The Relative Biological Efficiency of X Rays Generated at 220 kVp and Gamma Radiation from a Cobalt 60 Therapy Unit

Established techniques with broad bean roots have been used to measure the relative biological efficiency (R.B.E.) of cobalt 60 γ rays from a kilocurie therapy unit, and X rays generated at 220 kVp. The value of the R.B.E. was found to be 0·84 with 95 per cent confidence limits of ±0·05. There was no significant difference in the R.B.E. when the irradiation with the cobalt 60 γ rays was carried out 1·5 cm or 20 cm deep in a water-filled phantom.