Researchers unveiled methylation-based circulating tumor DNA (ctDNA) monitoring designed to track metastatic breast cancer disease progression and therapeutic response during treatment with CDK4/6 inhibitors. The approach uses methylation patterns in ctDNA as a readout to capture tumor dynamics over time. The report positions the method as a more precise companion to ongoing treatment response monitoring, supporting individualized oncology decision-making. The study highlights ctDNA as a potential mechanism-level biomarker rather than relying solely on imaging and clinical endpoints. If validated across larger cohorts, methylation ctDNA could become a stronger option for longitudinal management in CDK4/6-treated patients, particularly where standard response metrics arrive late or are ambiguous.