Assessing brain metastasis response to immunotherapy: a pictorial review of atypical responses and intracranial adverse events.

Immunotherapy, particularly immune checkpoint inhibitors, plays a crucial role in the treatment of brain metastases in various primary cancers. Response assessment encompasses atypical patterns, including pseudoprogression, hyperprogression, or dissociated response, which present greater complexity than classical patterns defined by standardized response assessment criteria. Additionally, intracranial adverse events like hypophysitis or encephalitis may resemble tumor progression. Accurate evaluation and management of brain metastases during immunotherapy requires that radiologists are familiar with both classical and atypical response patterns, as well as potential intracranial adverse events. Brain MRI and advanced imaging techniques serve as essential tools for this purpose. CRITICAL RELEVANCE STATEMENT: Assessing brain metastases response to immunotherapy accurately is fundamental for therapeutic decision-making. Radiologists must recognize classical and atypical responses and adverse events associated with immunotherapy to ensure optimal patient management. KEY POINTS: Immunotherapy response assessment in brain metastases is complex due to atypical patterns including pseudoprogression, hyperprogression, and dissociated responses. Immunotherapy-induced intracranial adverse events, such as hypophysitis and encephalitis, must be accurately identified. Brain MRI, complemented by advanced imaging techniques (perfusion MRI, MRS, and amino acid PET), is crucial for distinguishing these complex scenarios.