S2268
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
performed with adapt-to-position (ATP)[1], to correct for rigid tumor displacements, on the first 4D_MRI scan during a fraction. GTV to PTV margins of 10- 15mm were applied, depending on BM amplitude. Deformable image registration (ADMIRE Research Version 3.48, Elekta, Stockholm, Sweden) was used to measure BM in 4D-MRI and to generate a 3D mid- position representation with BM removed. Subsequently, the drift was determined with respect to the ATP-MRI, followed by registration of the ATP- MRI to the planning CT to capture AC. BM, drift and AC are not independent: BM is measured in scans subject to drift, which are in turn subject to anatomy changes. Therefore we (mathematically) reordered the cascade of deformations vector fields (DVFs), allowing to single out individual sources of uncertainty (Figure1).. The ATP dose was mapped and accumulated for 4 scenarios: all uncertainties combined (accumulated), and individually (AC, Drift, BM). Accumulated dose was evaluated against the ATP doses, averaged over the 3 fractions.Evaluated DVH parameters for target (Dose Evaluation Volume[2], DEV) and selected OARs were a subset of the planning objectives (D98, D2, D0.035cc) evaluated in physical dose. Results: In 48/52 patients (92%) target minimum dose decreased, DEV-D98: -3.5Gy [-6.2, -1.8] (median [IQR]) as a result of drift. Accumulated DEV-D2 (near max dose) discrepancy was smaller: -1.0 Gy [-1.6, -0.7], resulting from BM dose-blurring. The ATP procedure was effective in managing target related AC. In OARs, small population discrepancies were present, however, in individual patients substantial changes in D0.035cc were established, e.g. accumulated GREAT_VESS: -0.6Gy [-3.6, -0.8], accumulated HEART: - 0.3 Gy [-3.1, +2.4], see figure 2. OARs dose-changes resulted from either AC or Drift, BM contribution was consistently smallest. Conclusion: Dose accumulation for patients treated with MRgSBRT showed that target dose was primarily affected by intra-fraction motion (drift) and may benefit from trailing. For OARs, no population trends were present, however, in individual cases both inter-fraction AC (to be leveraged with online replanning) and drift lead to substantial deviations. Management of BM (gating) is expected to give a minor improvement in dose delivery accuracy. References: [1] T. van de Lindt e.a., 2021, PMB, Validation of a 4D-
physical dose distribution for each fraction. Accounting for fraction dose distribution variations through BED accumulation may provide a more accurate representation of the delivered biological dose and could support further biological optimization in online adaptive workflows.Supported by SNSF grant 10001696 and partly by Varian, a Siemens Healthineers Company. References: 1. Bachmann N, Ahmadsei M, Hürlimann M, et al. Cone-beam computed tomography-based online adaptive radiotherapy of esophageal cancer in the neoadjuvant setting: Dosimetric analysis, toxicity and treatment response. Radiother Oncol. 2025;209:110981. doi:10.1016/j.radonc.2025.1109812. Rueckert, et al.. (1999). Nonrigid registration using free-form deformations: Application to breast MR images. IEEE Transactions on Medical Imaging, 18(8), 712–721. doi:10.1109/42.7962843. Zobrist B, Bertholet J, Frei D, et al. Monte Carlo dose calculation for photon and electron radiotherapy on dynamically deforming anatomy. Med Phys. 2025;52(2):1281-1292. doi:10.1002/mp.17472 Keywords: BED, dose accumulation, online ART Digital Poster Highlight 4203 Dose accumulation for patients with liver metastases treated with MRI-guided SBRT: impact of geometrical uncertainties S.R. van Kranen, J.-E. Peltenburg, T. Janssen, J.-J. Sonke Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands Purpose/Objective: To assess the impact of inter-fraction anatomy changes (AC), intra-fraction motion (drift) and breathing motion (BM) on the accumulated dose in patients with liver metastases treated with MRI-guided SBRT (MRIgSBRT). Material/Methods:
Following IRB approval, 52 patients with liver metastases were selected for dose accumulation. Patients were treated with 60 Gy in 3 fractions on a 1.5T MR Linac between 2019-2022.IGRT was
Made with FlippingBook - Share PDF online