ESTRO 2025 - Abstract Book

S4359

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

would be desirable, not only to improve the reliability of treatment plans but also to speed up the comparison of clinical results between different institutions.

Keywords: protontherapy; robust planning;

2505

Digital Poster Correlation between Gradient Index and Target Volume: Single Institution Experience in Linac-based Radiosurgery of Brain Metastasis Fernando M. Costa 1,2,3 , Alice Alves 1 , Fátima Aires 1 , João Casalta Lopes 1 , Pedro Soares 1 , Rosa Patrício 1,2 , Daniela Saraiva 1 , Anabela Gonçalves 1 , Patrícia Ferreira 1 , Gabriel Farinha 1 , Rita Figueira 1 , Armanda Monteiro 1,2 , Lígia Osório 1 1 Radiotherapy, ULS São João, Porto, Portugal. 2 Medical Imaging and Radiotherapy, E2S - P. Porto, Porto, Portugal. 3 Applied Physics, School of Engineering, University of Vigo, Vigo, Spain Purpose/Objective: Brain metastases occur in approximately 20% of patients with malignant neoplasms. Radiosurgery has become a staple in the management with Linac-based SRS treatment as a widely used modality. 1 The steep dose gradient outside the radiosurgical target is one of the factors that makes radiosurgery possible, playing a predominant role on the protection of the surrounding healthy brain tissue. The Paddick Quality Index is an important quality criterion in treatment planning that quantifies dose fall-off. 2–4 The purpose of this study is to evaluate whether there is a correlation between the Paddick Gradient Index and the Target Volume in Linac-based SRS. Material/Methods: Sixty-six plans for patients with brain metastases treated at our institution between January 2021 and August 2024 were analysed. For each of the patients, the target was defined on CT-MR deformable registered images. Patient’s treatment plan were performed using Eclipse TM treatment planning system version 15.6 (Varian Medical Systems, Palo Alto, CA) with 6 MV Flattening Filter Free (FFF) beams by non-coplanar partial Volumetric Modulated Arc Therapy technique. Target Volume (TV), Paddick Conformity Index (PCI), Homogeneity Index (HI), Selectivity Index (SI), Paddick Gradient Index (PGI) and Gradient Measure (GM) were extracted to describe the sample. Pearson product-moment correlation coefficient (two-tailed, p<0.001) was used for statistical analysis. A least-square method was used for non-linear regression. Results: A Pearson product-moment correlation was run to determine the relationship between PGI and TV in SRS Linac-based treatment. There was a strong, negative correlation between them, which was statistically significant ( r = -0.628, n = 66, p = .001). The non-linear regression analysis (Fig. 1) showed a PGI reduction with the increase of TV, through a power function (PGI=4,6614(TV) -0,25 ).