Stage confusion. Stage and staging tells you where the cancer is, what the prognosis is, and what the treatment might be with options. You do not change stage once treatment begins. The tumor either responds or doesn't. Complete repsonse (?Remission in leukenmia?) is the goal, shrinkage is ok. The idea is that as long as the cancer responds, you stay on that regimen...Maybe.
Response. Initial staging is used to plan treatment and gauge prognosis. After treatment is started the terms used to describe the progress include remission, complete response and partial response. You can go from stage 4 to no evidence of disease (very rare), you will be considered a stage 4 in complete remission.
Treatment response. When a patient with stage IV cancer who has received treatment and responded to the treatment and achieved some clinical improvement, this improvement will not change the initial stage to a lower stage. Merely, we call it as a treatment response- which could be divided as a partial response, major response or complete response/complete remission- which can happen in stage IV cancer.