Circulating Tumor Function: A Systems Biology Framework for Liquid Biopsy in Genitourinary Cancers

Liquid biopsy enables minimally invasive detection and longitudinal monitoring of tumor-derived material in blood and urine. In genitourinary cancers, most applications have focused on genomic alterations in circulating tumor DNA (ctDNA), together with circulating tumor cells (CTCs), extracellular vesicles (EVs), and cell-free RNAs. These measurements are clinically informative but are often interpreted as isolated, predominantly descriptive biomarkers and therefore incompletely represent the adaptive processes that determine progression and treatment response. We propose circulating tumor function (CTF) as a systems biology framework for integrating tumor-derived and host-derived genomic, regulatory, metabolic, redox, and immune signals obtained through serial liquid biopsy. CTF is not a single analyte or assay; rather, it is an inference model intended to generate interpretable functional states, including proliferative activity, immune evasion, metastatic potential, metabolic stress, and therapeutic adaptation. We review the contributions and limitations of ctDNA, ncRNA networks, EV-mediated signaling, redox biomarkers, and tumor-host crosstalk in prostate, bladder, renal, and testicular cancers. We also outline the analytical and clinical validation required to determine whether integrated CTF models provide incremental value over established single-analyte approaches. This framework may help reposition liquid biopsy from molecular detection toward functional precision oncology.