S2738
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
determines the additional setup variation introduced when using soft tissue image matching compared to FM matching and quantifies any additional PTV margins required for optimised delivery. Material/Methods: To evaluate the impact of image matching without FM, 10 consecutive liver SABR patients were retrospectively selected. Each patient had been planned and treated in end exhale breath-hold (EEBH) and had FM implanted in the vicinity of the target. For each treated fraction, three therapeutic radiographers independently performed retrospective EEBH CBCT to planning CT image matching using (i) FM-match, (ii) liver-match (ignoring FM), and (iii) liver segment-match (ignoring FM). The set up variation between the CBCT and planning CT scan were recorded. Using a multi- observer symmetric matching methodology [1], the observed registration displacements were used to estimate systematic and random variation for image matching. Results: Setup variations were used to calculate image matching contributions to PTV margins for each matching technique, in all translational axes. An additional PTV margin would be required for soft tissue matching over FM-matching for both techniques. Liver-matching requires an additional 1.4mm in the vertical, 0.2mm in the longitudinal, and 0.6mm in the lateral directions. Segment-matching requires an additional 1.0mm in the vertical, 1.3mm in the longitudinal, and 0.5mm in the lateral directions. Conclusion: FM is the most accurate method of image-matching for patients referred for CBCT-imaged liver SABR treated in EEBH at our centre. There is increased uncertainty with soft tissue matching when no FM are present. With appropriate margin adaptation, safe and effective EEBH liver SABR delivery remains achievable without FM, using either a liver or liver-segment based soft tissue match. References: [1] Tudor, George & Bernstein, David & Riley, Stephen & Rimmer, Yvonne & Thomas, Simon & Herk, Marcel & Webster, Amanda. (2020). Geometric Uncertainties in Daily Online IGRT: Refining the CTV-PTV Margin for Contemporary Radiotherapy. 10.1259/geo-unc-igrt. Keywords: PTV margin, image-matching, inter- operator
Conclusion: Our institutional data demonstrate that a tattoo-less workflow is advantageous in term of setup time and magnitude of translational setup variations and corroborate the results supporting SGRT-based setup workflows. Keywords: breast, tattoo-less
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Liver SABR without implanted fiducials. A review of on-set image-matching variation and PTV margin recommendation. Conor Hudin 1 , Marina Khan 2 , Sunita Mahto 2 , Sophia Ahmed 2 , Christopher Thomas 1 1 Medical Physics and Clinical Engineering, Guy's and St. Thomas' NHS Foundation Trust, London, United Kingdom. 2 Radiotherapy, Guy's and St. Thomas' NHS Foundation Trust, London, United Kingdom Purpose/Objective: Liver stereotactic ablative radiotherapy (SABR) delivers a highly concentrated dose of radiation to a tumour in the liver while minimising damage to surrounding healthy tissues. Fiducial markers (FM) are commonly implanted to aid in cone beam CT (CBCT) to planning CT image registration to ensure consistent, reproducible and accurate treatment. However, a subset of patients are unsuitable for FM due to medical contraindications or the need for expedited treatment. In these cases, image matching is performed using soft tissue anatomy.This study
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A simple "void-and-no-fluid" protocol ensures clinically acceptable bladder volume control during online adaptive radiotherapy
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