Prognostic and predictive value of circulating tumor cell (CTC) counts in metastatic castration-resistant prostate cancer (mCRPC). (a) Survival curves (Kaplan-Meier) of a group of 276 patients with mCRPC who were about to start a new line of chemotherapy, showing how patients with <5 CTCs per 7.5 ml blood (‘favorable’ CTC count; green line) had a better prognosis than the group of patients with higher counts (red line). (b) Differential survival curves representing outcomes for patients according to the change in their CTC counts after receiving treatment, which show that patients with a conversion from an ‘unfavorable’ to a ‘favorable’ count during the response to treatment (blue line) experience longer survival than those without a CTC drop (red and orange lines). The green line is the survival curves for those patients who have low CTC counts at the beginning to that study and whose counts remained low throughout the study. OS, overall survival; HR, Haza-ratio; CI, confidence interval. Reprinted from  de Bono JS et al. Clin Cancer Res 2008, 14:6302-6309. (Copyright by American Association for Cancer Research).