon catheter containing iodine, lead-shot 

 catheter, lipiodol, or gelatin-barium pellet, or by barium- or 

 thorium-air contrast, according to the site. Skin markings are 

 used to give two lines which intersect at this point. Or the 

 vertical depth below a skin marking can be found by standard 

 radiographic methods. This localization must be done with the 

 patient in the exact position he is to occupy during treatment. ^^ 



The X-ray tube can then be set to angles measured by a 

 ''parallelogram beam director" or "arc beam director," which 

 is removed before adjustment of the tube, or to lines scribed on 

 a protractor spanning the patient, or arc attached to the tube. 

 Simplest of all, a sheet of cardboard is cut to fit the contour of 

 the body, and the lines along which the applicator should be 

 directed are drawn on it. 



A second method is the use of a calliper, fixed to the tube, 

 which carries a pointer coincident with the beam axis, which 

 can be made to slide to touch the patient at the point of emergence. 

 Green's calliper will also indicate points at known distances 

 normal to the beam axis — a help when setting glancing fields — 

 while Grimmett has adapted a calliper to give audible warning 

 if the patient moves appreciably from the correct set- 

 ting.^- ^' !-• ^^' -^^ 



Mayneord ^^ has described an optical device which shows 

 the exit point of the beam axis by a light spot on the patient. 

 A small lamp can be arranged in an applicator to give a beam 



