Refining the PTV margin determination for VMAT SBRT lung treatment through moving target dose model to account for respiratory motion uncertainty.

[OBJECTIVE] Adding a Planning Target Volume (PTV) margin remains a straightforward and effective strategy to ensure adequate target coverage under various uncertainties. Appropriately reduced margins can minimize treatment-related toxicity without compromising tumor control. However, respiratory motion introduces complex interactions with Internal Target Volume (ITV) management, making conventional PTV recipes less reliable. This study aims to refine the definition of PTV margin for lung stereotactic body radiotherapy (SBRT) using a moving target dose (MTD) model to account for respiratory motion uncertainty under free breathing, and to compare it with alternative margin calculation methods.

[METHODS] Data from 31 patients with non-small-cell lung cancer (15 in the upper lobe, 16 in the lower lobe) were retrospectively analyzed. All patients underwent Four-Dimensional (4D) CT simulation and tumor motion tracking during treatment, represented by three-dimensional coordinates extracted from log files generated by Synchrony Respiratory Tracking System, which were used to characterize displacement along the superior-inferior (S–I), anterior-posterior (AP), and left-right (L–R) axis. The MTD model is proposed to assess the accumulative dose of the moving target considering both motion and dose fall-off outside of the PTV from VMAT treatment, then margins were calculated to ensure Gross Tumor Volume (GTV) receives a minimum of 95% of the prescribed dose during treatment, stratified both by anatomical direction and tumor location.

[RESULTS] While the results of different methods exhibit a consistent pattern across pathological locations and different directions, they show significant discrepancies in magnitude. The smallest result is derived from the MTD model, whereas the spatial-temporal (ST) and adjusted van Herk models yield slightly larger result than the others. As calculated using the MTD model, for the upper-lobe group, the margins were 3.10 mm (S–I), 0.98 mm (A–P), and 0.41 mm (L–R), whereas for the lower-lobe group, the corresponding margins were 5.28 mm (S–I), 1.00 mm (A–P), and 0.80 mm (L–R), respectively. Compared with MTD model, upper-lobe margins from ST model was larger by 1.47 mm (S–I), 0.84 mm (L–R), and 1.31 mm (A–P), whereas lower-lobe margin increased by 2.00 mm (S–I), 1.12 mm (L–R), and 1.49 mm (A–P). Notably, the margins would generally increase with prolonged treatment time, majorly caused by respiratory baseline drift effects and short-term error fluctuations.

[CONCLUSION] Our study provided a method of incorporating dosimetric factors into PTV definition for lung SBRT to account for respiratory motion-induced uncertainties. Margins derived solely from spatial-temporal analysis are relatively conservative, whereas the MTD model provides a margin definition relevant to the accumulated dose by the target, potentially balancing GTV cumulative dose coverage with reduced normal tissue exposure. For clinical robustness, future calculation should incorporate device/therapist-specific uncertainties via variance summation to refine margin recipe.

[CLINICAL TRIAL NUMBER] Not applicable.

[SUPPLEMENTARY INFORMATION] The online version contains supplementary material available at 10.1186/s13014-026-02795-z.