Circulating Tumor DNA in Breast Cancer: Diagnostic Insights From a Case Series.

Circulating tumor DNA (ctDNA), the tumor-derived fraction of cell-free DNA, has emerged as a valuable biomarker for monitoring treatment response, detecting minimal residual disease (MRD), and identifying early cancer recurrence. While histologic tissue diagnosis remains the gold standard for confirming malignancy, guidelines from organizations such as the National Comprehensive Cancer Network (NCCN) and the European Society for Medical Oncology (ESMO) acknowledge that ctDNA may serve as a supplemental tool in rare instances where tissue is unobtainable. In such cases, results should be interpreted alongside clinical and radiologic findings, with tissue confirmation pursued whenever possible. This case series presents three distinct breast cancer cases in which ctDNA-based MRD detection was instrumental in identifying recurrence and guiding precision therapy based on actionable genomic alterations when biopsy was not feasible or inconclusive.