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

PURPOSE: Retrospective study of 3D clinical treatment plans based on radiobiological considerations in the choice of the reference dose level from tumor dose-volume histograms. METHODS AND MATERIALS: 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. RESULTS: 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. CONCLUSIONS: 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.