This sqadia.com short video clip is a brief explanation of Tumourigenesis.
Tumourigenesis is a multistage process that results from accumulated mutation. Tumours arise from single cell, which proliferate to form a clone of cells with identical abnormalities. As tumours develop, they undergo further somatic mutations, cell with loss of suppressor gene develops, cell with rapid division develops, genetically different from each other, but which are part of the same tumour: this is heterogeneity.
Chemotherapy will kill the majority of tumour cells. However, tumour cells resistant to chemotherapy will survive and be selected for resulting in the regrowth of a tumour that is resistant to chemotherapy