S2195
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Digital Poster 4970
Initial clinical experience of RayStation-based online adaptive RT using Radixact for endometrial cancer. Irena Blasiak-Wal 1 , Emily Hogg 1 , Alex Dunlop 1 , Natalie Bleackley 1 , Michael Thomas 1 , Benjamin Thomas 2 , Susan Lalondrelle 2 1 Radiotherapy Physics, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 2 Radiotherapy and Imaging, The Royal Marsden NHS Foundation Trust, London, United Kingdom Purpose/Objective: The Radixact (Accuray, Wisconsin) platform offers great potential for online adaptive radiotherapy (oART) due to excellent quality imaging system (ClearRT®) as well as long imaging and treatment fields. Endometrial cancer patients are standardly treated with large PTV margins (15 mm Sup/Inf and 10 mm circumferentially for vault, 7 mm uniformly for nodes) due to significant inter-fraction motion. We have developed an oART technique for the Radixact platform that uses the RayStation TPS (RaySearch Laboratories, Stockholm) that incorporates PTV margin reduction and inherently reduces the irradiated volume and OAR doses. Our first oART patient was treated in March 2025 for endometrial cancer. In this work we review the target volumes and OAR doses for the first 11 patients
treated with oART. Material/Methods:
Figure 1: Axial dose distribution adapted (top) v’s scheduled (bottom) fraction. Results: A total of 225 oART fractions were performed. On all but one fraction (99.6%) the daily adapted plans were selected for treatment. PTV volume was reduced on average by 25.7% (range 22.1 – 29.0%). OAR doses were analysed on three representative patients. Bladder, rectum and space bowel mean doses were reduced by an average of 2.4 Gy, 6.3 Gy and 2.3 Gy respectively. The V30 Gy and V40 Gy parameters were also reduced. A summary of the OAR dose reduction is shown in Table 1.
11 post-surgical endometrium cancer patients were treated with oART between May and October 2025 prescribed to 45Gy median dose to PTVnv in 25 fractions delivered daily. The reduced PTV margins were 7 mm for vault and 5 mm for nodal uniform expansion. During each oART fraction CTV targets and OARs (bladder, rectum and space bowel) were delineated on daily kV CT images. Daily plans were optimised using RayStation Treatment Adaptation module. A plan comparison was performed between daily oART and scheduled (reference plan calculated on daily scan); PTVnv coverage, dose conformity and homogeneity were reviewed as shown in Figure 1, OARs and PTVnv clinical goals and DVHs were assessed and the most appropriate plan was used for treatment.
Table 1. OAR dose reduction Conclusion: oART for endometrium cancer patients is feasible
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