Why chemotherapy unable to cure stage 4 cancer even if the tumor is cleared?

Chemo limitation: Cancer cells or micrometastasis are unique cells which can continue to function and grow despite exposure to chemotherapy and radiation. Their dna adapts and finds new pathways for cell survival and proliferation. Sometimes it relies on the cells of the microenvironment to grow and flourish. We are getting better in understanding these pathways and finding targeted therapies to treat cancers.

Many tumors are: Quite responsive to systemic therapy, and even disappear. Unfortunately, there are residual resistant cells that lurk beneath detection, and they re-emerge with time. Small cell lung cancer is the classic tumor that acts this way. So, the answer is resistant clones, not able to be eradicated with current therapy.