ITV versus mid-ventilation for treatment planning in lung SBRT: a comparison of target coverage and PTV adequacy by using in-treatment 4D cone beam CT

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 ITV-based 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 cone-beam 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 ITV-based PTVs using isotropic margins of 5 mm (PTV(ITV + 5mm)), 4 mm (PTV(ITV + 4mm)) and 3 mm (PTV(ITV + 3mm)). The in-treatment 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.