S2905
RTT- RTT operational practice and workflow innovations
ESTRO 2026
Keywords: Pelvic Radiotherapy ， Bladder volume ， Setup error
Digital Poster Highlight 1096 Replacing the rescan? Dose evaluation of HyperSight CBCT for adaptive offline replanning in head & neck cancer Sjoerd Hoek, Paul van Duijvenvoorde, Yvonne van Herten, Abrahim Al-Mamgani, Anna Dinkla Department of Radiation Oncology, Amsterdam UMC, Amsterdam, Netherlands Purpose/Objective: Anatomical changes during radiotherapy for head and neck cancer can compromise dose delivery, traditionally necessitating repeat CT scans for offline re-planning. HyperSight CBCT provides fast imaging with Hounsfield Unit accuracy comparable to planning CT, enabling accurate offline re-planning while potentially eliminating repeat CTs. This may optimize workflow efficiency and reduce workload while maintaining dosimetric accuracy. Material/Methods: Fourteen head and neck cancer patients treated with VMAT were included. A 6 s HyperSight CBCT acquired during the first fraction on an Ethos linac was used. CBCT datasets were reconstructed using iCBCT Acuros or iCBCT MAR, and patients were selected based on close anatomical and positional correspondence with their planning CT. The original plan was recalculated on the HyperSight CBCT dataset. Dose differences ( Δ = CT – CBCT) were expressed as percentage differences for mean dose and D0.1 cc (spinal cord and bone mandible). Analyses included target volumes (CTV, PTV) and OARs (oral cavity, parotid and submandibular glands, spinal cord, bone mandible). Deviations exceeding 3–5 % were considered clinically relevant. Inter-patient variability was visualized using boxplots of Δ mean dose. Results: Table 1 presents minimal mean dose differences for target volumes (CTV High 0.10 ± 0.50%, CTV Low 0.05 ± 0.38%, PTV High 0.27 ± 0.53%, PTV Low − 0.05 ± 0.46%). Most OARs showed limited deviations, with statistically significant differences for only the oral cavity ( Δ Mean − 1.45 ± 2.51%, Δ Range − 9.2–0.2%, p = 0.01) and right parotid ( Δ Mean − 1.09 ± 1.49%, Δ Range − 4.6–0.8%, p = 0.01), reflecting minor anatomical deviations in air cavities and body contour, with a few outliers.
Other OARs, including the right submandibular gland and left parotid, showed minor to moderate, non- significant deviations. D0.1 cc differences were small (spinal canal − 0.55 ± 1.20%, bone mandible − 0.61 ± 1.71%). Figure 1 shows Δ mean dose boxplots, illustrating tightly clustered differences around zero with few outliers.
Conclusion: Recalculating on HyperSight CBCT appeared to accurately reproduce target and most OAR doses, though a few OARs exhibited larger deviations related to minor variations in air cavities and the body contour. These results support the feasibility of HyperSight CBCT for plan verification and suggest it could potentially replace the rescan in offline re- planning workflows. Future studies including larger and more diverse patient cohorts are warranted to confirm these findings and support clinical implementation. Keywords: Hypersight, Offline re-planning, Head and Neck Mini-Oral 1238 Ending the Tyranny of the Full Bladder: MR-Guided Adaptive SBRT for Prostate Cancer JESUS DOMINGUEZ, JULIA RODRIGUEZ RODRIGUEZ, ELOISA OLLERO, DAVID SANZ, PATRICIA VELASCO, DANIELA GONSALVES, ABRAHAMS OCANTO, GLORIA GUARDIA, EDUARDO MEILAN, ALBERTO GOMEZ, NATALIA BARJA DE QUIROGA, FELIPE COUÑAGO
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