S2253
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Oncology, Copenhagen University Hospital - Rigshospitalet, Copenhagen, Denmark
Purpose/Objective: In the STRICT-Lung trial (NCT05354596), patients with centrally located lung lesions receive stereotactic body radiotherapy (SBRT) using an intentionally inhomogeneous dose distribution—delivering up to 85 Gy to the gross tumour volume (GTV) and 56 Gy to the planning target volume (PTV) in 8 fractions—while adhering to strict organ-at-risk (OAR) constraints [1]. These constraints create steep dose gradients, making treatment highly sensitive to intra-fractional motion. Given prior reports of severe toxicity in central SBRT [2,3], this study evaluates the impact of intra-fractional position correction in STRICT-Lung patients. Material/Methods: Forty-seven consecutive patients treated according to the STRICT-Lung protocol were analysed. Treatment was based on soft - tissue alignment to the planning CT (pCT). Three cone-beam CTs (CBCTs) were acquired per fraction: a pre-treatment CBCT (CBCT-before) for setup and anatomical assessment, a mid-treatment CBCT (CBCT-midway) after the first of six fields for intra-fractional position correction, and a post- treatment CBCT (CBCT-after) to evaluate baseline shifts. Target displacements between CBCTs were quantified for each fraction. Dose was recalculated on CBCT-midway and CBCT-after (752 CBCTs in total) and accumulated on the pCT to estimate changes in maximum OAR dose (D0.3cc) for the organ receiving the dose closest to its constraint. Results: The median [interquartile range] baseline shift from CBCT-before to CBCT-after was 2.7 mm [1.6, 4.2], which decreased to 1.3 mm [0.7, 2.3] after correcting the treatment position following the first field (p < 0.05, Wilcoxon signed-rank test) [Figure 1]. The baseline shift between CBCT-before and CBCT-midway was significantly larger than that between CBCT- midway and CBCT-after, though the latter time interval was longer, indicating gradual soft-tissue relaxation. Dose recalculations on CBCT-midway and CBCT-after showed similar results [Figure 2], confirming that position correction after the first field reduced residual baseline shifts to levels with negligible dosimetric impact. The cohort exhibited a low toxicity rate, with only five grade 3 treatment-related events (dyspnoea, fatigue, lung infection, atrial fibrillation, and subclavian-vein thrombus) and no events above grade 3 at 1–3.5 years follow-up.
Conclusion: Real-time dose-guided ART was implemented experimentally for the first time. The delivered doses were comparable for dose-guided ART and geometry- guided MLC tracking which both demonstrated improved dosimetric accuracy compared to no tracking. While dose-guided ART has demonstrated improvements over geometry-guided MLC tracking in silico, remaining technical limitations must be addressed to realise these benefits clinically. References: [1] Grimm, J., et al., High Dose per Fraction, Hypofractionated Treatment Effects in the Clinic (HyTEC): An Overview. International Journal of Radiation Oncology*Biology*Physics, 2021. 110(1): p. 1-10[2] Jaffray, D.A., et al., Accurate Accumulation of Dose for Improved Understanding of Radiation Effects in Normal Tissue. International Journal of Radiation Oncology*Biology*Physics, 2010. 76(3, Supplement): p. S135-S139[3] Mejnertsen, L., et al., Dose-based optimisation for multi-leaf collimator tracking during radiation therapy. Physics in Medicine & Biology, 2021. 66(6): p. 065027. Keywords: dose-guided, MLC tracking, MRI linac Intra-fractional position correction improves accuracy and safety in SBRT for central lung lesions in the STRICT-Lung trial (NCT05354596). Simon N Thomsen 1,2 , Hjørdis H Schmidt 1 , Ditte S Møller 1,2 , Marianne M Knap 1 , Mette Pøhl 3 , Lone Hoffmann 1,2 1 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 3 Department of Mini-Oral 3677
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