ESTRO 2025 - Abstract Book

S2815

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

References: 1 RCR, “Postoperative radiotherapy for breast cancer: UK consensus statements”, 2016, Available: rcr.ac.uk 2 Netherton, et al 2018. Interplay effect on a 6 ‐ MV flattening ‐ filter ‐ free linear accelerator with high dose rate and fast multi ‐ leaf collimator motion treating breast and lung phantoms. Medical physics , 45 (6), pp.2369-2376 3 Ranger, et al 2018. A dosimetric comparison of breast radiotherapy techniques to treat locoregional lymph nodes including the internal mammary chain. Clinical Oncology , 30 (6), pp.346-353 4 Yamauchi, et al 2020. The influence of respiratory motion on dose distribution in accelerated partial breast irradiation using volumetric arc therapy. Physica Medica , 80, pp.23-33

2392

Digital Poster Dosimetric Impact of Different CT Image Sets on Lung SBRT Planning

Helena Vivancos Bargalló 1 , Natalia Tejedor Aguilar 1 , Núria Jornet Sala 1 , Jaime Pérez-Alija Fernández 1 , Cristina Ansón Marcos 1 , Pedro Gallego Franco 1 , Eva Maria Ambroa Rey 2,1 , Marta Barceló Pagès 1 , Agustín Ruiz Martínez 1 , Alejandro Domínguez Perea 1 , Víctor Riu Molinero 1 , Javier Roda García 1 , Núria Farré Bernardó 3 , Pablo Carrasco de Fez 1 1 Medical Physics, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain. 2 Medical Physics, Consorci Sanitari de Terrassa, Terrassa, Spain. 3 Radiotherapy Oncology, Hospital de la Santa Creu i Sant Pau, Barcelons, Spain Purpose/Objective: A 2022 ESTRO survey [1] highlighted significant variability in the CT image sets used for dose calculation in lung SBRT. Nearly half of respondents employed the Average Intensity Projection (AIP), while others used slow CT, maximum ventilation, mid ventilation, minimum ventilation, or Maximum Intensity Projection (MIP), showing no clear preference. This study investigates the dosimetric impact of different CT image sets on dose calculation. Material/Methods: Ten peripheral lung cancer patients treated with SBRT at our center were selected. Each underwent 4DCT and a slow CT during free breathing. The ITV was delineated on the MIP image and refined using 4DCT phases as needed. Treatment plans were created with 2-4 semi-arcs of 6 MV FFF beams in Eclipse v15.6, based on three CT images: MIP, Mid-Ventilation (Mid-V) reconstructions from 4DCT, and slow CT. Dose calculations were performed using the Acuros XB (AXB) algorithm in dose-to-medium mode with a 0.125 cm grid. Plans were designed to achieve V100% = 95% of the prescribed dose (PD) for the PTV while meeting OAR constraints. Each plan was then recalculated with fixed MU in the AIP (used as ground truth as it is the most used image for dose calculation), and differences in PTV coverage and dose distribution were analysed. Results: PTV coverage between initial and AIP recalculated plans varied. MIP planning resulted in a lower V100% than expected when recalculated in AIP, while Mid-V planning showed a larger V100%. The Slow CT plans exhibited minimal dose differences (Figure 1a). The Paddick conformity index (CI), homogeneity index (HI), and dose gradient index (DGI) were evaluated for all plans in the original image set and their recalculations in AIP (Table 1). Significant differences were found when using a t-test (p<0.05) except for the CI Paddick and DGI in Slow CT plans. Dose distribution analysis in the AIP recalculation revealed dosimetric differences in MIP and Mid-V plans. Meanwhile, Slow CT plans showed minimal dose variability. Figure 1b illustrates the percentage of PTV with dose differences larger than 5% of the PD. MIP and Mid-V recalculations exhibited greater variability than Slow CT recalculations.