Application of the NSD concept in radiotherapy. Limitations due to inaccuracies in clinical dosimetry.

Time dose fractionation schemes in radiotherapy have been developed on empirical lines over a period of several years. Ellis proposed the NSD formula to facilitate a meaningful comparison of clinical results from different centres and different fractionation regimes. Recent studies indicate that there is a steep tumour dose response curve which necessitates that the delivered doses must have an accuracy within +/- 5%. The ranges of the variations of the parameters in the NSD formula which still give dose estimates within this accuracy have been studied in this paper. It is concluded that because of the uncertainties involved in dose output calibration and other physical and empirical constants, extreme caution has to be exercised in routinely using the NSD concept and those of TDF, CRE, PT and BIR, which have the NSD concept as their basis.