Intra-adaptational motion during online adaptive radiotherapy: analysis of couch shifts and implications for margin design

Purpose Online adaptive radiotherapy (oART) minimizes inter-fractional variation, yet intra-fractional motion (IFM) remains a source of uncertainty. We quantified intra-adaptational motion (IAM) using routinely recorded couch shifts between pre-adaptation (CBCT1) and pre-treatment (CBCT2) scans to derive motion-related components of the CTV–PTV margins in a large clinical cohort. Methods We retrospectively analyzed 1378 fractions from 69 prostate cancer patients treated with 60 Gy in 20 fractions as oART on the Varian Ethos platform (vision 1.1; Varian Medical Systems, Palo Alto, CA, USA). Couch shifts were calculated based on rigid registration targeting the CTV. Systematic and random errors were calculated for right–left (RL), anterior–posterior (AP), and superior–inferior (SI) directions. Margins were estimated using the van Herk formula. Correlations between couch shifts and the CBCT1–CBCT2 time interval were assessed using Spearman’s ρ. Results Couch shifts showed substantial inter-patient variability. Population mean, systematic, and random errors ranged from 0.05 to 1.4 mm, 0.6 to 1.3 mm, and 1.0 to 1.4 mm, respectively. The mean adaptation time was 17.5 ± 5.1 min (median: 16.6 min, interquartile range [IQR]: 14.0–20.3 min). The resulting IAM-related margins were 2.3 (R), 2.2 (L), 2.2 (S), 3.2 (I), 2.9 (A), and 5.7 mm (P). Positive correlations were observed between the time interval and motion in AP (ρ = 0.26), SI (ρ = 0.30), and vector magnitude (ρ = 0.29; all p < 0.001). Conclusion Intra-adaptational motion varies substantially between patients, supporting the consideration of patient-specific margins. Verification CBCT2 is essential in oART due to potentially large positional shifts occurring during the adaptation process. The calculated margins represent upper bounds for motion-related margins, as they incorporate registration errors and cover longer intervals than actual beam delivery.