Differential diagnosis of imaging alterations after stereotactic radiotherapy of brain metastases: A narrative review.

PURPOSE: Stereotactic radiotherapy is a key treatment for brain metastases, alongside surgery and systemic therapies. A substantial proportion of patients develop radiation necrosis after stereotactic radiotherapy. Differentiating radiation necrosis from tumour progression is critical, as they require distinct therapeutic strategies. While magnetic resonance imaging remains the mainstay of surveillance, its limitations emphasize the need for alternative, non-invasive diagnostic modalities. This narrative review aimed to summarize recent imaging techniques for distinguishing radiation necrosis from tumour progression after stereotactic radiotherapy for brain metastases. MATERIALS AND METHODS: A systematic literature search was conducted in Embase and Medline (using the search engine PubMed) for studies published between January 2012 and February 2025. The terms "radiation necrosis," "brain metastasis," and "diagnosis" were combined. Eligible studies included adult patients with brain metastases treated with stereotactic radiation, with differentiation of radiation necrosis from tumour progression assessed by histopathology or imaging follow-up. Studies exclusively on primary brain tumours or without separate brain metastases analysis were excluded. Both prospective and retrospective designs were considered. RESULTS: Fourty studies met inclusion criteria: magnetic resonance imaging (n=21), nuclear medicine (n=13), and radiomics (n=6). Advanced magnetic resonance imaging (e.g., perfusion, intravoxel incoherent motion, treatment response assessment maps) and aminoacid positron-emission tomography showed the highest diagnostic accuracies. Radiomics, especially multiparametric approaches, improved performance but lacked external validation. CONCLUSION: Combined advanced magnetic resonance imaging, aminoacid positron-emission tomography, and radiomics approaches offer the greatest potential. Prospective studies are needed.