Messaritakis I, et al. - For this investigation, peripheral blood was obtained from 66 consecutive-patients before chemotherapy administration, after one-cycle and at relapse to examine the presence of Bcl-2⁺CTCs in chemotherapy-naïve SCLC patients and their clinical relevance during front-line treatment. A significant association was found between Bcl-2⁺/CD45-cells after one-treatment cycle and shorter OS. Based on the co-expression of Bcl-2, cytokeratins (CK), vimentin (Vim) and M30 in SCLC patients, these outcomes showed an important phenotypic CTCs heterogeneity. According to the findings obtained, the changes of Bcl-2⁺/CD45- CTCs during treatment appears to be a dynamic biomarker related to treatment efficacy and patients’ clinical outcome.