Comparison between the ideal reference dose level and the actual reference dose level from clinical 3D radiotherapy treatment plans.

Abstract

Retrospective study of 3D clinical treatment plans based on radiobiological considerations in the choice of the reference dose level from tumor dose-volume histograms. When a radiation oncologist evaluates the 3D dose distribution calculated by a treatment planning system, a decision must be made on the percentage dose level at which the prescribed dose should be delivered. Much effort is dedicated to deliver a dose as uniform as possible to the tumor volume. However due to the presence of critical organs, the result may be a rather inhomogeneous dose distribution throughout the tumor volume. In this study we use a formulation of tumor control probability (TCP) based on the linear quadratic model and on a parameter, the F factor. The F factor allows one to write TCP, from the heterogeneous dose distribution (TCP{(epsilon(j),D(j))}), as a function of TCP under condition of homogeneous irradiation of tumor volume (V) with dose D (TCP(V,D)). We used the expression of the F factor to calculate the "ideal" percentage dose level (iDL(r)) to be used as reference level for the prescribed dose D delivery, so as to render TCP{(epsilon(j),D(j))} equal to TCP(V,D). The 3D dose distributions of 53 clinical treatment plans were re-evaluated to derive the iDL(r) and to compare it with the one (D(tp)L) to which the dose was actually administered. For the majority of prostate treatments, we observed a low overdosing following the choice of a D(tp)L lower than the iDL(r.) While for the breast and head-and-neck treatments, the method showed that in many cases we underdosed choosing a D(tp)L greater than the iDL(r). The maximum difference between the iDL(r) and the D(tp)L was -3.24% for one of the head-and-neck treatments. Using the TCP model, the probability of tumor control is compromised following an incorrect choice of D(tp)L; so we conclude that the application of the F factor is an effective tool and clinical aid to derive the optimal reference dose level from the dose-volume histogram (DVH) of each treatment plan.