Fig. 1.

Events in the life trajectory of a tumor cell.

Transformation of a normal epithelial cell to a tumor cell followed by stages of tumor initiation, progression, malignancy, invasion and systemic spread are multi-step processes involving multiple cell types following the clonal evolution as well as CSC hierarchy model. The dynamic transition between these cycling/dormant and drug resistant/sensitive CSC phenotypes determines their survival in circulatory system and subsequent extravasation into congenial secondary organs or sites of inflammation. The emergence of secondary tumors or recurrences at new sites post metastatic events, marks the revival of an entire initiation phase and an unending journey of a tumorigenic cancer cell.