Metastatic relapse in patients with solid tumors is the consequence of cancer cells that disseminated to distant sites, adapted to the new microenvironment, and escaped systemic adjuvant therapy. There is increasing evidence that hematogeneous dissemination starts at an early stage of cancer progression with single tumor cells or cell clusters leaving the primary site and entering the blood circulation. These circulating tumor cells (CTCs) can extravasate into secondary tissues where they become disseminated tumor cells (DTCs). Patients might relapse years after initial resection of the primary tumor when DTCs become overt metastases. Current diagnostic strategies for stratification of therapies against metastatic cells focus on the primary tumor tissue. This approach is based on the availability of stored primary tumors obtained at primary surgery, but it ignores that the DTCs might have evolved over years, which can affect the antimetastatic drug response. However, taking biopsies from metastatic tissues is an invasive procedure, and multiple metastases located at different sites in an individual patient show marked genomic heterogeneity. Thus, capturing CTCs from the peripheral blood as a "liquid biopsy" has obvious advantages in particular when repeated sampling is required for monitoring therapies in cancer patients. However, the biology behind tumor cell dissemination and its contribution to metastatic progression in cancer patients is still subject to controversial discussions. This manuscript reviews current theories on the genetic traits behind the spread of CTCs and progression of DTCs into overt metastases.
Keywords: Cancer; Circulating tumor cells; Disseminated tumor cells; Genetic progression; Metastasis.