Let me do more than count the ways: what circulating tumor cells can tell us about the biology of cancer

Tumor cells in the circulation of patients with advanced cancers have been described for over a century, but only recently have methods become available to reproducibly and robustly detect these cells in patients with cancer. A variety of methods have been developed to study this phenomenon, reflecting a broad interest in the field. The presence of circulating tumor cells (CTCs) in the peripheral blood of patients with metastatic cancer has been found to be of prognostic significance, and changes in CTC numbers over time appear to reflect treatment outcome. The ability to detect and study CTCs suggests that CTC concentration in blood may be able to be used as an intermediate biomarker in therapeutic trials of novel therapies in cancer patients and that molecular changes in patients' tumors may be able to be detected and addressed with appropriate therapeutic interventions. Studies in patients with early, nonmetastatic cancers are beginning, and some studies indicate that circulating tumor cells can predict outcome in this setting. While the ability to count and characterize circulating tumor cells holds much potential for the future, improvements in and standardization of assay methods need to be made before the potential of this technology is fully realized.