Some Idiosyncrasies of Electron Behavior The familiar concept of a sharply collimated beam of x-rays or gamma rays in air, with no significant amount of scattering outside its predetermined cross-sectional area, needs to be considerably modified in the case of electron radiation. Individual tracks of relatively low-energy electrons in a cloud chamber (1) are characterized by erratic wanderings, with numerous abrupt changes of direction even in a path length of a few centimeters. If the electron energies are increased from kilovolts to megavolts, and the range of observation from centimeters to meters, somewhat similar behavior patterns can be demonstrated on a correspondingly enlarged scale. Such air-scattering phenomena tend to diffuse and dissipate the initial collimation of an electron beam rather rapidly with increasing distance from the source, and thereby introduce additional complications to the problem of personnel protection. Since completely “shadowed” areas are not geometrically projected by intervening objects, certain types of primary shielding barriers which have been found quite satisfactory for x-rays may prove to be of comparatively little value when electrons are used. The late Dr. Failla once remarked (2) that electrons would “sneak around behind a shield and hit you in the back.” Entry mazes are therefore relatively ineffective, and exposure room doors need to be tightly fitted. On one particular occasion, after a 2-Mev Van de Graaff machine had been converted for electron operation, it was found necessary to add layers of rubber weather stripping all around the edges of the previously adequate lead door. This remarkable infiltrating ability as a result of multiple scattering of electrons seems almost analogous to gaseous diffusion. Bremsstrahlung production has been suggested as another factor to be considered. Unless the electron beam is concentrated directly on a target of high atomic number, however, the efficiency of this process is small. For example, a Victoreen chamber, wrapped in lead rubber to an equivalent thickness of approximately 4 gm. per square centimeter, required a dose of 100 to 110 rads of 6-Mev electrons on the outer surface of the package to record 1 roentgen of x-rays. That conversion ratio of slightly less than 1 per cent is further reduced, to the order of 0.2 per cent, when 4 cm. of water or any tissue-equivalent material is substituted for the lead rubber. Some Therapeutic Considerations The application of such electrons to therapeutic purposes introduces an additional problem of shielding critical organs of the patient. In the particular case of the 6-Mev linear accelerator (Varian) at U.C.L.A. Medical Center, external electrons are made available by magnetically deflecting the sharply focused beam away from the gold x-ray target, to emerge through a metal scattering window.