There is at present no standard method of specifying tumour dose and differences in terminology can lead to difficulty in the comparison of clinical prescriptions between radiotherapy departments.For instance, if in a particular case the dose distribution is such that the limits of tumour dose are 3,600 and 4,400 rads (i.e.a total variation of 20 per cent), this dose may be stated according to several different conventions as:(a) 4,000 rads ± 10 per cent,i.e.themedian dose.(b) 4,400 rads,i.e.themaximum dose.(c) 3,600 rads,i.e.theminimum dose.It is evident that, in the interests of comparison of dose levels and efficiency of techniques some standardisation is necessary and it may be useful to consider what information is desirable, albeit this may be difficult to determine fully in practice. Thelimitsof tumour dose are obviously of interest, but themedianvalue usually given may be misleading. For a range of 3,600–4,400 rads the distribution may be such that the majority of the tumour receives either 3,700 or 4,300 rads. In both cases themediandose would be 4,000 rads but theaveragedoses (the average values of the energy dissipated per gram throughout the tumour) would differ by nearly 20 per cent, with consequent markedly different radiobiological effects (Oliver and Lajtha, 1961).If the dose distribution were known completely it would be possible to construct a distribution curve for the energy dissipation per gram (Fig. 1) from which theaverage dosecould be determined.