Choroid plexus asymmetry on routine MRI as an imaging signature of breast cancer brain metastasis

Background. Brain metastasis affects 15-30% of patients with breast cancer, yet brain-specific biology underlying central nervous system colonisation is poorly captured by available clinical measures. The choroid plexus, an MRI biomarker of neuroinflammation and a primary immune-cell gateway implicated in brain metastasis, has not been examined in this setting. Methods. In a real-world, cross-sectional study of 1,000 women with breast cancer (190 with brain metastasis), bilateral choroid plexus volumes on contrast-enhanced brain MRI were quantified using a deep-learning pipeline. Guided by the principle that hemispheric asymmetry indices are sensitive to focal pathology, this study derived from an exploratory bilateral volume comparison a parameter-free Choroid Plexus Asymmetry Index (CPAI = |L-R|/(L+R)). Multivariable logistic regression, propensity-score matching, laterality and grade-stratified analyses assessed robustness. Results. The results show that total choroid plexus volume does not differ between groups, whereas left and right volumes change asymmetrically and CPAI is elevated in patients with metastasis (median 0.088 vs 0.059; Cliff's delta = 0.20, 95% CI 0.10-0.29). CPAI is minimally correlated with 10 peripheral markers and with histological grade. The association is robust under multivariable adjustment (adjusted OR 1.58 per +1 SD, 95% CI 1.32-1.88) and propensity-score matching, with |CPAI| scaling with metastatic burden, independent of metastasis side. Conclusions. Choroid plexus morphological asymmetry on routine brain MRI is associated with breast cancer brain metastasis independently of peripheral inflammation, tumour grade and conventional markers.