RADIATION DOSi: 



same biological response arc unequal. If the dose-response curves are of the 

 same shape, i.e. if they may be superimposed by a simple change in the dose 

 scale for one of the radiations, the inverse ratio of the doses which produce 

 identical effects is defined as the relative biological efficiency (R.B.E.) of 

 the two radiations (I.C.R.U. 1956, p. 7). Some seventy-five determinations of 

 R.B.E. of supervoltage X radiation relative to ordinary deep-therapy X 

 radiation have recently been reviewed by Kohn^"*. Plotted as a histogram, 

 these observations show a Gaussian distribution with a mean at 0-85 and 

 standard deviation of about 0-15. The difference from unity is small, and it 

 is unfortunately the case that no inconsiderable fraction of the 300 scientific 

 man-years of labour, represented by this histogram, is wasted through 

 inadequate dosimetric precision. After doubtful results have been discarded, 

 there nevertheless remains a substantial body of data indicating an R.B.E. 

 of mega voltage relative to 200 kV X radiation of about • 8. This is indicated 



y-rays 

 ° 8 min 



III' I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 



40 80 120 200 320 A 00 520 600 



y and X-ray dose in roentgens 

 K-ray dose in energy units 



Figure 5. Inhibition of growth produced by 8, X and a radiation 



by the very careful work of Kohn himself, and his colleagues in San 

 Francisco^^'^®, and of a group of investigators at the Christie Hospital, 

 Manchester ^^. 



Large differences in R.B.E. are only found when radiations of very 

 different quaUty are compared, e.g. when X and y radiation on the one hand 

 is compared with neutron or a radiation on the other. Figure 5 shows a 

 family of experimental curves for growth inhibition by a-rays. X-rays, and 

 y-rays^^. The a-ray curve is dose-rate independent over the range investi- 

 gated. The X- and y-ray curves have already been discussed in connection 

 with dose-rate dependence. It is evident that in this case R.B.E. of any two 

 radiations cannot be represented by a single parameter. A horizontal line, 

 representing the growth rate ratio of irradiated to control roots of 0-8, inter- 

 sects the curves in a series of doses which have quite different ratios from the 

 intersections with a horizontal line at the 0-15 level. R.B.E. is thus seen to 

 be a function of dose and dose rate. It is also a function of the physiological 

 condition of the cells and the oxygen tension in their environment at the time 

 of irradiation. Since the effects of densely ionizing particles are much less 

 influenced by oxygen tension than those produced by X or y radiation, 



8 



