MULTIPLE 2-DIMENSIONAL VERSUS 3-DIMENSIONAL PTV DEFINITION IN TREATMENT PLANNING FOR CONFORMAL RADIOTHERAPY

Purpose: To demonstrate the need for a fully three-dimensional (3D) co mputerized expansion of the gross tumour volume (GTV) or clinical targ et volume (CTV), as delineated by the radiation oncologist on CT slice s, to obtain the proper planning target volume (PTV) for treatment pla nning according to the ICRU-50 recommendations. Materials and methods: For 10 prostate cancer patients two PTVs have been determined by expa nsion of the GTV with a 1.5 cm margin, i.e. a 3D PTV and a multiple 2D PTV. The former was obtained by automatically adding the margin while accounting in 3D for GTV contour differences in neighbouring slices. The latter was generated by automatically adding the 1.5 cm margin to the GTV in each CT slice separately; the resulting PTV is a computer s imulation of the PTV that a radiation oncologist would obtain with (th e still common) manual contouring in CT slices. For each patient the t wo PTVs were compared to assess the deviations of the multiple 2D PTV from the 3D PTV. For both PTVs conformal plans were designed using a t hree-field technique with fixed block margins. For each patient dose-v olume histograms and tumour control probabilities (TCPs) of the (corre ct) 3D PTV were calculated, both for the plan designed for this PTV an d for the treatment plan based on the (deviating) 2D PTV. Results: Dep ending on the shape of the GTV, multiple 2D PTV generation could local ly result in a 1 cm underestimation of the GTV-to-PTV margin. The devi ations occurred predominantly in the cranio-caudal direction at locati ons where the GTV contour shape varies significantly from slice to sli ce. This could lead to serious underdosage and to a TCP decrease of up to 15%. Conclusions: A full 3D GTV-to-PTV expansion should be applied in conformal radiotherapy to avoid underdosage. (C) 1998 Elsevier Sci ence Ireland Ltd. All rights reserved.