S1997
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
designed to reduce dose to surrounding normal tissues during inverse planning. NTO can be used either in user-defined (manual) or automatic mode. This study aimed to evaluate the impact of different NTO configurations on the quality of volumetric modulated arc therapy (VMAT) plans for
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Analysis on the selection of optimal planning methodology, dose reporting for the PTV and associated uncertainties in lung SABR. Jose Solomon Raj Lourdhurajan 1 , Prasana Sarathy Nariyangadu 1 , Karen Venables 1 , Daniel Megias 2 , Roeum Butt 2 , Chirag Lakhani 2 , Suraiya Dubash 2 , Nihal Shah 2 , Anup Vinayan 2 1 Radiotherapy Physics, Mount Vernon Cancer Center, East and North hertfordshire NHS trust, London, United Kingdom. 2 Radiotherapy, Mount Vernon Cancer Center, East and North hertfordshire NHS trust, London, United Kingdom Purpose/Objective: Average intensity projection (AIP) scans are routinely used in the planning of linac-based lung SABR [1]. The blurred edges of the ITV raises discussion regarding linac-based SABR plans: whether the planning strategy should prioritise homogeneity or boost the ITV with hotspots (heterogeneous plan) [2]. The study aims to highlight the planning uncertainties linked to AIP scans, as well as provide recommendations on planning methodology and PTV coverage reporting. Material/Methods: A retrospective analysis of ten patients treated with linac SABR was chosen for this study. The V100%, Dmin, and Dmean for the PTV and ITV were calculated for phase 0 (maximum inhale), phase 50 (maximum exhale), and AIP scans using two planning methodologies. The heterogenous plans were made in such a way that 120% of the prescribed dose covered the ITV; on the other hand, the homogenous plans were created to ensure the prescribed dose covered the ITV and the PTV with the random hotspots within
endometrial cancer. Material/Methods:
This dosimetric study included 32 patients with similar treatment prescription doses (45 Gy in 25 fractions, 1.8 Gy per fraction). Treatment plans were generated using Varian Eclipse TPS Version 16.1. For each patient, three VMAT plans were created: without NTO (NTO- OFF), with manually defined NTO parameters (NTO- MAN), and with automatic NTO configuration (NTO- AUTO).Plan quality was evaluated by comparing PTV coverage, Conformity Index (CI), Homogeneity Index (HI), organ-at-risk (OAR) doses (bladder, rectum, bowel), and monitor units (MU).Dosimetric parameters were compared using the Friedman test, with pairwise comparisons performed using the Wilcoxon signed- rank test when significant differences were found. A p- value < 0.05 was considered statistically significant. Results: All the plans were clinically acceptable plans according to the PTV coverage.The HI was significantly improved in NTO-MAN plans compared to both NTO-OFF and NTO-AUTO plans (p=0.0001). The CI was significantly better with both manual and automatic NTO compared to NTO-OFF (p < 0.0001), with no difference between the two optimized plans.VMAT with NTO- AUTO better spares OAR, especially the bladder (bladder V40 Gy, p=0,006 ; bladder V30 Gy, p=0,007) and the rectum (rectum V40 Gy, p=0.001) compared to NTO-MAN and NTO-OFF.The average number of monitor units (MU) was significantly lower in NTO-OFF plans compared to NTO-AUTO (p =0.001), with no significant difference between NTO-OFF and NTO- MAN. Conclusion: Both manual and automatic NTO improved VMAT plan conformity for endometrial cancer compared to plans without NTO. Manual NTO provided better dose homogeneity, while automatic NTO offered superior OAR sparing, particularly for the bladder and rectum. Overall, automatic NTO provides an efficient and clinically practical balance between target coverage and normal tissue protection. Keywords: endometrial cancer, Normal Tissue Objective , VMAT
the PTV. Results:
The variation of the PTV V100%, Dmin, Dmean for each patient for each planning methodology is shown in table 1. Patient 1 and patient 5 showed large differences in reported parameters between AIP, phase 0 and phase 50. The mean PTV V100% on the AIP for the heterogeneous plans was found to be 96.0 ± 0.9% and the homogeneous plans was 94.5 ± 1.2%. The ITV V100% the AIP for the heterogenous plans was found to be 100.0 ± 0.1% and the homogenous plans was 97.7 ± 7.4%. The ITV Dmin on the AIP for the heterogenous plans was found to be 104.4 ± 7.5% and homogenous plans was 98.8 ± 5.4%.The V100% was less than 85% for tumours in which the density of the PTV was low. The PTV V100% standard deviations for the homogenous plans was found to be 6.3% and for heterogenous plans was 3.3%, respectively.
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