The dependence of build-up ratio on the field size and the source-surface distance.

Abstract

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 Details The measurements were made on a Hunslet dual purpose cobalt unit with a cobalt-60 source 2 cm in diameter and a source-diaphragm distance of 35 cm, and on a 4 Mev Orthotron linear accelerator where the source-diaphragm distance is 60 cm. The ionization chamber employed consists of a block of Perspex 5.5 cm in diameter and 3.5 cm thick, with a measuring volume 8 mm in diameter and 1 mm deep. The front wall is made from aluminized Melinex film of 1.2 mg per square centimeter. The chamber was designed and constructed in the Physics Department at Cookridge Hospital, Leeds, England, and was connected to either a Farmer substandard dosimeter or a Vibron electrometer. For both the maximum and the surfacedose measurements the chamber was in the same position relative to the source-diaphragm system. When determining the maximum dose the build-up material was placed over the front of the chamber; the readings of the maximum dose were therefore corrected, using the inverse square law to relate them to the constant source-to-surface condition. Five hundred milligrams per square centimeter of Perspex were used to provide a build-up for cobalt-60 radiation, and for the linear accelerator the thickness was 980 mg∕cm2. In the case of cobalt, the current from the ionization chamber was measured, while for measurements on the accelerator the charge was read for the same integrated output on the dose monitor of the linear accelerator. The measurements were obtained with the ionization chamber “free in air,” i.e., without phantom material. We had found from previous experiments, confirming those of Johns and his associates (2), that a phantom has little effect on the build-up ratio, its presence affecting equally both the surface and maximum readings.