S2189
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
daily CBCT doses occasionally exceeded clinical constraints, the accumulated dose remained within tolerance. Weak correlations were observed between transit dosimetry and dose deviations, with the strongest found for bladder D_mean ( ρ = –0.38, p = 1 × 10 ⁻ ⁶ ). Table 1 - Relative percentage difference between accumulated and planned doses for D2%, D98%, and Dmean.
Research and The Royal Marsden Hospital, London, United Kingdom
Purpose/Objective: Using oART, large CTV to PTV margins of 1 to 1.5cm conventionally required to account for interfraction motion of target structures (vaginal vault (CTVv) and pelvic lymph nodes (CTVn)[1] in post-operative radiotherapy (PORT) for endometrial and cervix cancer can be safely reduced[2], although intrafraction motion still needs to be adequately addressed. We implemented daily CT-based oART using Radixact® (Accuray, Wisconsin, USA) tomotherapy system with Raystation® (RaySearch Laboratories, Stockholm, Sweden) treatment planning system (TPS) with CTVv 7mm and CTVn 5mm margins based on earlier departmental analysis and published data[2]. Online planned versus delivered dose was analysed to assess currently implemented margins and intrafraction motion, and if further margin reduction is possible. Material/Methods: Patients having PORT for endometrial or cervix cancer (45Gy in 25 fractions) with CT-based oART were included for analysis. A daily session fan-beam CT (FBCTsession) was used for oART planning, followed by a verification FBCT (FBCTverif) before treatment to confirm positioning and perform couch shifts if needed, and a weekly FBCT immediately following treatment (FBCTpost). Images were exported to the TPS and CTVv and CTVn were recontoured on FBCTpost. Delivered dose was estimated by recalculating FBCTsession dose on FBCTpost and dosimetrically compared using Wilcoxon signed-rank tests. CTVv and CTVn were copied from FBCTsession to FBCTpost with 1mm incremental isotropic margins to simulate PTV, and percentage of geometric overlap between simulated PTV and recontoured CTVv and CTVn on FBCTpost were recorded, mandating 95% geometric overlap in greater than 90% of fractions as acceptable. Statistical analysis was performed in R (version 4.5.1) using RStudio. Results: For 22 fractions in 11 patients, there was no statistically significant difference between online planned and delivered dose for CTVv D98%, D95%, D50%, D2% (Figure 1), and CTVn D98% and D95%, however CTVn D50% and D2% dose delivered was statistically significantly higher than planned: median 45.16Gy (IQR 0.08Gy) vs 45.26Gy (IQR 0.24Gy) (padj = 0.02) and 46.20Gy (IQR 0.10Gy) vs 46.39Gy (IQR 0.27Gy) (padj = 0.005) respectively (Figure 2). A 6mm isotropic margin of CTVv, and 3mm isotropic margin of CTVn achieved greater than 95% coverage in 90.9% of fractions. Conclusion: CT-based oART planned with CTV to PTV isotropic margins of 7mm for CTVv and 5mm for CTVn are
Figure 1 - Sagittal dose difference map showing accumulated vs. planned dose with CTV, anorectum, and bladder contours. Conclusion: The proposed automated workflow enables efficient prostate dose accumulation and results show good agreement between planned and accumulated doses. The weak correlation with transit dosimetry suggests that EPID-based monitoring does not adequately capture internal interfractional variations. Future work will assess the influence of contouring and deformation accuracy and extend this approach to other anatomical sites. Keywords: dose accumulation, prostate, transit dosimetry How low can we go? Margin validation of CT-based online adaptive radiotherapy (oART) in the post- operative treatment of endometrial and cervix cancer Benjamin J Thomas 1,2 , Alex Dunlop 3 , Shabanaz Boodhoo 1 , Irena Blasiak-Wal 3 , Emily Hogg 3 , Simeon Nill 3 , Susan Lalondrelle 1,2 1 Department of Radiotherapy, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 2 Division of Radiotherapy and Imaging, The Institute of Cancer Digital Poster 4580 Research, London, United Kingdom. 3 The Joint Department of Physics, The Institute of Cancer
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