Background: The internal target volume (ITV) approach and the mid-ventilation (MidV) concept are the two main respiratory motion-management strategies under free breathing. The purpose of this work was to compare the actual in-treatment target coverage during volumetric modulated arctherapy (VMAT) delivered through both ITVbased and MidV-based planning target volume (PTV) and to provide knowledge in choosing the optimal PTV for stereotactic body radiotherapy (SBRT) for lung lesions. Methods and materials: Thirty-two lung cancer patients treated by a VMAT technique were included in the study. For each fraction, the mean time-weighted position of the target was localized by using a 4-dimensional conebeam CT (4D-CBCT)-based image guidance procedure. The respiratory-correlated location of the gross tumor volume (GTV) during treatment delivery was determined for each fraction by using in-treatment 4D-CBCT images acquired concurrently with VMAT delivery (4D-CBCT in-treat ). The GTV was delineated from each of the ten respiratory phase-sorted 4D-CBCT in-treat datasets for each fraction. We defined target coverage as the average percentage of the GTV included within the PTV during the patient's breathing cycle averaged over the treatment course. Target coverage and PTVs were reported for a MidV-based PTV (PTV MidV ) using dose-probabilistic margins and three ITVbased PTVs using isotropic margins of 5 mm (PTV ITV + 5mm ), 4 mm (PTV ITV + 4mm ) and 3 mm (PTV ITV + 3mm ). The intreatment baseline displacements and target motion amplitudes were reported to evaluate the impact of both parameters on target coverage. Results: Overall, 100 4D-CBCT in-treat images were analyzed. The mean target coverage was 98.6, 99.6, 98.9 and 97.2% for PTV MidV , PTV ITV + 5mm , PTV ITV + 4mm and PTV ITV + 3mm , respectively. All the PTV margins led to a target coverage per treatment higher than 95% in at least 90% of the evaluated cases. Compared to PTV ITV + 5mm , PTV MidV , PTV ITV + 4mm and PTV ITV + 3mm had mean PTV reductions of 16, 19 and 33%, respectively.Conclusion: When implementing VMAT with 4D-CBCT-based image guidance, an ITV-based approach with a tighter margin than the commonly used 5 mm margin remains an alternative to the MidV-based approach for reducing healthy tissue exposure in lung SBRT. Compared to PTV MidV , PTV ITV + 3mm significantly reduced the PTV while still maintaining an adequate in-treatment target coverage.

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