Assessing Response of High-Grade Gliomas to Immune Checkpoint Inhibitors

Background: Immunotherapeutic agents, especially checkpoint inhibitors, have emerged as the mainstay of therapy for several solid and hematological malignancies. These therapies are under investigation for the treatment of high-grade gliomas and brain metastases. Methods: This article reviews the unique challenges encountered when evaluating changes on magnetic resonance imaging (MRI) of glioblastomas seen in response to immunotherapy and checkpoint inhibitors and how to effectively incorporate MRI findings into the response assessment of high-grade gliomas to these emerging therapies. Results: An increase in tumor size or the appearance of new lesions on MRI may represent either an immune-mediated inflammatory response or true tumor progression, which may precede the subsequent stabilization or response of high-grade gliomas to immunotherapy. These MRI findings should not result in the mandatory cessation of immunotherapy in patients with high-grade glioma. Conclusions: Although immunotherapy Response Assessment for Neuro-Oncology criteria have been developed to assist with response assessment of high-grade gliomas to immunotherapy and to provide guidance with treatment decisions, these criteria have not been validated in prospective clinical trials. In patients with brain tumors who are receiving immunotherapy, MRI findings suggestive of disease progression should be evaluated with caution to prevent premature discontinuation of potentially beneficial therapies. Close, clinical monitoring with appropriate short-term, follow-up imaging is often necessary, and histopathological analysis may be required in some cases to confirm disease progression before a decision on continuation of these novel therapies can accurately be made.