Influence of Delivery Geometry and Beam Modulation on Dose Gradient and Organ-at-Risk Sparing in Lung SBRT: A Comparative Planning Study
Timur Koca, Yemliha Enes Tüten, İsmail Hakkı Sarpün, Yılmaz Bilek, Umur Kağan Kahya, Aylin Fidan KorcumAbstract
Purpose
To compare target coverage, selected plan-quality indices, organ-at-risk (OAR) dose parameters, and planned beam-on delivery time among coplanar VMAT (C-VMAT), non-coplanar VMAT (NC-VMAT), unblocked helical tomo-therapy (UB-HT), and directionally blocked helical tomotherapy (DB-HT) in lung stereotactic body radiotherapy (SBRT).
Methods
Ten patients with early-stage non-small cell lung cancer or oligometastatic lung lesions were retrospectively replanned. For each patient, seven plans were generated using the same planning CT dataset and structure set: C-VMAT, four NC-VMAT configurations with couch rotations of 45°, 60°, 75°, and 90°, UB-HT, and DB-HT. Plans were evaluated using target coverage, gradient index (GI), conformity index (CI), homogeneity index (HI), selected OAR dose metrics, and planned beam-on time. Comparisons were performed using the Friedman test.
Results
All techniques achieved clinically acceptable target coverage, with mean PTV V50 coverage close to the planning objective. Although PTV V50 showed a statistically significant overall difference, the absolute differences were small. Significant overall differences were observed for GI and HI, whereas CI did not differ significantly. Planned beam-on time showed the clearest technique-related difference, with VMAT-based plans requiring substantially shorter times than HT-based plans. HT-based plans, particularly DB-HT, showed lower mean lung V20 and mean lung dose values. DB-HT also showed the lowest mean heart and aorta D0.1cc values; however, these findings were interpreted as descriptive overall differences rather than pairwise evidence of superiority.
Conclusion
Under standardized planning conditions, all evaluated techniques produced acceptable lung SBRT plans. VMAT-based techniques provided shorter planned beam-on times, whereas DB-HT showed lower mean values for selected lung and mediastinal OAR parameters. Larger studies with anatomical stratification and spatial gradient metrics are needed to clarify the clinical relevance of these dosimetric differences.