Melanoma Brain Metastasis Pseudoprogression after Pembrolizumab Treatment.

The role of immunotherapy in treatment of brain metastases is unknown because most trials exclude patients with active brain lesions. As new immunomodulating agents gain approval for many malignancies, it is important to know if they have unique effects in the central nervous system (CNS). Here, we present a case of a patient with progressing brain metastases treated with a single cycle of pembrolizumab, who presented with mental status changes 11 days thereafter. MRI of the brain showed enlargement of CNS lesions with intense central enhancement and diffuse perilesional edema. Histologic evaluation of a resected lesion revealed isolated clusters of tumor cells surrounded by reactive astrocytosis, scattered inflammatory cells, and an abundance of microglial cells. Given the increasing use of immune checkpoint inhibitors in patients with brain metastases from melanoma and other diseases, recognition of pseudoprogression and management with immune suppression are essential.