Fractionation procedures in radiation therapy: a computerised approach to evaluation

The effects on the cell populations of normal tissues (skin) and tumours (epidermoid cancer), of various fractionation schemes for radiation therapy, were computed on the basis of a three-component model. The relatively minor effects attributable to induced mitotic synchrony, accumulation of irreparable injury, and the presence of an anoxic component were neglected. Iso-effect curves were generated for various intervals between fractions, ranging from 0·125 day to 32 days, by specifying the lowest permissible fraction of normal-cell survival, for three field sizes. The iso-effect curve for “daily” treatment is shown to be in very good agreement with observed tolerance doses. Iso-effect curves for intervals of 0·25 day or less agree closely with clinical results for continuous irradiation. The model used is probably not valid for intervals of more than four days, for reasons which are discussed. The results can be interpreted in terms of the relative efficacies of various fractionation schemes, so that optimal schemes may be selected.