On the validity of the NSD concept

The nominal standard dose depends mainly upon the skin reaction, the field size, and the location. In particular, different degrees of skin reaction cause variations in the "tolerance dose" up to a factor of 2. The influence of these parameters must be taken into account carefully in order to attain valid conclusions. The NSD concept has been verified by re-evaluation of Strandqvist's investigation. An exponent of 0.28 +/- 0.04 has been calculated, which is at the lower limit of Ellis' value (0.35). The use of the NSD formula may fail for radiation regimens which are not bound to skin reaction. Several radiation schemes in the treatment of lung cancer exhibit worse tumour control though NSD values are higher. For this a strong correlation between survival rate and fractionation frequency has been found.