Not a doctor, just a colon cancer survivor, so from my experience, oncologists use CT (and other) scans and a CEA (blood) test to monitor effectiveness of the chemotherapy. There is also a blood test called Signatera (I've not had that one.)

To simplify, the scans show if tumors are growing/shrinking (the therapy is not working/is working), the CEA/Signatera indicates the activity (aggressiveness) of the cancer.

My understanding, which could be wrong, is that oncologists want you to complete a full course of treatment (when your body can handle it) then reevaluate if another therapy or doing another course of treatment is necessary.

Stopping in the middle of a course seems unusual unless there are problems tolerating it, it is found to be ineffective or the cancer has spread elsewhere.

Let's see what others have to say.