Inter-operator variability in organs at risk delineation: Their effects on dose-volume histograms

Abstract

Introduction As recommended by the ICRU report 83, while planning treatment in radiotherapy (RT), organs at risk (OAR) must be systematically delineated. As several studies have shown, interoperator variability implies differences in volume once rebuilt from computed tomography images. This work aims to evaluate the impact of this variability on dose-volume histograms (DVH). Materials and methods Using a treatment planning system (TPS) Isogray (Dosisoft®), three typical treatment plans, for 3 different pathologies, were performed: a case of carcinoma of the prostate in 3D conformational radiotherapy (3DCRT), one case of Hodgkin lymphoma Involved Field and in a case of breast carcinoma 3DCRT. To model changes in volumes of organs at risk induced by delineation, margins, implemented by the TPS were defined around reference delineations. These margins have been defined using results of several authors who have studied inter-operator variability in delineation. In addition, reference delineations were validated by two radiation oncologists from the Gustave Roussy Institute. The resulting DVH were analyzed to compare the dose-volume metrics for all treatment plans and all organs. Results Preliminary results relate to the D98%, D50% and D2%. Whatever the position of the OAR from the irradiated volume, defined by the 50% isodose of the prescribed dose, D50% of each organ at risk does not vary significantly with the volume in the volume range defined. D98% decreases of about 9% when delineation tends to overestimate the volume of interest at the maximum. However D2% increases of about 7% when delineation tends to overestimate volume of interest at the maximum. Conclusion Except for D50%, inter-operator variability in OAR delineation may induce a different interpretation of the DVH in the treatment planning process. These results have to be taken into account during the validation of a clinical treatment plan. In addition, to reduce inter-operator variability, this work recommends to define standardized protocols of delineation for each organ.