Introduction
Cancer is a highly complex disease that is characterized by evolving pathophysiological processes [1]. Hence, treatment decisions are typically made according to the manifestation and progression of the primary tumor [2]. Currently, tissue biopsy technology, which excises or punctures diseased tissue from the patient for pathological examination, is considered the “gold standard” for cancer diagnosis and treatment [3]. Nevertheless, as various tumor entities originate from internal organs, acquiring diseased tissue can be highly risky and challenging. Furthermore, clones differentiate into various subpopulations during the biological process of tumorigenesis, resulting in tumor heterogeneity [4,5]. This diversity and plasticity cannot be detected in single-tissue biopsy samples owing to their spatial and temporal heterogeneity, making it difficult to acquire comprehensive information on the genetic and epigenetic variability of cancer [6].
Compared to tissue biopsy, liquid biopsy (LB) is a minimally invasive diagnostic tool that is less wound-inducing and can be conducted frequently, allowing longitudinal monitoring of the dynamic changes in tumors and thereby its malignancy [7]. Thus, a straightforward assessment of tumor metastases and the biochemical changes during blood-borne cancer dissemination becomes possible by cell or molecular monitoring. In contrast to tissue biopsy, LB mainly involves analyzing circulating tumor cells (CTCs), circulating cell-free nucleic acids, and exosomes from the peripheral blood of patients with cancer (Fig. 1) [8,9]. CTCs are cancer cells that actively or passively escape from the primary tumor site to the bloodstream and are regarded as the culprits of cancer metastasis. CTC analysis has the potential to provide disease-related information on tumor composition, invasiveness, drug susceptibility, and treatment resistance [10,11]. Circulating cell-free nucleic acids, which mainly comprise circulating cell-free DNA (cfDNA), are typically discharged from apoptotic and necrotic tumor cells into the bloodstream [12]. Most cfDNA originates from non-malignant cells, whereas circulating tumor DNA (ctDNA) derived from tumor cells typically accounts for 0.1–10% of the total cfDNA, depending on the tumor burden, inflammatory status, cellular turnover, and accessibility of cancer cells to the blood vessels [13]. Therefore, cell-free nucleic acid analysis can provide detailed information relevant to cancer diagnosis and therapy [14]. Exosomes are extracellular vesicles secreted by healthy and tumor cells, containing proteins, RNA, and DNA from patients’ tumors [15]. Thus, tumor-derived exosomes can indicate the nature and status of tumor entities, as they contain all cellular contents originating from their tumor cells.