II. Dose-time Relationships in Radiotherapy and the Validity of Cell Survival Curve Models

The increase of dose with overall time in radiotherapy could a priori be due either to slow tissue repair, or to the relatively smaller effectiveness of smaller individual fractions, or to both. From experimental results with pig skin, and on general grounds which are discussed, the second explanation appears to be the important factor. We have investigated the extent to which experimental mammalian cell survival curves can explain the observed clinical dose-time relationships. It is concluded that such a simple theoretical model could explain the observed clinical data only if curves with extrapolation numbers of about 10 or with shapes that continue to show downward curvature with increasing dose are appropriate.