Systematic gravity-induced posterior tumour drift in free-breathing lung stereotactic ablative radiotherapy.

Background and purpose: Intra-fractional respiratory tumour motion is a known challenge in lung stereotactic ablative radiotherapy (SABR), as it can compromise target dose coverage. Gravity-induced tumour baseline drift during free-breathing carries the same risk but remains uncharacterised. This study aimed to quantify posterior tumour drift in peripheral lung SABR, investigate predictive factors, and evaluate tumour dose coverage. Materials and methods: In total, 97 patients (102 tumours) treated with 67.5 Gy in 3 fractions were analysed. Pre- and post-treatment cone beam computed tomography (CBCT) imaging quantified intra-fractional shifts at the first and second fraction. Temporal linear regression determined patient-specific drift rates. Correlations between drift rates and patient and tumour parameters were assessed. Plans were recalculated for worst-case drifts exceeding the 4 mm posterior planning target volume (PTV) margin. Results: Systematic posterior drift was observed, with mean vertical shifts of -1.45 mm at the first fraction and - 1.24 mm at the second fraction (both Conclusions: Gravity-induced posterior drift represents a systematic, clinically significant phenomenon in free-breathing lung SABR. In 15.7% of patients, drifts exceeded planning margins, with potentially severe target underdosage. Routine post-treatment CBCT verification is recommended to identify at-risk patients, enabling intervention at subsequent fractions to ensure treatment accuracy.