The impacts of mid-treatment CBCT-guided patient repositioning on target coverage during lung VMAT.

INTRODUCTION: The purpose of this study is quantify intrafraction motion (IFM) during lung volumetric-modulated arc therapy (VMAT) and evaluate the impact of mid-treatment cone beam computed tomography (CBCT)-guided patient repositioning on target coverage.

METHOD: This analysis included lung tumours treated with VMAT to 50-60 Gy in 3-5 fractions. Treatment planning was based on four-dimensional CT scans from which internal tumour volumes (ITV) were derived. An isotropic 5 mm margin was added to obtain the final planning target volume (PTV). Patients were treated supine with a customized dual vacuum immobilization device (BodyFIX, Elekta, Sweden). All patients underwent pre and mid-treatment CBCTs. Following each CBCT, a rigid registration was performed by a radiation oncologist. IFM was defined as the target displacement from pre to mid-treatment CBCT. For patients with an IFM vector ≥5 mm, a post hoc dose calculation analysis was performed to assess the dosimetric impact of CBCT-guided repositioning.

RESULTS: Ninety-seven patients (367 fractions) were included. Mean (±SD) overall treatment time was 53:02 ± 13:08 min. Mean time for mid-treatment CBCT scan acquisition and patient repositioning was 15:49 ± 4:14 min. Mean IFM vector was 1.5 ± 1.4 mm (max = 8.1 mm) and was <5 mm in 354/367 (96%) of fractions. For all 13 fractions with an IFM vector ≥5 mm, dose calculation analysis of worst-case scenario indicates that ITV coverage would have remained ≥95% without mid-treatment repositioning.

CONCLUSION: For 96% of fractions, the IFM vector was within the 5 mm PTV margin. Mid-treatment CBCT-guided couch repositioning did not significantly impact ITV coverage and prolonged treatment duration.