S2270
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Conclusion: Vertical abdominal displacements showed a statistically significant correlation with mediastinum displacements; therefore, sporadic mediastinum displacement can be discovered by monitoring Δ A. Maintaining a controlled level of TP and AP could potentially reduce the positional variability of the mediastinum during DIBH. Keywords: DIBH, reproducibility, protons Digital Poster 4392 Evaluating PTV Margin Reduction in MR-Guided Prostate SBRT Benjamin Vanspeybroeck, Tim Everaert, Jacques Bezuidenhout, Sven Van Laere, Guy Soete, Thierry Gevaert, Mark De Ridder Radiotherapy UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium Purpose/Objective: MR-guided online adaptive radiotherapy (MRgART) with cine-MRI gating may allow further reduction of planning target volume (PTV) margins. Previous studies suggest margins as small as 2 mm are feasible and could lower treatment-related toxicity. This study aimed to evaluate the adequacy of our current 3 mm clinical target volume (CTV) to PTV margin in MR- guided prostate stereotactic body radiotherapy (SBRT) and assess the potential for further reduction. Material/Methods: In this retrospective analysis, intrafraction prostate motion was evaluated in 10 patients treated in the phase II Proseven study (Vanspeybroeck et al., 2025) ¹ . Each patient received five fractions delivering 36 Gy to the PTV and 40 Gy to the CTV using focal boosting over seven days, on a 0.35 T MR-Linac (ViewRay Systems, Inc.). High-resolution TRUFI MRI sequences acquired prior to contouring and plan adaptation were fused with post-treatment TRUFI images. Table shifts applied after online adaptation were incorporated as translational adjustments. Prostate volumes were contoured on pre- and post-treatment scans, yielding 50 analyzed fractions. An automatic beam-hold was triggered if the prostate deviated >3 mm in the sagittal plane. Intrafraction motion was quantified by comparing post-treatment prostate volumes with pre- treatment volumes expanded by 1 mm, 2 mm, and 3 mm. The fraction of the post-treatment prostate volume encompassed by each expansion was calculated. A 95% threshold was used to define adequate coverage. Cumulative distribution functions (CDFs) and Pearson correlation coefficients between translational displacements and CTV coverage were computed.
whether vertical displacements of the abdomen ( Δ A) at an abdominal point (AP) can explain the sporadic deviation in DIBH position of the mediastinum ( Δ M). Material/Methods: For 14 patients with mediastinal Hodgkin and non- Hodgkin lymphoma treated with proton radiotherapy in DIBH, the correlation between Δ A and Δ M was assessed for weekly control CT scans performed in DIBH. The Δ T was managed in the same way as the clinical routine, meaning it was fed back to the patient via visual guidance and required to be within a 3 mm gating window from the treatment planning session to permit the initiation of a CT acquisition. Results: In 9% (7 of the total 75 CTs), the mediastinum deviated more than 5 mm from the planned position, even though the values of Δ T were within the gating window for all cases, and in 5% (4 CTs), the mediastinum deviated more than 10 mm. The maximum deviation was 24 mm. A statistically significant linear correlation was found between Δ A and Δ M (r = 0.6, R2 = 0.607).
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