Variability of Gross Tumor Volume Delineation for Stereotactic Body Radiotherapy of the Lung With Tri-(60)Co Magnetic Resonance Image-Guided Radiotherapy System (ViewRay): A Comparative Study With Magnetic Resonance- and Computed Tomography-Based Target Delineation

INTRODUCTION: To evaluate the intra-/interobserver variability of gross target volumes between delineation based on magnetic resonance imaging and computed tomography in patients simulated for stereotactic body radiotherapy for primary lung cancer and lung metastasis. MATERIALS AND METHODS: Twenty-five patients (27 lesions) who underwent computed tomography and magnetic resonance simulation with the MR-(60)Co system (ViewRay) were included in the study. Gross target volumes were delineated on the magnetic resonance imaging (GTV(MR)) and computed tomography (GTV(CT)) images by 2 radiation oncologists (RO1 and RO2). Volumes of all contours were measured. Levels of intraobserver (GTV(MR)_RO vs GTV(CT)_RO) and interobserver (GTV(MR)_RO1 vs GTV(MR)_RO2; GTV(CT)_RO1 vs GTV(CT)_RO2) agreement were evaluated using the generalized κ statistics and the paired t test. RESULTS: No significant volumetric difference was observed between all 4 comparisons (GTV(MR)_RO1 vs GTV(CT)_RO1, GTV(MR)_RO2 vs GTV(CT)_RO2, GTV(MR)_RO1 vs GTV(MR)_RO2, and GTV(CT)_RO1 vs GTV(CT)_RO2; P > .05), with mean volumes of GTVs ranging 5 to 6 cm(3). The levels of agreement between those 4 comparisons were all substantial with mean κ values of 0.64, 0.66, 0.74, and 0.63, respectively. However, the interobserver agreement level was significantly higher for GTV(CT) compared to GTV(MR) (P <.001). The mean κ values significantly increased in all 4 comparisons for tumors >5 cm(3) compared to tumors ≤5 cm(3) (all P < .05). CONCLUSION: No significant differences in volumes between magnetic resonance- and computed tomograpghy-based Gross target volumes were found among 2 ROs. Magnetic resonance-based GTV delineation for lung stereotactic body radiotherapy also demonstrated acceptable interobserver agreement. Tumors >5 cm(3) show higher intra-/interobserver agreement compared to tumors <5 cm(3). More experience should be accumulated to reduce variability in magnetic resonance-based Gross target volumes delineation in lung stereotactic body radiotherapy.