RBE and Quality of Electromagnetic Radiation at Depths in a Water Phantom

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Abstract
The biological effectiveness of X-rays from (HVL 5.0 Al to 3.2 Cu) and Co-60 gamma rays were determined using HeLa cell "survival" as an end-point. These measurements were made at various depths within a water phentom to determine the influence of radiation degradation by passage through the scattering medium. Absorbed doses upon which the RBE values are based were calculated from ionization measurements and were also measured using chemical dosimeters. To facilitate calculations, measurements of radiation quality in terms of effective energy were made and the exact composition of the growth medium determined. Radiation quality measurements were made with a dual ionization chamber. Methods of calibration of this dual ion chamber and results are included and compared with results of other investigators. All cell irradiations were carried out for several dose levels and depths. Mean lethal doses and associated standard deviations are included. Results of the studies are as follows: (1) There is no change in the biological effectiveness of any of the electromagnetic radiations with depth in scattering material. (2) All X-ray beams have the same biological effectiveness at all depths. (3) The RBE of the cobalt-60 gamma radiation with respect to any X-ray was 0.90[plus or minus]0.02 at the 95% confidence level.