Harnessing minimal residual disease as a treatment decision and monitoring in solid tumor

Minimal residual disease (MRD) is an emerging biomarker that reflects residual tumor burden following therapy and plays a significant role in predicting relapse in patients with solid tumors. This report presents two cases reports demonstrating the clinical utility of MRD in guiding post-treatment decisions and monitoring disease progression. In the first case, a 54-year-old male with stage T2N0M0 sigmoid colon cancer underwent hemicolectomy without adjuvant chemotherapy but later developed lung metastases after seven years. Although PET/CT showed a complete metabolic response to systemic chemotherapy, persistent MRD prompted continuation of treatment until molecular remission was achieved. The second case involved a 57-year-old woman with HR-positive, HER2-negative breast cancer who showed a complete response after neoadjuvant chemotherapy and surgery, yet MRD remained detectable through circulating tumor DNA (ctDNA) analysis. Subsequent targeted hormonal therapy led to MRD clearance. These cases underscore that, even in the presence of favorable clinical and radiological outcomes, MRD evaluation provides critical insights into occult disease persistence and may warrant treatment continuation or intensification. Integrating MRD into the post-treatment evaluation strategy enables real-time, individualized therapeutic decisions that minimize relapse risk and reduce unnecessary treatment burden. The use of MRD as a sensitive predictive biomarker marks a paradigm shift toward more precise and dynamic cancer management in solid tumors.