256 Applied Biophysics 



HVL), while the experimental beam was 1,000 kilovolts (10 

 millimeters Cu HVL). The dose required in one sitting to pro- 

 duce the same skin reaction was 50% greater with the 1 000 

 kilovolt than with the 300 kilovolt beam. Theoretically, only 

 part of this alteration in skin response can be accounted for by 

 the reduction in the photoelectric absorption in the sulphur in 

 the skin with the shorter wave lengths, the remainder being so 

 far unexplained, unless it is due to a radiosensitivity change. 

 The results conform well with those encountered in gamma- 

 ray treatment. This reduction in skin response also opens up 

 improvements in technique, but more especially makes possible 

 a reduction in the skin reaction which has undoubtedly an 

 indirect effect on the patient's well-being, during and after 

 treatment. 



Modifications in 200 Kilovolt Techniques Possible by 

 Employing Million-volt X-rays 



1. Whereas it was impossible to employ small fields in the 

 treatment of small lesions buried deep in the body, e.g., rectal 

 carcinoma, owing to the poor depth dose of such fields, at 1,000 

 kilovolts, it becomes possible and economical to employ multiple 

 small fields, even through the remote lateral skin surfaces. 



2. In intrinsic carcinoma of the larynx, it is customary and 

 necessary at 200 kilovolts to employ three fields — two opposed 

 laterals, and an anterior field. At 1,000 kilovolts only the two 

 opposed laterals are necessary, which simplifies and increases 

 the accuracy of the technique. 



In this type of case with two opposed beams, it is found that 

 blocks of tissue up to 14 centimeters thick receive nearly uniform 

 irradiation throughout by two opposed million-volt X-ray beams. 



3. In many cases at 200 kilovolts, it is found necessary to 

 employ beams angulated in three dimensions (spinal cord and 

 bladder). So far, at 1,000 kilovolts, it has not been found neces- 

 sary to employ such beams except in a few brain cases where 

 the eye has to be avoided. Setting up beams accurately in three 

 dimensions and calculating the necessary isodoses is a difiicult 



