The Origin of Cancer-Associated Fibroblasts (CAFs) in Brain Metastases: Seven Hypotheses and Current Evidence

Brain metastases have traditionally been investigated primarily with regard to interactions between tumor cells, the surrounding brain microenvironment, and associated inflammatory responses. By contrast, the presence of a true intratumoral stromal compartment in brain metastases has received much less attention, partly because the normal brain parenchyma lacks classical interstitial fibroblasts. However, recent histological, transcriptomic, and spatial studies suggest that at least a subset of carcinoma brain metastases contains fibroblast-like or cancer-associated fibroblast-like cells associated with collagen-rich extracellular matrix deposition. This raises a fundamental biological question: where do these cells come from? In this review, we discuss seven non-mutually exclusive hypotheses for the origin of cancer-associated fibroblast (CAF)-like cells in brain metastases, including pseudostromal mimics, meningeal or perivascular fibroblast-related cells, vascular mural cells, glial cells, cancer stem cell plasticity, tumor-stroma clusters, and circulating mesenchymal precursors. We compare the current findings for each hypothesis and highlight their major limitations. Overall, the available data support a heterogeneous and context-dependent model rather than a single universal origin. A better understanding of these stromal cell states may help refine the biological classification of brain metastases and support the future development of more targeted stromal therapies.