Million-Volt Therapy 249 



each, and operating from the a.-c. mains. All vacuum and elec- 

 trical operations are indicated on a power-station type of illumi- 

 nated diagram, facilitating fault finding. 



The treatment and high-tension rooms are enclosed in walls 

 built of some 125 tons of interlocking barytes bricks, so effec- 

 tively preventing the egress of X-rays, that it is possible to store 

 films within a few feet of the treatment room. 



In this equipment, we have a simple, controllable, safe source 

 of high-voltage X-rays, not quite as hard as the gamma rays 

 from radium, but equal in intensity, under the same geometrical 

 conditions, to 7,000 grams of radium. 



During the war no development work on X-ray tubes and 

 equipment has been possible in Britain, luit in the United States, 

 a number of different types of high-voltage X-ray equipments 

 have been produced, one in particular being very compact, tube 

 and resonating transformer being housed in a tank some 6 feet 

 [1.8 meters] long and 4-9- feet in diameter. It is also of interest 

 to note that during the German occupation of Norway, Nor- 

 w^egian engineers and physicists constructed and operated a 1.5 

 million volt Van de Graaff generator and multiacceleration tube. 



Physical Investigations on Operating Conditions 



When the treatment of patients with the million-volt plant 

 commenced, there were few physical data available regarding 

 the properties of the short-wave length rays so generated, and 

 a complete investigation had to be made to find the optimum 

 operating conditions to attain ( 1 ) the shortest economical wave 

 length and (2), at the same time, the best geometric arrange- 

 ment to give the highest % depth dose in the patient, with a 

 reasonable X-ray intensity. Since the primary object of the 

 whole investigation was to find whether the radiosensitivity of 

 malignant cells, in vivo, increased with reduction in the X-ray 

 wave length, the tendency w^as to bias (1) in preference to (2). 



The properties of generation of X-rays, by the stopping of 

 high-speed electrons by a target, are such that, although the 

 electrons have all, in our case, a million volts equivalent velocity, 



