S2706
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
1312 Interobserver agreement among RTTs for IGRT matching in fiducial-free patients with hepatobiliary malignancies Amanda Webster 1,2 , Amy Dodd 1 , Kathryn McGeady 1 , Justhna Motlib 1 , Harikrishnan Nair Venugopalan 1 , Kathryn Osborn 1 , Shabnam Petkar 1 , Samaneh Shoraka 1 , Lauren Taylor 1 , Catharine Clark 3,2 , Maria A Hawkins 2,4 1 Radiotherapy and Proton Beam Therapy, UCLH, London, United Kingdom. 2 Medical Physics and Biomedical Engineering, UCL, London, United Kingdom. 3 Radiotherapy Physics, UCLH, London, United Kingdom. 4 Oncology Department, UCLH, London, United Kingdom
and 1.12 mm. The overall spread of residuals across matches was –2.1 to +2.0 mm vertically, –2.5 to +2.3 mm longitudinally, and –2.0 to +1.9 mm laterally, with the largest variability in the cranio-caudal direction. Across all matches, 58.7%, 54.2%, and 67.1% of residuals lay within ±1 mm vertically, longitudinally, and laterally, respectively; proportions within ±2 mm were 90.7%, 83.1%, and 89.8%. Conclusion: Given the difficulty of direct target visualisation and the limitations and risks of fiducials, whole-liver soft- tissue matching yields consistent RTT matching, with sub-millimetre systematic error, ~1–1.5 mm random error, and ≥ 83% matches within ±2 mm. Greatest variability occurred cranio-caudally, consistent with motion [4]. These data indicate good interobserver agreement among RTTs for whole-liver IGRT and support its use where fiducials are undesirable or Nadarevic, T., et al., Computed tomography for the diagnosis of hepatocellular carcinoma in adults with chronic liver disease. Cochrane Database Syst Rev, 2021. 10(10)2. Seppenwoolde, Y., et al., Treatment precision of image-guided liver SBRT using implanted fiducial markers depends on marker– tumour distance. Physics in Medicine & Biology, 2011. 56(17)3. Wang, C.P., et al., Tumour seeding after impractical. References: 1. percutaneous cryoablation for hepatocellular carcinoma. World J Gastroenterol, 2012. 18(45)4. Webster, A., et al., A systematic review of the impact of abdominal compression and breath-hold techniques....in patients with hepatobiliary and pancreatic malignancies. Radiother Oncol, 2024. 201 Keywords: IGRT, matching variability, fiducials Digital Poster Highlight 1321 Introducing 3D Optical Scanning and VMAT as a Novel Technique for Precision Low-Dose Radiotherapy in Osteoarthritis Treatment Luke Williams, Anna Huynh, Michael Chao Radiation Oncology, GenesisCare, Melbourne, Australia Purpose/Objective: Osteoarthritis (OA) is a prevalent degenerative joint disease with limited non-surgical options for patients with refractory symptoms, particularly in anatomically complex sites like the hands1. This study introduces a novel low-dose radiotherapy (LDRT) technique combining 3D optical surface scanning, custom silicone bolus, and Volumetric Modulated Arc Therapy (VMAT) for improved treatment viability. The objective was to compare the efficiency, accuracy, and
Purpose/Objective: Target visualisation in the upper abdomen is
challenging due to soft-tissue similarity and motion [1]. For patients with hepatobiliary malignancies, centres may insert fiducials, but accuracy depends on implantation site and proximity to the target, and the procedure carries a 1–4% complication risk [2, 3]. Many, therefore, use the whole liver for daily 3D IGRT soft-tissue matching. However, it is unknown if RTTs match consistently when they do not have fiducials to use. This study evaluated interobserver agreement among RTTs for whole-liver IGRT matching without fiducials. Material/Methods: Approval was granted, and ethics was not required as data use is covered by consent. Five hepatobiliary patients were randomly selected; all treated with abdominal compression. Ten RTTs were invited to participate. Matching was performed retrospectively, blinded, and limited to two minutes per case to replicate the online workflow.Each RTT performed rigid CT–CBCT registration followed by manual soft- tissue adjustment of the liver in three translational axes (no rotations), according to departmental protocol. The mean shift per dataset and axis was taken as the reference. Directional residuals (RTT minus reference) were used to calculate per-observer systematic (mean) and random (SD) errors. These were summarised across RTTs to derive the population systematic error (SD of observer means) and random error (root-mean-square of observer SDs) [8, 9]. Additional outcomes included the interobserver range and the proportions of matches within ±1 mm and ±2 mm. Results: Nine RTTs completed the study, each matching 25 datasets. The population systematic error was 0.40 mm, 0.41 mm, and 0.38 mm in the vertical, longitudinal, and lateral directions, respectively. The corresponding random error was 1.15 mm, 1.44 mm,
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