Until recently, most roentgenotherapy was practiced with voltages up to approximately 100,000 applied to the roentgen-ray tube. The roentgen rays generated at these voltages produced definite biologic reactions at or near the surface of the body. Because of their comparatively low penetrating power, however, these rays were rapidly absorbed by the tissues, and it was difficult to deliver any effective amount to a point much below the surface without injury to the overlying tissue.
Figure 1 gives a visual picture of absorption of roentgen rays as they pass through matter. It shows a cube of lead glass which was placed beneath a half inch opening in a lead diaphragm and exposed for several hours to the rays from a tube operating with four milliamperes at 200,000 volts (peak). This glass becomes brown under the influence of roentgen rays, and the intensity of the coloration indicates the intensity of the roentgen-ray