Diffusion Tensor Imaging (DTI) Biomarker Alterations in Brain Metastases and Comparable Tumors: A Systematic Review of DTI and Tractography Findings

INTRODUCTION: Brain metastases (BM) are the most frequent tumors of the central nervous system (CNS). Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) technique that provides insights into brain microstructural alterations and tensor metrics, and generates tractography to visualize white matter (WM) fiber tracts based on diffusion directionality. This systematic review assessed evidence from DTI biomarker alterations in BMs and comparable tumors such as glioblastoma (GBM). METHODS: PubMed, Scopus, and Web of Science were searched, published between January 2000 and August 2023. The key inclusion criteria were studies reporting DTI metrics in BMs and comparisons with other tumors. Data on study characteristics, tumor types, sample details, and main DTI findings were extracted. RESULTS: 57 studies with 1592 BM patients and 1578 comparable brain tumors were included. Peritumoral fractional anisotropy (FA) consistently differentiates BMs from primary brain tumors, whereas intratumoral FA shows limited discriminatory power. Mean diffusivity (MD) increased in BMs versus comparators. Intratumoral metrics were less consistent, but revealed differences in BM origin. Axial and radial diffusivity (AD and RD, respectively) have provided insights into the effects of radiation, tumor origin, and infiltration. AD/RD differentiated tumor infiltration from vasogenic edema. Tractography revealed anatomical relationships between WM tracts (WMTs) and BMs. In addition, tractography-guided BM surgery and radiotherapy planning are required. Machine learning models incorporating DTI biomarkers/metrics accurately classified BMs versus comparators and improved diagnostic classification. CONCLUSION: DTI metrics provides non-invasive biomarkers for distinguishing BMs from other tumors and predicting outcomes. Key metrics included peritumoral FA and MD.