Central-axis depth-dose data for phantoms of 0- to 25-cm thickness in roentgen, cesium-137, and cobalt-60 therapy.

Abstract

In Respect to x-ray generators, cesium, and cobalt units operating at a fixed target or source-skin distance, the percentage depth dose along the central axis has been taken from data for semi-infinite phantoms (1, 3, 14). These tables, however, apply to ideal situations in which there are homogeneous water-equivalent materials of unit density and maximum side- and back-scatter. In clinical work, discrepancies between the actual and standard central axis depth-dose data occur because of the lack of the previously mentioned ideal conditions. The effect of limiting the tissue thickness has been estimated only partially for medium energy x-rays and cesium-137 and cobalt-60 gamma rays (6) and for 1,000 kVp x-rays (9, 10). In certain cases, the effect of limiting the tissue thickness was given only for the exit surface dose (2, 4, 5, 12, 13). A preliminary work for 15 × 15-cm fields has been presented previously for 1.0 to 4.0 mm Cu h.v.I. x-rays, as well as for cesium-137 and cobalt-60 gamma rays (6). From th...