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2064 Background: Grade II and III meningiomas are invasive tumors which tend to recur following initial treatment and are associated with shorter survival (relative to grade I). Hemangiopericytoma (HPC) is a dural based tumor that originates in the pericytes in the walls of capillaries. Treatment options for these tumors include surgical resection and radiation. There are no defined standard treatments after failure of these modalities, and efficient systemic treatments are lacking. This trial concept was formed following previous reports of high PD-L1 levels in anaplastic meningioma. We currently report our initial experience with pembrolizumab for refractory atypical/anaplastic meningioma (RAM) and HPC patients. Methods: This is a single arm, single institute, phase 2 trial. Inclusion criteria included patients with RAM or HPC whose all previous lines of treatment were exhausted. All patients were treated with pembrolizumab (200mg every 3 weeks) until disease progression. Primary endpoint was 6- and 12-months progression free survival (PFS). The study was planned for 25 patients and we report here preliminary results. Results: Through February 2021, 12 patients were enrolled to this study (two with HPC, 10 with RAM). After a median follow up of 18.5 months, the 6 and 12 months PFS were 25% and 16.7% respectively, with a median PFS of 2.75 months. Two patients had a partial response while all others progressed, making the overall response rate 16.7%. One of the responders had HPC and has been stable for 30 months, while the second had atypical meningioma and was stable for 13 months. Median survival has not been yet reached; 1 year survival rate was 82.5%. Grade 3 toxicities included hyperglycemia, elevated liver enzymes and fatigue (that did not cause treatment discontinuation), with no grade 4 or 5 toxicities. PD-L1, mutation burden, MSI and other genomic analyses are still pending. Conclusions: Pembrolizumab induced a low response rate for RAM and HPC. However, a subset of patients might benefit from this treatment with a prolonged response period. To define this subpopulation further molecular studies are needed. Clinical trial information: NCT03016091.

2074Background: Ofranergene obadenovec (VB-111) is a gene-therapy specifically targeting angiogenic endothelial cells thereby leading to tumor starvation. Safety and efficacy of VB-111 in combinati...

2033 Background: The risk of VTE is very high for patients with brain tumors, with Glioblastoma (GB) considered one of the most at risk cancers. The aim of this study is to describe the occurrence ...