Monitoring response to therapy in patients with cancer: is circulating DNA the answer?

One of the most frequent uses of tumor markers is in monitoring response to therapy in patients with advanced malignancy. Although serial imaging is the gold standard method for assessing response, the use of circulating markers has several advantages including non-invasive measurement, relatively low costs and convenience for patients. Proposed markers for monitoring therapy in patients with breast cancer include CA 15-3, CEA, TPA, TPS and circulating tumor cells (CTC) (1,2). Although the best marker for monitoring remains to be determined, CA 15-3 is the most widely used in clinical practice.