ステージI非小細胞肺癌に対する胸部放射線療法における中間換気（MidV）PTVアプローチ：単一システムワークフローの実装

The Mid-Ventilation (MidV) approach is an alternative to internal target volume (ITV) for generating a planning target volume (PTV) in lung stereotactic ablative radiotherapy (SABR). It has been shown to reduce PTV and volume of normal lung tissue irradiated, whilst maintaining high local control rates. However, this approach has not been widely adopted due to difficulties in determining the MidV phase in commercially available treatment planning systems. We hypothesised that a fully integrated MidV workflow could be developed and validated within a single vendor platform, producing accurate contours and dosimetric improvements compared with the ITV based method.

A MidV workflow was implemented using Eclipse/Aria, incorporating deformable image registration (DIR) of 4DCT images, automated 4D contour statistics for MidV phase selection and an embedded PTV margin calculation spreadsheet. Ten patients from a multi-centre Phase II lung SABR study were retrospectively analysed. The following metrics were used to compare the DIR method with the manually contoured iGTV: mean distance to agreement (MDA), dice similarity coefficient (DSC) and accurate selection of the MidV Phase (±10 %). Corresponding ITV and MidV PTV based plans were generated and compared using absolute PTV volumes, mean lung dose (MLD) and V20Gy.

DIR generated iGTVs demonstrated high geometric accuracy (mean MDA 0.86 mm; DSC 0.86). MidV phase selection aligned with manual selection in all patients. MidV based planning produced smaller PTVs than ITV based in 8/10 cases (mean 47.0 cm³ vs 54.6 cm³). Lung dose metrics improved consistently: MLD decreased by 14 % (3.46 Gy to 2.97 Gy) and V20Gy by 16 % (3.78 % to 3.19 %).

A streamlined, vendor integrated MidV workflow was successfully developed and validated, producing accurate DIR based contours and meaningful dosimetric reductions compared with ITV based method. These findings support broader clinical implementation of MidV based lung SABR.