A 62-year-old man with poorly differentiated papillary thyroid carcinoma with extensive brain, lung, lymphatic metastases post thyroidectomy, radioiodine therapy, chemotherapy with dabrafenib and trametinib, and stereotactic radiation surgery presented with recurrence of brain metastases one year after the diagnosis of a 2.1 cm metastatic lesion in the left parieto-occipital region causing right homonymous hemianopsia. The metastases initially improved in size and number with radiation and serial brain MRIs were stable. Biopsy of the mets revealed a poorly differentiated carcinoma. He was subsequently treated with pembrolizumab for six months. Two months after treatment initiation, he reported episodic behavioral arrest, confusion, and expressive aphasia concerning for seizures. Continuous EEG monitoring revealed a left-sided focus without seizures, and the episodes persisted despite levetiracetam and clobazam. Four lumbar punctures revealed lymphocytic pleocytosis (10-14 cells), elevated protein (48–68 mg/dl), negative cytology, flow cytometry and viral studies including JC virus. Paraneoplastic panels in serum and cerebrospinal fluid were negative. Repeat MRIs findings were most consistent with radionecrosis and noted improvement of the metastatic lesions. Suspicion was raised for pembrolizumab-induced encephalitis and he received high-dose steroids with minimal response however, clinical improvement noted with reduced episode frequency after intravenous immunoglobulin induction therapy and rituximab maintenance therapy.