220 



Applied Biophysics 



h. Multiple fields. When it is desired to produce a relatively 

 uniform dose distribution through a volume, or to dose a tumor 

 at a depth to a greater degree than the skin at the area of entry 

 of the X-rays, it is evident that a number of beams must be used 

 which all include the tumor, but enter through different skin 

 areas. The simplest case is that of two oppositely directed beams. 

 This is useful in the treatment of the lip, eyelid, or nose, by con- 

 tact therapy, and gives a fairly uniform dose distribution.^^ It 

 has been discussed by Smithers ^* and by Wilson. ^^ With the 

 usual deep-therapy conditions — 40 to 100 centimeters FSD 

 (focus-skin distance), about 1 millimeter copper HVL (half 

 value layer) — a dose varying between 90% and 105% of the 

 skin dose (the sum of contributions from both fields) can be 

 obtained through a thickness of about 12 centimeters, i.e., the 

 diameter of the average neck. 



Two fields at right angles give a region of maximum dose on 

 the bisector, and nearer to the apex of the angle than the point 

 of intersection of their axes, \\jlson ^^ has shown that this can be 

 put to advantage, for example, in the treatment of a tumor of 

 the lung, situated near the anterior chest wall (figure 2). 



FIG. 2. FiEi.ns Prearranged Using Dose Contours 



Tumor uniformly irradiated with maximum dose equal to 120% of maximum 



skin dose. 2 10 X 8 cm fields only 



• 



Irradiation of a tumor of the lung near the anterior chest wall by two fields at right 



angles. The maximum dose occurs on the bisector of the angle between the fields, but 

 nearer to the apex of the angle than the point of intersection of the axes of the two 

 beams. The fields are arranged to give this region of maximum dose at the site of 



the tumor (Wilson *^) 



