Inflammatory Reaction Secondary to Immune Checkpoint Inhibitor Therapy Mimicking a Post-Operative Brain Abscess

Abstract Background Immune checkpoint inhibitors have revolutionized the treatment of many cancers, including melanoma, non-small cell lung cancer, and renal cell carcinoma. These therapeutics increase the activity of T cells against neoplastic cells, however, the immune response generated also has the potential to target normal cells resulting in immune related adverse events (irAEs). Most irAEs occur outside of the nervous system, but cases of limbic encephalitis, hypophysitis, optic neuritis, and pseudoprogression have been reported. Case Description Here, we present a case of an intracranial irAE after neoadjuvant stereotactic radiosurgery and craniotomy for resection of a left parietal lobe metastasis. The patient presented with headache, right-sided apraxia, and a pronator drift two weeks after surgery. Imaging findings were suggestive of an intracranial abscess. However, the lack of fever, normal white blood cell count, and benign clinical appearance in the setting of combination nivolumab and ipilimumab therapy argued in favor of an irAE. Following initiation of dexamethasone, the neurologic deficits resolved and the magnetic resonance imaging of the brain normalized over 7 weeks. Conclusions This is the first report of an acute surgical site immune response following stereotactic radiosurgery and craniotomy in a patient receiving nivolumab and ipilimumab. These immune mediated responses can be treated with corticosteroids and close observation.