Fig. 2

Chemoresistance in SCLC. A the duration of a treatment-free interval (TFI) has been recognized as a caliber to identify patients proper for subsequent chemotherapies: those with a TFI shorter than 2 months are most likely to be refractory to salvage second-line chemotherapy with worse outcome. For patients undergoing relapse with more than 2 months after the first-line treatment are considered sensitive relapse, who tend to be responsive to subsequent chemotherapies. B Factors that may contribute to chemoresistance in SCLC, which include MDR proteins, DNA repair, heterogeneity, aberrant metabolism and cancer stem cells