S2034
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
modest collimator margin (~7 mm) is preferable to mitigate excessive dose peaks and maintain plan quality. Keywords: SBRT, ultra-central lung tumors, collimator margin Digital Poster 5195 Developing a Multi-Isocentre Treatment Planning Technique for Craniospinal and Long Limb Irradiation on a C-arm Linac Rubina Begum, Mark McGovern Radiotherapy Physics, Guy's & St.Thomas NHS Hospitals, London, United Kingdom Purpose/Objective: Long-volume cancers had been regularly treated within our department with Tomotherapy (Accuray, USA) which uses a continuous helical delivery, eliminating the need for field overlaps and isocentre shifts, thereby reducing uncertainties during treatment. Due to end-of-life status of Tomotherapy an alternative volumetric modulated arc therapy (VMAT) technique on a Truebeam c-arm linac (Varian Medical Systems, CA, USA) was developed, utilizing two or more isocentres with overlapping fields and auto- feathering to help reduce treatment delivery uncertainty. Material/Methods: The Eclipse planning system v15.6 (Varian Medical Systems, CA, USA) was used to create multi-isocenter plans, with the auto-feathering (AF) tool [1] utilized to create smoother dose gradients in overlapping field regions. Sites including craniospinal, leg, and arm were created using up to 3 isocentres, with 2 VMAT arcs per isocentre. Overlapping field lengths were set as greater than 5cm [2]. Target volumes and organs-at- risk (OARs) doses were compared with Tomotherapy and 3D-conformal moving junction plans.Duplicate plans were produced with one isocentre shifted superiorly/inferiorly with respect to other isocentres whist keeping the same beam modulation to assess delivery uncertainty. The cases were also re-planned without auto-feathering to analyse differences in dose distribution and to assess dose gradients in beam overlap regions.Field junction dosimetry was verified by point dose measurements on the linac, with dose distribution assessed using Gafchromic film. Measurements were repeated with set-up error of 3mm and 5mm to verify that the ±3% tolerance and any set-up uncertainty of 3mm would be acceptable, and agreed with the treatment plan. Results: Treatment plans without auto-feathering were found to have steeper dose gradients (figure 1). Overlapping field lengths of 8cm reduced large areas of overdose
Radiation Oncology, American University of Beirut, Beirut, Lebanon
Purpose/Objective: Stereotactic body radiotherapy (SBRT) is well established for peripheral lung tumors, yet its application in ultra-central disease remains challenging due to the proximity of critical thoracic organs. This study presents a single-institution dosimetric comparison of three planning strategies for ultra-central lung tumors: intensity-modulated radiotherapy (IMRT), 3D forward planning with a 0-mm collimator margin (3D_0mm), and 3D with a 7-mm margin (3D_7mm). We hypothesized that the planning technique and collimator margin in 3D planning influence compliance with dose-constraint criteria and hotspot intensity, with larger margins mitigating hotspots without compromising organ-at-risk (OAR) sparing. Material/Methods: Twelve patients with ultra-central lung tumors, defined per Guillaume et al. as planning target volumes (PTVs) abutting or overlapping the trachea, proximal bronchial tree, esophagus, heart, or great vessels, were retrospectively analyzed. Three plans were created for each patient on Eclipse TPS (v18.0.1): IMRT, 3D_0mm, and 3D_7mm. All plans were prescribed to 60 Gy in 8 fractions. Planning objectives and OAR limits followed the SUNSET trial constraint set for 60 Gy/8. The primary endpoint was overall constraint adherence (CA). Secondary endpoints included OAR- specific compliance for the proximal bronchial tree (PBT), heart, and esophagus, as well as Dmax%, defined as the maximum point dose normalized to the prescription dose, representing hotspot intensity. Comparisons were performed using paired t-tests. Results: IMRT achieved the highest overall CA, with mean (±SD) pass-rates of 0.956 ± 0.071 compared to 0.909 ± 0.084 for 3D_7mm and 0.900 ± 0.122 for 3D_0mm. CA was significantly higher with IMRT versus 3D_7mm (p = 0.043, d = 0.66) and trended higher versus 3D_0mm (p = 0.092, d = 0.53). No difference was observed between 3D_0mm and 3D_7mm (p = 0.695). Hotspot intensity (Dmax%) differed significantly: IMRT 126.2 ± 6.2%, 3D_0mm 148.3 ± 13.5%, and 3D_7mm 123.3 ± 10.5% (overall p < 0.001). 3D_0mm generated higher hotspots than both IMRT (p < 0.001) and 3D_7mm (p < 0.001), whereas 3D_7mm and IMRT were comparable (p = 0.33). Conclusion: In ultra-central lung SBRT, IMRT achieved the highest dose-constraint adherence. 3D planning with a 7-mm collimator margin provided comparable OAR sparing and hotspot control to IMRT, while a 0-mm margin markedly increased hotspot intensity. In low-resource settings where IMRT is not available, 3D SBRT with a
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