S849
Clinical - Lung
ESTRO 2026
both treatment sides; organ doses showed strong laterality effects, with higher cardiac and GI doses for left-sided plans and higher hepatic doses for right- sided cases. Six of the 10 patients required at least one replan during the course of their treatment. Four of these patients required replan due to changes in pleural effusion already present when treatment began. All 4 patients underwent aspiration to drain the effusions followed by replans. In 2 cases the patients were rested between draining of the effusion and starting the replan. The other 2 patients had other anatomical changes that affected the dosimetry sufficiently to warrant a replan. Conclusion: All 10 patients that started treatment after randomisation completed treatment as planned. Anatomical changes, particularly pleural effusions, were the dominant trigger for replanning, underlining the need for close adaptive monitoring in hemithorax proton therapy. HIT-Meso continues to recruit patients. Keywords: Mesothelioma, Proton, Trial Digital Poster 4613 Three-Dimensional Evaluation of Lung Displacement: Implications for Plan Uncertainty Calculation in Lung Radiotherapy ABEL RODRIGUEZ ARANDA 1 , ADRIAN ANDRADES 2 1 Medical-Physics, Hospital Universitario de Lanzarote, Arrecife, Spain. 2 Radiotherapy, las Palmas University Hospital, Las Palmas, Spain Purpose/Objective: This study provides a detailed assessment of lung displacements along the X, Y, and Z axes during the respiratory cycle. The aim is to determine representative values along each axis to calculate plan uncertainty in lung radiotherapy treatments and evaluate dosimetric robustness. Material/Methods: A cohort of 32 patients underwent respiratory-phase CT scans. A three-dimensional (3D) matrix representing delta displacements along X, Y, and Z during maximum inhalation and exhalation was generated using Eclipse TPS. Profiles were traced every 4 cm along axial (perpendicular to Z- longitudinal), sagittal (perpendicular to X-lateral), and coronal (perpendicular to Y-vertical) planes, producing a 3D matrix of Hounsfield Unit (HU) levels as a function of position for each lung point. Delta displacements were obtained from differences in point positions between inhalation and exhalation. Mean delta displacements were calculated across all patients and classified by lung (left or right) and axis: Z-axis was divided into three zones; X and Y axes were
support its integration into practice, although prospective studies are needed to confirm its long- term clinical impact. Keywords: Adaptive radiotherapy, Lung cancer, Organs at risk
Digital Poster Highlight 4601
Early dosimetric and adaptive experience from the first patients on the UK HIT-Meso (NCT05655078) clinical trial Andrew Gosling 1 , Reza Abdullah 2 , Danya Abdulwahid 3 , Frances Charlwood 4 , N Counsell 5 , Steven Court 1 , Kimberley Durno 6 , David Edge 1 , Toby Hillman 2 , David Lines 4 , Caroline Maguire 1 , Emma Patel 1 , Vasileios Rompokos 7 , Crispin Hiley 8 1 Proton Therapy, UCLH, London, United Kingdom. 2 Respiratory Medecine, UCLH, London, United Kingdom. 3 Clinical Oncology, The Christie, Manchester, United Kingdom. 4 Proton Therapy, The Christie, Manchester, United Kingdom. 5 CTC, UCL, London, United Kingdom. 6 Clinical Oncology, Barking, Havering, and Redbridge, London, United Kingdom. 7 Radiotherapy, Royal Marsden, London, United Kingdom. 8 Clinical Oncology, UCLH, London, United Kingdom Purpose/Objective: To present the first 10 patients that completed treatment on the multi-centre randomised controlled trial HIT-Meso (NCT05655078, funded by Asthma + Lung UK, Mesothelioma UK (grant reference number MCTA22F\7)). HIT-Meso compares proton therapy pencil-beam scanning (PBS) hemithorax irradiation to treat pleural mesothelioma versus observation which is the current standard of care. This represents the first prospective evaluation of proton hemithorax irradiation in pleural mesothelioma in the UK. We present the dosimetry achieved for the target and organs of interest, report on the replan rate, and causes of replans. Material/Methods: An audit was performed of the dosimetry of the first 10 patients that completed treatment after randomisation to the treatment arm of HIT- Meso. Two patients had left sided disease, 8 patients had right sided disease. The PBS IMPT treatment plans were created in the Varian Eclipse treatment planning system. Recorded was coverage of the treatment target, organ of interest doses compared to the trial dose guidelines, number of replans, and the reasons for replans. Results: Dosimetry results are presented in the table below. The dose achieved shows a dependence on which side is treated. Target coverage was consistent for
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