Dosimetric robustness of tumour- and fiducial-based tracking in Radixact lung stereotactic body radiotherapy: a four-dimensional CT-based simulation using imaging-derived tracking-error scenarios

OBJECTIVES: To compare the dosimetry and robustness of two Radixact Synchrony lung stereotactic body radiotherapy (SBRT) tracking strategies: tumour-based lung tracking with respiratory modelling (LwR) and fiducial-based tracking with respiratory modelling (FwR). METHODS: Ten stage I non-small-cell lung cancer patients were retrospectively analysed. SBRT plans (42 Gy in 4 fractions) were generated on maximum-exhalation computed tomography (CT) using planning target volumes (PTVs) created by 3-7 mm isotropic gross tumour volume (GTV) expansion. Each plan was mapped to all four-dimensional CT (4DCT) phases via isocentre shifts from phase-specific GTV (LwR) or individual fiducial displacement (FwR). Deformable dose accumulation yielded LwR, nearest FwR, and farthest FwR simulated plans. GTV/PTV V100%, representative organ-at-risk (OAR) doses, and robustness versus target localisation error, tumour-fiducial distance, and tumour volume were evaluated. RESULTS: With a 3-mm PTV margin, FwR showed larger losses in GTV coverage and PTV robustness than LwR (maximum decreases of 9.0% and 26.0%). FwR was less robust, particularly with unfavourable tumour-fiducial geometry. Larger PTV margins progressively mitigated FwR coverage degradation while maintaining acceptable OAR doses. CONCLUSION: For lung SBRT delivered with Radixact Synchrony, LwR was more robust than FwR in this cohort, for which all tumour-fiducial distances exceeded 20 mm. When FwR is required, particularly under suboptimal tumour-fiducial geometry, a 3-mm margin should be used cautiously, and patient-specific evaluation of 5-7 mm margins within OAR dose constraints may be necessary. ADVANCES IN KNOWLEDGE: A 4DCT-based simulation using imaging-derived tracking-error scenarios reveals dosimetric robustness differences between LwR and FwR tracking.