THE DEPENDENCE OF BUILD-UP RATIO ON THE FIELD SIZE AND THE SOURCE-SURFACE DISTANCE

In a high-energy radiation beam the maximum ionization does not occur at the surface of an air-tissue interface but, depending on the energy of the radiation, at a depth of a few millimeters or centimeters. The ratio of this maximum ionization and the “surface” ionization as measured by a “thin-windowed” ionization chamber is termed the build-up ratio. A considerable amount of experimental work has been undertaken (1–3) with cobalt-60 teletherapy units to find the optimal conditions which under clinical circumstances give the highest ratio. This published information has been restricted to data for constant diaphragm settings and for diaphragm-surface distances of up to 80 cm. The studies reported herein extend this data to distances of 300 cm for cobalt-60 radiation and additionally to 240 cm for x rays from a 4 Mev linear accelerator. The results are presented to emphasize the variation of the build-up ratio with distance at constant field size rather than at a constant diaphragm setting. Experimental D...