S2209
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Digital Poster 848
Quantification of laryngeal motion for patients with early stage glottic cancer receiving focal magnetic resonance (MR)-guided SBRT in a phase I trial Kamal Singhrao 1 , Marvin Kinz 1,2 , Alexander Droznin 3 , Sejal Sunil Chirmade 3 , Jennifer Campbell 3 , Sara Boyle 3 , Danielle N. Margalit 1 , Roy B. Tishler 1 , Ritchell van Dams 1 , Raymond H. Mak 1 , Thomas L. Carroll 4 , Zhaohui Han 1 , Jonathan D. Schoenfeld 1 , Jonathan E. Leeman 1 1 Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Boston, USA. 2 Department of Physics and Astronomy, Heidelberg University, Heidelberg, Germany. 3 Department of Radiation Oncology, Brigham and Women's Hospital, Boston, USA. 4 Department of Otolaryngology-Head and Neck Surgery, Brigham and Women's Hospital, Harvard Medical School, Boston, USA Purpose/Objective: We completed a single institution phase 1 clinical trial for magnetic resonance-guided radiotherapy (MRgRT) for patients with early stage glottic cancer1. This enables MRI-guided hypofractionated/SBRT treatment combined with real-time motion tracking/gating by continuously imaging the larynx during radiotherapy with cine-MRI2. Continuous cine-MRI enables a unique assessment of intrafraction gross tumor volume (GTV) position and physiologic motion during treatment delivery (Figure 1). This could help inform head and neck radiotherapy using standard image guidance — where intrafraction physiologic motion is not tracked. Using intrafraction cine-MRI, we quantified the intrafraction GTV displacement, characterized beam-interruption displacement events (BIDEs) such as swallowing, and tested the correlation between GTV and skin surrogate motion. Material/Methods: Intrafraction cine MRI images were acquired for 5 patients with stage Tis/I/II squamous cell carcinoma of the glottic larynx undergoing definitive RT in a single- institution phase I trial. The radiotherapy prescription was 42.5 Gy in 5 fractions delivered every other day for subjects not currently smoking with treatment volume <10cc. Treatment was delivered using gated MRgRT on the ViewRay Systems MRIdian system with continuous target tracking. A GTV and 3-mm gating region-of-interest (ROI) structure were drawn on sagittal/coronal cine-MRI for beam-gating guidance. Beam interruption occurred if >5% of the GTV left the ROI boundary. An in-house optical-character- recognition and computer-vision-based point tracking software was developed to read beam delivery data and quantify larynx motion in intrafraction cine-MRI images. Tracked points included the GTV and skin
Results: For a spherical tumor the maximum displacement theoretically allowed was equal to its radius. When displaced inside a spherical lung domain, as seen in Figure 1, the maximum displacement theoretically allowed was dependent on an expression of the two radii, but always smaller than the tumor radius.This was confirmed in the numerical experiments on both the spherical test geometry and the patient geometry where larger tumor shifts introduced negative determinants of the Jacobian of the DVF in front of the shifted tumor, row 7 in Figure 1. Conclusion: Laplacian DVFs are valid for simulating small tumor position shifts but the displacement should not exceed the radius of the shifted volume. References: Olovsson, N., Wikström, K., Flejmer, A., Dasu, A., 2025. Robust treatment planning and displacement vector field editing for the probabilistic evaluation of proton radiotherapy of small lung tumors. Phys. Med. Biol. https://doi.org/10.1088/1361- 6560/ae19c7Vandemeulebroucke, J., Sarrut, D., Clarysse, P., 2007. The POPI-model, a point-validated pixel-based breathing thorax model. XVth international conference on the use of computers in radiation therapy (ICCR) 2, 8. Keywords: Deformation model, Lung cancer, Respiratory motion
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