ESTRO 2023 - Abstract Book

S1821

Digital Posters

ESTRO 2023

The four optimization methods give similar CI values, for lesions < 1cm in size the CI values > 1.2. Regarding to homogeneity, for small lesions there is no difference between the optimization methods. However, for lesions 1-2cm or larger, more homogeneous plans are obtained with the optimization by the Ring method.

Conclusion Optimization with the NTO Ring method generates less complex plans, with a better gradient index and, for not very small lesions, with good homogeneity, using reduced optimizer times.

PO-2049 Verification of hybrid VMAT plan robustness

P. Weso ł owska 1 , E. Fujak 1 , A. Walewska 1 , D. Bodzak 1 , P. Kuko ł owicz 1 , A. Zawadzka 1 , M. Fillmann 1

1 The Maria Sklodowska-Curie National Research Institute of Oncology, Medical Physics Department, Warsaw, Poland

Purpose or Objective This study aims to evaluate the robustness of hybrid VMAT (H-VMAT) plans for breast patients treated with lymph node region. The main focus is put on the dose distribution measurement in the junction of abutting 3D/VMAT fields where undesirable cold/hotspots may occur. Materials and Methods The H-VMAT plan consists of four static fields and two VMAT arcs. The plan was calculated in Eclipse TPS (AAA v.15.6, Varian) for TrueBeam linac (Varian) equipped with Millenium 120 MLC. To assess the influence of jaw positions on the dose distributions, several measurements were done. Firstly, measurements of the H-VMAT plan were performed in chest phantom with EBT3 films in two transverse planes; one in the junction of the fields and second: 3 cm farther. Secondly, the static part of H-VMAT plan, and also the plans with an Y jaws offset of 1 and 2 mm were measured with EBT3 films positioned in a solid water phantom for 0° gantry angle (Fig 1). Additionaly the junction of the two open abbuting fields was evaluated, as it is susceptible to even small geometric misalignment of the jaws. Three dosimetrically equivalent linacs’ jaw settings were compared using simple plan with two open abutting fields. The junction in X and Y directions was tested for 0°/90° collimator and 0° gantry angles using EBT3 film positioned in a solid water phantom. Differences in relative dose in junction region were evaluated. The short-term reproducibility (ten consequtive irradiations) and long- term reproducibility (one irradiation per day for a week) was verified on one linac.