Circular RNAs in Plasma and Beyond: Potential Biomarkers for Breast Cancer.

Breast cancer (BC) continues to be a major cause of cancer-related mortality among women, and early diagnosis remains critical for improving survival outcomes. Conventional tissue biopsy and imaging techniques are constrained by invasiveness and limited sensitivity in early-stage disease, whereas routine serum tumor markers lack sufficient specificity for reliable early detection. Circular RNAs (circRNAs) have increasingly been recognized as promising non-invasive biomarkers, owing to their remarkable stability and detectability in plasma. Here, we summarize the current landscape of plasma circRNAs as diagnostic, prognostic, and chemoresistance-related biomarkers in BC, emphasizing their clinical relevance in therapy selection, subtype stratification, treatment monitoring, and recurrence prediction. Aberrant circRNA expression in the plasma of BC patients has been associated with tumor size, stage, and molecular subtype, and functionally linked to cell survival, metastasis, and drug resistance. Importantly, circRNAs may complement conventional approaches including imaging and serum markers, and their integration with other circulating analytes could enhance diagnostic precision and support individualized therapeutic decisions. Our work consolidates recent advances in circRNA research across preoperative, postoperative, drug resistance, and recurrence settings, providing a framework for non-invasive diagnosis, clinical decision-making, and outcome prediction in BC management. This review aims to bridge tissue-based discoveries and plasma-based applications for the clinical translation of circRNAs.