Cesium-137 teletherapy units have been installed in a number of institutions around the world (1, 7). These units for the most part are designed for treatment distances of greater than 30 cm. Whitmore (11) has described a cesium-137 unit used at 15 cm. source-to-skin distance (SSD). Radium short-distance units have been used for the treatment of patients with tumors relatively close to the skin (4), and cobalt 60 has more recently been employed for the same purpose (10). A cesium-137 teletherapy unit, designed for treatment distances between 10 and 30 cm., was manufactured by Atomic Energy of Canada Limited (AECL) and installed in The University of Texas M. D. Anderson Hospital and Tumor Institute in March 1959. This machine is intended for the treatment of tumors which lie less than 5 cm. below the skin. The dose to deeper lying tissues can be significantly less than the tumor dose because of the “inverse-square” reduction possible at short treatment distances. Basic Design A photograph and drawing of th...