Interpretation of changes in Circulating Tumor Cell counts

The presence of circulating tumor cells (CTC) in the blood of cancer patients may guide the use of therapy. We investigated how to evaluate a reduction in the number of CTC after administration of therapy. CTC were enumerated with the CellSearch system in 111 metastatic breast and 185 metastatic prostate cancer patients before start of a new line of chemotherapy and after initiation of therapy. Different means to express changes in CTC counts were evaluated with respect to overall survival (OS). A static CTC cut-off is the best method to determine whether a therapy is effective. This is exemplified by the highest Cox hazard ratio of 2.1 for OS; three methods to express relative differences performed worse. A look-up table is provided from which the significance of a change in CTC can be derived. Aim of therapy should be the elimination of all CTC. A period of 10-12 weeks of therapy is needed to reach the treatment effect on CTC