Abstract
BackgroundThe 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 materialsThirty-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-CBCTin-treat). The GTV was delineated from each of the ten respiratory phase-sorted 4D-CBCTin-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 (PTVMidV) using dose-probabilistic margins and three ITV-based PTVs using isotropic margins of 5 mm (PTVITV + 5mm), 4 mm (PTVITV + 4mm) and 3 mm (PTVITV + 3mm). The in-treatment baseline displacements and target motion amplitudes were reported to evaluate the impact of both parameters on target coverage.ResultsOverall, 100 4D-CBCTin-treat images were analyzed. The mean target coverage was 98.6, 99.6, 98.9 and 97.2% for PTVMidV, PTVITV + 5mm, PTVITV + 4mm and PTVITV + 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 PTVITV + 5mm, PTVMidV, PTVITV + 4mm and PTVITV + 3mm had mean PTV reductions of 16, 19 and 33%, respectively.ConclusionWhen 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 PTVMidV, PTVITV + 3mm significantly reduced the PTV while still maintaining an adequate in-treatment target coverage.
Highlights
The internal target volume (ITV) approach and the mid-ventilation (MidV) concept are the two main respiratory motion-management strategies under free breathing
All the planning target volume (PTV) margins led to a target coverage per treatment higher than 95% in at least 90% of the evaluated cases
When implementing volumetric modulated arctherapy (VMAT) with 4D cone-beam computed tomography (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 stereotactic body radiotherapy (SBRT)
Summary
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. Stereotactic body radiotherapy (SBRT) is considered a standard treatment for inoperable early-stage non-small lung cell cancer (NSLCC) and oligometastases [1]. One treatment solution for lung SBRT is volumetric modulated arc therapy (VMAT) combined with respiration-correlated 4D cone-beam computed tomography (4D-CBCT) image guidance. Respiration-correlated 4D-CBCT is considered one of the optimal volumetric image-guided technologies for lung SBRT treatment [3]. With 4DCBCT, the mean position, trajectory, and shape of a moving tumor can be verified in the treatment unit and the respiration-induced geometrical uncertainties for mobile targets can be reduced [4]
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