222 Applied Biophysics 



More complicated cases of summation of three and of four 

 beams, whose axes are not coplanar, have been given by Lamer- 

 ton and Mayneord ^ ^ and l^y Ungar '^^ respectively. Ungar de- 

 velops methods of treating vertebrae with 200 kilovolt radiation 

 which give a dose at the lesion about 1.4 times as great as that 

 at any skin area, except for certain small field overlaps not 

 exceeding 5 square centimeters. 



The method of rotating the patient (or tube) carries the 

 multiple-beam technique to the limit, where the skin area of entry 

 of the beam becomes a continuous belt round the patient. 

 Nielsen -^ has described the application of this method in the 

 treatment of cancer of the esophagus. The patient sits on a 

 stool which rotates him once in about 15 minutes about an axis 

 along the esophagus, which is 50 centimeters from the tube 

 focus. A narrow beam is used, and to insure that it includes the 

 esophagus the shadow pattern of this beam is viewed on a 

 fluorescent screen. With radiation of 0.9 millimeter copper 

 HVL, the skin dose on the anterior and posterior surfaces is 40% 

 to 509c, and in the axillae 25% to 35%^ of the central dose. 

 The longer radius from the axis of rotation to the axilla gives 

 the skin in this region a greater linear velocity, so that it more 

 quickly crosses the X-ray beam. Jensen ^' has described irradia- 

 tion of the pelvis with a tube which rotates through 180° about 

 an axis in the supine (and then prone) patient. \'arious modi- 

 fications are possible in these methods — the shutter can be closed 

 during part of the rotation, the angular velocity can be varied 

 at different parts of the arc, and by tilting the beam axis at an 

 angle to the axis of rotation, first in one direction and then in 

 the other, the tumor can be irradiated through two zones of 

 skin to provide a still greater ratio of tumor to skin dose. In 

 the last case, however, the position of the maximum dose may 

 be shifted along the axis of rotation away from the point of 

 intersection of the beam axis. 



c. Wedge fields. VAVis and Aliller ' have shown that an X-ray 

 beam can be so modified by a wedge-shaped filter that two such 

 fields at right angles, with the thick edges of the wedges con- 

 tiguous, give a fairly uniform dose distribution through the block 



