S2028
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Keywords: Liver SBRT, MRI, tumour delineation
Purpose/Objective: To assess the role of magnetic resonance imaging (MRI) in liver stereotactic body radiotherapy (SBRT) planning, identify optimal sequences for tumour delineation, and evaluate challenges in co-registration with computed tomography (CT). Material/Methods: We retrospectively reviewed 21 patients with unresectable liver malignancies treated between 2013–2015. All underwent CT simulation and diagnostic MRI. MRI quality based on the needs of radiation oncology and radiology was scored on a 5- point scale by two radiologists, two radiation therapists, and one radiation oncologist; inter-rater reliability was assessed using Cohen’s Kappa. The highest-ranked sequences were compared to CT for tumour visibility and planning utility. Results: Fifty-two tumours were identified across Primary CT, contrast-enhanced CT (CE-CT), and MRI. Visibility was highest on MRI (98.1%, 51/52), compared to CE-CT (81.3%) and Primary CT (48.1%). Seventeen tumours (32%) were only visible on MRI. MRI informed planning in 15/21 patients (71.4%), with co-registered sets used for delineation in 11 patients. Common sequences included T2 SSFSE, T2 FRFSE with fat saturation, and T1 LAVA Flex post-contrast. T2 FRFSE with respiratory gating achieved the highest median score (5) but was acquired in only two patients. T1 LAVA Flex Water post-contrast ranked second (median score 4) and was imported for planning in 64.7% of patients receiving MR contrast. Inter-rater concordance was 82.8% (Cohen’s Kappa = 0.66). Conclusion: MRI significantly improves tumour visualization for liver SBRT planning compared to CT. We recommend standardizing imaging protocols and routinely acquiring two sequences: axial T2 SSFSE (fast, non- contrast) and T1 LAVA Flex Water post-contrast (high tumour visibility). Future research should explore respiratory-triggered gating and prospective validation of MRI-based planning protocols. References: Dietrich O, Biffar A, Baur-Melnyk A, Reiser MF. Technical aspects of MR diffusion imaging of the body. Eur J Radiol. 2010 Dec [cited 2017 Jul 22];76(3):314–22. Worm ES, Hoyer M, Fledelius W, Hansen AT, Poulsen PR. Variations in magnitude and directionality of respiratory target motion throughout full treatment courses of stereotactic body radiotherapy for tumors in the liver. Acta Oncol. England; 2013 Oct;52(7):1437– 44.Niekel, M C, Bipat S, Stoker J. Diagnostic Imaging of Colorectal Liver Metastases with CT, MR Imaging, FDG PET, and/or FDG PET/CT: A Meta-Analysis of Prospective Studies Including Patients Who Have Not Previously Undergone Treatment. Radiology. 2010;257(3):674–84.
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Hybrid Single-Arc VMAT-SIB: Achieving Cardiac and Pulmonary Sparing in Hypofractionated Breast Radiotherapy Ismaell Massalha 1,2 , Adham Hijab 1 , Yonina Tova 1 , Shlomi Alani 1 1 Radiation oncology, Rivka Ziv Medical Center, Tzfat, Israel. 2 The Department of Microbiology, Immunology, and Genetics, Faculty of Health Sciences, Ben-Gurion University, Beer-Sheva, Israel Purpose/Objective: Breast-conserving surgery followed by whole-breast irradiation (WBI) reduces local recurrence and breast cancer mortality in early-stage disease. ¹ Conventional tangential WBI with sequential boost prolongs treatment and risks geometric uncertainty. Simultaneous integrated boost (SIB) shortens the duration while improving conformity; however, full-arc VMAT-SIB increases low-dose exposure to the heart, lungs, and contralateral breast—especially the left- sided structures.Hybrid techniques combining tangents for WBI with VMAT boost preserve geometric robustness while enhancing precision. Although dual- arc hybrids show improved homogeneity and OAR sparing, the simplified single-arc hybrid SIB remains unevaluated.This study assesses dosimetric performance, OAR sparing, and delivery efficiency of single-arc hybrid VMAT-SIB (tangential WBI + single VMAT arc) versus full-arc VMAT-SIB across two hypofractionated regimens (26 Gy/5 fx; 40.05 Gy/15 fx). Material/Methods: Twenty patients with early-stage breast cancer (10 left- sided, 10 right-sided; 5 + 5 per regimen) underwent hybrid planning: whole breast 26 Gy/5 fx or 40.05 Gy/15 fx ( α / β = 4 Gy; BED ≈ 86 Gy ₄ for 40.05/48 Gy) with a SIB to 30 Gy or 48 Gy, respectively. Plans were compared with full-arc VMAT-SIB using paired t-tests for target coverage and quality (PTV V95%, conformity index [CI], homogeneity index [HI]) and for organ-at- risk parameters, including mean heart and LAD dose, ipsilateral lung V5 Gy, mean contralateral breast dose, and beam-on time. Results: All hybrid plans achieved PTV V95% > 98%, CI < 1.2, HI < 0.15, and Dmax ≤ 107%. The hybrid approach significantly reduced ipsilateral lung V5 Gy (17.3 ± 3.5% vs 20.1 ± 2.8%; 21.6 ± 2.2% vs 23.0 ± 1.3%; p < 0.05), left-sided mean heart dose (0.70 ± 0.2 Gy vs 0.83 ± 0.6 Gy; 0.73 ± 0.36 Gy vs 0.80 ± 0.35 Gy; p < 0.05), and left- sided LAD mean dose (5.3 ± 3.0 Gy vs 6.7 ± 2.7 Gy; 5.7 ± 3.3 Gy vs 7.2 ± 1.9 Gy; p < 0.05). Contralateral breast
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