question, cancer
Because all Circulating Tumor Cells (CTC's) do not cause cancer or metastasis. Cancer cells can be in various phases of cell cycle called G, S, M etc. If the cancer cell is in resting phase called G0, it is very difficult to eliminate it. These resting cells may or may not get activated in the future. That is the reason why recurrences happen or do not happen!! Apart from Surgery as a treatment of cancer, Chemotherapy, Radiotherapy, Targeted therapy (molecular therapy) all rely on active cancer cell or a Chat Doctor. Resistance to treatment is a topic for another day! Hope the above description answers your question.