S1967
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster Highlight 3954
Dosimetric impact of anatomical change on VHEE and VMAT plans: A comparative robustness study Fabio S D'Andrea 1,2 , Robert Chuter 2,3 , Adam H Aitkenhead 2,3 , Roger M Jones 1,4 1 Department of Physics and Astronomy, The University of Manchester, Manchester, United Kingdom. 2 Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom. 3 Division of Cancer Sciences, The University of Manchester, Manchester, United Kingdom. 4 Accelerator Science and Technology, The Cockcroft Institute, Daresbury, United Kingdom Purpose/Objective: Anatomical change during radiotherapy degrades plan quality and the therapeutic ratio; adaptive radiotherapy (ART) can mitigate this but is resource- intensive, limiting routine use [1]. Prior work [2,3] suggests very high-energy electrons (VHEE) are less sensitive to heterogeneities, attributed to differences in dose-deposition and reduced sensitivity to density interfaces, yet evidence largely derives from simple geometries (e.g. slab phantoms, air cavities) rather than complex, patient-specific, optimised treatment plans. This study compares VMAT and 200MeV VHEE for sensitivity to inter-fraction anatomical change in retrospective, adaptation-triggered clinical cases, constituting a modality stress test to assess whether the purported insensitivity of VHEE holds beyond Four retrospective cases (two thoracic, one uterine, one rectal) that prompted mid-course adaptation were analysed. For each case, the clinical 6MV VMAT plan was compared with a dosimetrically matched 200MeV spot-scanned VHEE plan using multiple static fields [4]. VHEE plans were optimised using an in-house Monte Carlo framework (GATE/Geant4) experimentally validated at 200MeV at CERN CLEAR. The original VMAT and VHEE plans were recalculated on the rigidly registered mid-treatment CT without re-optimisation to isolate anatomy-driven dosimetric effects. Dosimetric change was assessed against mandatory institutional constraints. Endpoints included the number and type of constraint breaches, changes in PTV and ITV/CTV D95%, and high-dose volumes in OARs. simple geometries. Material/Methods:
Results: VMAT breached dose constraints in 3/4 cases; VHEE breached one constraint in 1/4. In Lung-1 (tumour regression), VMAT breached three constraints: PTV D95%=88.4% (limit ≥ 90%); PTV D1cc,max=107.7% (limit ≤ 107%); spinal cord PRV D1cc,max=49.4 Gy (limit ≤ 48 Gy). In Uterus (organ contraction), VMAT increased bowel V100% fivefold, from 51.3 to 259.3cm3, and produced a PTV hotspot (D1cc,max=113.3%). In Rectum (weight loss), neither modality breached constraints; VMAT increased bowel V100% from 30.6 to 280.1cm3, while VHEE increased less. VHEE preserved PTV/ITV D95% in all four cases; its sole breach was a PTV hotspot (D1cc,max=110.0%) in Lung-2 (uniform shrinkage), where VMAT also breached hotspot limits and lost ITV coverage (D95%=92.3%; limit ≥ 95%).
Conclusion: Across four clinically challenging, adaptation-triggered cases, 200MeV VHEE showed greater intrinsic dosimetric robustness than 6MV VMAT to mid-course anatomical change, with more stable target coverage
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