Foundation of the Ellis-formula

The Ellis-formula is used in radiation therapy to derive the nominal dose. This nominal dose is an index of effectiveness applicable to different fractionation schemes. Its critical value, the standard nominal dose, corresponds to the tolerance of normal tissue. The Ellis-formula is first compared to the earlier equation of Strandqvist. It is then explained how the so-called TDF-factor is applied to derive the nominal dose for the combination of two partial fractionation series. The usual way to account for an extended interruption in a fractionation schedule is also considered. The common generalization of the Ellis-formula rests on approximations; its is also restricted to the combination of two partial series. In order to overcome this limitation a new numerical method is presented which provides nominal doses for irregular fractionation schedules. It does not apply to continuous irradiation. (orig./ORU)