ESTRO 2021 Abstract Book

S473

ESTRO 2021

37.3% but the linear trend did not reach significance [p = 0.102]. Headache decreased by 84.4 ± 18.2% with a significant linear trend over time [F(1) = 9.299, n = 8, p = 0.019, partial eta squared = 0.571] (figure 2, bottom). Functional status was up-trending but did not reach significance [p = 0.100]. Neurocognitive ability overall did not change [mean p = 0.313]. Five/10 patients weaned completely off steroids in the immediate post-treatment period and remained steroid-free during the remaining 12-months. There were significantly fewer days of steroid usage overall in the 12 months after IA bevacizumab compared to before [Z = -2.253, p = 0.024, r = -0.531]. 0/10 died and 0/10 exhibited AEs attributed to bevacizumab alone. 4 AEs of moderate severity were probably related to blood-brain barrier disruption (tonic/clonic seizures with altered mental status two hours post-procedure) or bevacizumab + BBBD (mono-ocular blurred vision with diplopia on day 1). 2/10 patients experienced RN recurrence requiring intervention (one patient surgery, the other patient 2 cycles IV bevacixumab) at 11 months.

Conclusion A single targeted low-dose intra-arterial injection of bevacizumab led to durable clinical and radiographic improvement of refractory radiation necrosis during the 12-month follow up. To our knowledge this is the first prospective report in the literature of this novel approach in adults. Targeted low-dose IA bevacizumab might potentially be a longer lasting safer alternative in the treatment of radiation necrosis of the brain as compared to multiple cycles of IV bevacizumab at higher doses. PH-0608 A national study on the inter-observer variability in delineation of organs at risk in the brain E.L. Lorenzen 1 , J.F. Kallehauge 2,3 , C.S. Byskov 4,5 , R.H. Dahlrot 6,2,7 , C.A. Haslund 8 , T.L. Guldberg 8 , Y. Lassen- Ramshad 2 , S. Lukacova 4 , A. Muhic 9 , P.W. Nyström 2 , L. Haldbo-Classen 4 , I. Bahij 2 , L. Larsen 8 , B. Weber 2 , C. Rønn Hansen 2 , C.R. Hansen 1 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 3 Aarhus University Hospital, Department of Clinical Medicine, Aarhus, Denmark; 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 5 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 6 Odense University Hospital, Department of Oncology, Odense, Denmark; 7 University of Southern Denmark, Institute of Clinical Research, Odense, Denmark; 8 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark; 9 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective Proton-based radiotherapy has the potential to reduce the dose to parts of the brain compared to photon- based radiotherapy. In Denmark patients are referred to proton radiotherapy based on a plan comparison performed in each of the four Danish neuro oncology centers. Consistent delineation of organs at risk (OAR) is therefore required to get a consistent evaluation of the potential gain of protons. The Danish Neuro Oncology Group (DNOG) had established national consensus guidelines for delineation of OAR structures based on the published literature. This study was conducted to evaluate the inter-observer variance of the delineated OAR structures, to review the existing guidelines and to create a validation atlas for auto-segmentation algorithms. Materials and Methods Initial validation of preliminary guidelines was performed by distributing an anonymized MRI of a brain cancer