[Model of biological equivalent dose distribution modified by volume effect in radiotherapy]

This paper demonstrates that a biologically equivalent dose distribution including volume effect can be generated. Since the time-dose-fractionation (TDF) concept is convenient for comparing various radiation treatment schedules, TDF distribution maps are made on the basis of the physical dose distribution. On the other hand, the dose volume histogram is useful to evaluate volume effect, but is not necessarily an easy approach owing to the absence of spatial linkage. If distribution maps also representing the volume effect could actually be made, it would become easier to simultaneously predict both tumor control probability and the normal tissue complication rate. Because such tools should be very useful for planning radiotherapy, we proposed an experimental volume effect model. In this, one pixel is affected by all its surrounding pixels and the effect depends on the distance between pixels, volume, and the irradiated dose of another pixel. When the model was adapted to the conventional power law model, we could acquire a new equation with mathematical analysis. This permitted us to calculate the volume effect on each voxel within the treatment volume. Using a personal computer and treatment planning system, we calculated the "TDF-volume" distribution and drew maps based on this equation and the TDF values of each voxel for radiotherapy of a pelvic tumor.