Clinical and Physical Aspects of Electron Beam Therapy

Abstract

Physical measurements of dose distribution and absorbed dose per unit of exposure are logical preliminary steps to be taken before the use of new apparatus for treatment. These data may be utilized to establish guide lines for treatment planning but should not be accepted as a substitute for biological measurements of dose response or close clinical surveillance of tumor regression and skin reactions, especially when a new modality is employed, such as high-energy electrons. Discrepancies between absorbed dose and biological response for different radiation modalities have long been recognized and are generally accounted for by introduction of the concept of RBE (relative biological effectiveness). Because of practical difficulties, absorbed dose measurements are infrequently made. More often, exposure dose, measured with ionization chambers, is taken as the reference point in making treatment plans. It is the purpose of this report to describe our work in progress to establish the RBE for a 35-Mev betatron electron beam, to correlate the reading of ionization chambers with absorbed dose, and to show the clinical significance of such studies. In order to obtain clinically significant data as quickly as possible, our initial experiments were concerned with establishing the lethal dose (LD 50/30 days) for mice exposed to the 35-Mev electron beam and 230-kv x-rays, using a Victoreen ionization chamber as a monitor for both radiation qualities. For these experiments type C57L/J male mice were irradiated when between nine and eleven weeks old, and all data were normalized to a ten-week age, in accordance with the age-sensitivity curve determined by Lindop and Rotblat (1) for SAS/4 mice. The animal irradiation chamber was made of solid Lucite, with the individual compartments only 1 inch in diameter, in order to minimize the amount of extraneous air space around the animals. Surface irradiations were in each case done with large fields and a maximum amount of back and side scatter. With the mouse irradiation chamber on the surface of the phantom, the long axes of the mice were 2 cm. below the surface. Exposure dose was measured with a Victoreen 250-r medium-energy chamber centrally located inside of the animal compartments. The chamber correction factor (1.05 for 2.0 mm. Cu h.v.l.) was applied to readings made on the 35-Mev electron beam as well as to those obtained with 230-kv x-rays. Other pertinent physical data were: A total of 125 mice were used in this experiment, 40 to establish the 230-kv lethality curve and 85 for the 35-Mev electron beam. More animals were used on the betatron because our initial estimates of lethal exposure dose proved to be too low, and also because it seemed desirable to double-check the surprisingly high values obtained. The results of this experiment are shown in Figure 1. The LD 50 as shown on the graphs is 745 r for 230-kv x-rays and 1,130 “r” for 35-Mev electrons.