The Specification of Tumour Dose

Abstract

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. the median dose. (b) 4,400 rads, i.e. the maximum dose. (c) 3,600 rads, i.e. the minimum 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. The limits of tumour dose are obviously of interest, but the median value 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 the median dose would be 4,000 rads but the average doses (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 the average dose could be determined.