Meningioma is the most common central nervous system (CNS) tumor and accounts for approximately 30% of all CNS tumors. For meningioma recurring after surgery and radiation therapy, there is no effective medical therapy. Repeat surgery or radiation therapy may be possible, but they are temporizing measures with limited durable relief. PD-L1 expression in meningioma is increased for recurrent tumors or prior radiation therapy, and a recent case study reported significant reduction of an intracranial meningioma after 6 months of PD-L1 blockade. Radiation has been shown to augment immune response when combined with PD-L1 blockade. Proton radiation therapy has higher relative biological effectiveness (RBE) and may further amplify the above immunological signals. Combination of proton radiation therapy administered concurrently with PD-L1 inhibitor may maximize immune response for recurrent meningioma. However, confirmation of the increased immunogenicity or increased tumor infiltrating lymphocytes using the combination of radiation therapy and PD-L1 blockade have not been confirmed in patients. The proposed study will be a single institution, single-arm, open-label, phase Ib study to combine neoadjuvant avelumab (a PD-L1 inhibitor) with hypofractionated proton therapy of 20 CGE (cobalt gray equivalent) over 5 fractions followed by planned surgery for recurrent radiation-refractory meningioma. This study is designed to provide proof of concept to demonstrate on-target effect of the combination to increase immunogenicity by directly examining the resected tumor for immune response and to evaluate preliminary clinical efficacy