Lumps of cancer at not homogenous. The majority of cells in that cancer are incapable of mitosis (cell division) or are only capable of dividing several times. So in theory you don’t even need to destroy them as they will die by themselves. They are largely immaterial to the process of metastasis. Metastatic cancers are spreading aggressively attacking the body on multiple fronts.
But 0.1% to 1% of that cancer depending on the type of cancer are cancer stem cells. These are the cause of the cancer.
There are also benign fast-dividing cells.
Small cells called circulating tumour cells (CTCs) are likely to be critical components of the process that triggers metastasis. Once they penetrate blood vessels or lymph nodes they travel throughout the body potentially creating new cancerous tumours at different locations. CTCs are scarce in the bloodstream – perhaps one tumour cell in 10 million normal lymphocytes.
Once most cancers have been detected they’ve already spread elsewhere.
Chemotherapy procedures work by killing of rapidly dividing cells stemming malignant uncontrolled growth. In many scenarios the cause of the cancer the cancer stem cells remain intact. They are very resistant to both radiotherapy and chemotherapy.