ESTRO 2021 Abstract Book

S879

ESTRO 2021

improves visual function and is associated with a low risk of ocular sequelae.

PO-1056 The use of Auto-Segmentation tools to assess the SUV-isocontour in FET-PET to GTV delineation A. Gawish 1 , M. Walker 1 , M. Kreißl 2 , B. Röllich 1 , T. Brunner 1 1 Medical Faculty University Hospital Magdeburg, University Clinic for Radiation Therapy, Magdeburg, Germany; 2 Medical Faculty University Hospital Magdeburg, University Clinic for Nuclear Medicine, Magdeburg, Germany Purpose or Objective The aim of the research is to determine the contribution of 18F-fluoro-ethyl-tyrosine (18F-FET) positron emission tomography (PET) to gross tumor volume (GTV) delineation in patients with GBM compared to magnetic resonance imaging (MRI) alone. Materials and Methods The study population consisted of 5 patients with GBM. Seven image segmentation techniques were used to delineate 18F-FET PET GTVs, and the results were compared to the manual MRI derived GTV (GTV-MRI). PET image segmentation techniques included autodelineation in 8 diferent isocontour thresholds as show in figure.1 10%, 20%, 30%, 40%, 50%, 60%,70% and 80%., Overlap analysis was also conducted to assess geographic mismatch between the GTVs delineated using the different techniques. Results Analysis of 40 GTV-PET and 5 GTV-MRT, Mean GTV-PET was 19,4 ml ( 0,2 – 96 ml). Contours defined using GTV- PET-30% provided the best successful delineation technically in all the patients, it resulted in a mean Dice of 0,84 . Overall, the majority of GTVs defined on PET-based techniques were usually smaller than GTV-MRI (75% of cases), while the GTV-PET-10 and 20 % were usually larger than GTV-MRI (25%). The use of auto-segmentation to create the GTV decreased the intra-observer variability and the human error with the colors despite the non-smooth ROIs.

Conclusion The selection of the most suitable 18F-FET PET-based segmentation algorithm is important as it affects both the volume and the form of the resulting GTV. Using auto-segmentation decreased the intra-observer variability and showed that 30% SUV isocontour is correctly performed and can be used for GTV delineation. The follow up and mapping the recurrence is a right way to confirm our findings. PO-1057 Stereotactic radiosurgery of postoperative resection cavity for brain metastases F. Sousa 1 , B. Castro 2 , A. Aguiar 1 , J. Rodrigues 3 , T. Viterbo 4 , J. Lencart 5 , A. Soares 6 , T. Ramos 1 , A. Pires 1 , S. Conde 1 1 Instituto Português de Oncologia do Porto Francisco Gentil, Serviço de Radioterapia, Porto, Portugal; 2 Instituto Português de Oncologia do Porto Francisco Gentil , Serviço de Radioterapia , Porto, Portugal; 3 Instituto Português de Oncologia do Porto, Grupo de Epidemiologia do Cancro, Porto, Portugal; 4 Instituto