Circulating tumor cells (CTC)s represent the point in the metastatic process of solid tumors when cells from a primary tumor invade, detach, disseminate, colonize, and proliferate in a distant site. Detection of elevated CTCs during therapy may be an accurate indication of subsequent rapid disease progression and mortality in breast, colorectal, and prostate cancer, noting that FDA labeling includes each of these neoplasms. Although some comparative cohort designs have been conducted to express the clinical utility of such testing, the vast majority of studies have been uncontrolled one-arm studies. As an illustration of the most sophisticated available evidence, Cristofanilli et al. (2004) and Tol et al. (2010) have reported two-armed studies in the prognostic/predictive assessment of metastatic breast and colorectal cancers, respectively. Two-armed prostate cancer studies have been performed, but are not available for review at this time, given their abstract-only availability. Paoletti et al. (2012) help to articulate such a current clinical niche as follows:
“At present, the most feasible use of CTCs is monitoring of patients with metastatic disease. Although technically “prognostic,” elevated CTC levels over time in a patient receiving therapy are essentially predictive of resistance to that therapy and suggest that a new therapy, if available, is indicated.”
There is limited coverage for the CellSearch® CTC (Veridex, LLC) assay. CTC testing is best reserved for the setting of a clinical trial where chemotherapy selection is more systematically subjected to revision.
The CTC assay is reported as a numerical result where five or more cells per 7.5 mL of whole blood predicts worse prognosis in individuals with known recurrent breast and prostate cancer, and three or more cells are predictive of shorter progression-free survival and overall survival in metastatic colorectal cancer.