Graphical representation of the effects on tumor and OAR for determining the appropriate fractionation regimen in radiation therapy planning.

Purpose: The authors propose a graphical representation of the relation between the effect on the tumor and the damage effect on an organ at risk (OAR) against the irradiation dose, as an aid for choosing an appropriate fractionation regimen.

Methods: The graphical relation is depicted by the radiation effect on the tumor E(1) versus that on an OAR E(0). By observing the features of the E(1) vs E(0) relation curve, i.e., convex or concave shape, one can judge whether multifractionation is better or not. This method is applied to the linear-quadratic model (with α and β parameters) as an example. Further, the method is extended to the general case for nonuniform dose distribution to the OAR, which is frequently seen in clinical situations.

Results: The criterion for selecting multi- or hypofractionation is based on the relation between the dose for the OAR and the α∕β ratio of the OAR to the tumor. It is also shown that the graphical relation enables us to estimate the final effect after multifractionated treatment by plotting a tangent line on the curve.

Conclusions: The graphical representation method is of use for improving planning in radiotherapy by determining the effective fractionation scheme.