LAUSR

BACKGROUND Immuno-oncotherapy (IO) has revolutionized systemic cancer care but remains experimental in brain tumors. IO treatment risks multi-organ autoimmune inflammatory responses that limit its use. The CNS is an immune specialized compartment with restricted cellular access, thus fewer cases reported for immune-mediated encephalitis. Interestingly, patients with history of blood brain barrier (BBB) compromise are potentially at higher risk for immune cell trafficking to the CNS. CASE DESCRIPTION We report the first case, to our knowledge, of a 70-year-old male with clear cell renal cell carcinoma with pulmonary metastases treated with lung irradiation, nephrectomy, and chemotherapy prior to switching to single-agent nivolumab immune-oncotherapy. The patient presented with new-onset generalized tonic-clonic seizure and left visual field-cut. Review of patient history revealed remote traumatic brain injury (TBI). Brain imaging noted a solid enhancing right occipital mass that was presumed metastasis versus lymphoma. Cerebrospinal fluid cytology was negative for malignancy but concerning for lymphoproliferative process not determined to be malignant. The patient started steroids and anti-epileptic therapy. After negative systemic cancer re-staging, IO was discontinued, and steroids were initiated with demonstrated patient clinical improvement. CONCLUSIONS We concluded the diagnosis of immune-mediated encephalitis secondary to IO with collection of reactive T-cells within the area of encephalomalacia. The area of encephalomalacia from prior TBI served to compartmentalize the reactive lymphocytes giving the appearance of a mass. Taken together, new onset seizure in patients on IO might signal encephalitis and CNS metastatic mimicry should be considered in patients with a prior history of TBI and encephalomalacia.