eet aay “gh tment Set ~ . .~ pibtaSayy Pe neeati Aat oeetalinnRa dn eas one oe = the value of Z alone. Then the RBE would be simply the ratio of the value of z for the standard, to that of the high-LET radiation, The result is shown in Fig. 10, in which an HSEF, f.e., the probability of a cell responding quantally, Pq, vs. 2 is plotted. the HSEF, In using the entire distribution of 2 is wultiplied by the HSEF to obtain values for the cell risk from a radiation of any quality. in the Fig, the RBE utilizes only the mean values uf However, as. seen z, and as such the RBE ratio provides an indication of the effectiveness of a radiation that delivers predomtnantly high cell doses, relative to the standard that delivers essentially only small cell doses. Thus it is seen that the RBE is at best simply a crude method of approximating in stepwise fashion what an aSCF presents as a continuous function. s It ts conceptually questionable because as used it is a confounded ratio, and employs only average values of cell dose, The latter would be valla only if the cell risk were proportional to cell dose, which Lt clearly is not. Nl 1 -Yooee ‘ (RBE) f P,, at z- RELATIVE BIOLOGICAL EFFECTIVENESS Fig. 19 kelationship between the HSEF and RBE, taken as the ratio of Z for the standard, to that for a high-LET radiation, The RBE is a crude approximation to the HSEF, in that it is the ratio of the mean of the relatively large cell doses delivered by a lulgh-LET radlation, to the mean of the relatively low cell doses delivered by the low-LET standard radiation. See text for additional explanation. -224- co: eve: ex; on]