ctDNA and immune biomarker testing for molecular residual disease surveillance and immune stratification in cancer

Molecular residual disease (MRD) refers to occult cancer persisting below the detection limit of conventional imaging and is a major source of later relapse. Circulating tumor DNA (ctDNA) provides a minimally invasive molecular surrogate of residual disease, but its interpretation is influenced by tumor burden, shedding, anatomic site, treatment-related cell death, sampling time, and analytical sensitivity. Immune biomarkers provide complementary information on whether residual disease is being recognized, contained, or permitted to escape by the host immune system. This Mini Review examines how ctDNA and immune biomarker testing can be integrated for MRD surveillance and immune stratification across solid tumors. We summarize tumor-informed and tumor-naïve ctDNA approaches, including mutation-, methylation-, fragmentomic-, and whole-genome-based assays; discuss tissue and blood immune biomarkers such as PD-L1, mismatch repair status, tumor mutational burden, antigen-presentation integrity, T-cell receptor repertoires, cytokines, and suppressive myeloid populations; and review recent evidence from colorectal, lung, urothelial, breast, and melanoma studies. We propose a conceptual, hypothesis-generating framework in which ctDNA reports tumor-derived molecular signal and shedding, whereas immune assays report immune pressure and escape. Joint interpretation may, if prospectively validated, distinguish biologically different MRD states, improve longitudinal risk assessment, and provide a laboratory foundation for biomarker-oriented immunotherapy and cell-therapy studies. However, clinical implementation requires standardized preanalytics, explicit assay limits, paired leukocyte controls, serial sampling, harmonized immune assays, and prospective trials demonstrating clinical utility rather than prognostic association alone.