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Proceedings of the British Institute of Radiology

At present it is difficult or impossible to compare the biological effects of radiotherapy to the cervix given by different regimes.

(A) For intermittent radiation this difficulty may be overcome by introducing a new unit (not previously discussed in this journal) called rad equivalent therapy (ret) which can be calculated as follows:

Total dose (in rads) = NSD (in ret) x N^0·24 x T^0·11 . (1) (where NSD=nominal standard dose, N=number of fractions, T=time in days), (Ellis, 1968; 1969). The NSD is the tolerance level for the normal tissues as used clinically. It depends on dosimetry and clinical judgment. In Oxford we use 1,800 ret.