During chemotherapy or other treatment, the doctors will be watching the size of the tumor as it appears during scans, which could be MRIs or CT scans, depending on where the tumor is and how easy it is to see. A complete response would mean not being able to see any tumor, but that rarely happens.
If you are going through chemotherapy and your tumor is not shrinking but also is not growing, your doctor will probably have you complete all the planned cycles of chemotherapy and reevaluate you at the end. If there is still no change, he will probably recommend a different chemotherapy regimen. While “no response” means that your tumor is not getting smaller, it also means it is not getting bigger.
If your tumor is growing at any stage, the treatment will be stopped and alternatives considered.