ESTRO 2020 Abstract book

S525 ESTRO 2020

were utilised as reference contours. Following local planning criteria 60 Gy in 30# treatment plans were generate for all individual PTVs, with dosimetric differences assessed through dose-volume histogram statistics of the delayed target volumes and organs at risk (OAR). Results One patient underwent rapid progression between MR acquisitions and was excluded. At a cohort level, GTV and PTV volumes decreased by 20 cm -3 and 71 cm -3 respectively for 1-week delayed MR delineation, whilst DSC values were approximately invariant (table 1) at 0.82.

The median OS among patients with RN was 21.3 months and among non RN 13.7 months, with no significant differences (p=0.15).

A broad range of PTV and CTV inter-operator dose differences (fig. 1) were determined, with the largest variations for primary tumours proximal to multiple OARs. Despite OAR dose dependency on location of target volumes, systematic reductions in all OAR doses were observed for delayed delineation, including a crucial median 3.3 Gy reduction in mean brain dose.

Conclusion We found no statistically significant associations between risk factors and radionecrosis, neither in survival between patients with and without necrosis, which could be partly due to the few cases of RN. With some more sample, statistically meaningful differences could be found. PO-0897 Dosimetric Consequence of Transient Post- Surgical Anatomical Effects for Glioblastoma M. Tyyger 1 , M. Nix 1 , L. Murray 1,2 , S. Currie 3 , C. Nallathambi 1 , R. Speight 1 , B. Al-Qaisieh 1 1 Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom ; 2 Radiotherapy Research Group, University of Leeds, Leeds, United Kingdom ; 3 Department of Neuroradiology, Leeds Teaching Hospitals NHS Trust, Leeds, United Kingdom Purpose or Objective This investigation aimed to quantify volumetric and dosimetric effects of employing <72-hour post-surgical assessment MRI, in contrast to 1 week delayed imaging for radiotherapy delineation. Exploitation of post-surgical assessment MRI for glioblastoma (GBM) delineation eradicates the necessity of acquiring an additional delayed MRI at time of CT- simulation. However, transient peritumoural oedema/inflammation and increased time for progression limits the reliability of post-surgical delineations to accurately represent tumour extent at treatment. Material and Methods Six patients with GBM underwent <72-hour and 1-week delayed post-surgical MRI, with gross tumour volumes (GTV) delineated independently by a neuroradiologist (O nr ), and two oncologists (O 01 , O 02 ). Clinical target volumes (CTV) were generated using 25 mm isotropic margins, adjusted for anatomical boundaries, and planning target volumes (PTV) were grown using 5 mm isotropic margins. Sørensen–Dice coefficient (DSC) and volumetric analysis was performed with in-house software, in which GTVs were discretised into individual voxels and O nr contours

Conclusion Tumour delineation at 1 week post-resection, once transient post-surgical effects dissipated produced smaller target volumes, alongside target dose increases and OAR dose reduction. Inter-operator variability remained similar whether employing post-surgical or delayed MRI. Additionally, delineation on post-resection MRI was disfavoured due to potential for under-treatment; due to progression and/or non-rigid deformation. PO-0898 Primary tumor-status relevance in NSCLC with brain metastases undergoing radiosurgery A. Gonzalez Lopez 1 , F. Suarez 1 , D. Büchser 1 , A. Frias 1 , A. Lasso 1 , E. Mayrata 1 , P. Gonzalez-Fernandez 2 , E. Boveda 1 , P. Bilbao 1 1 Hospital de Cruces, Radiation Oncology, Barakaldo, Spain ; 2 Hospital de Cruces, Endocrinology, Barakaldo, Spain