This may be from retrospective reports of a doctor's experience or from a well-designed clinical trial. The more rigorous the study design the more accurate are the predictions. Yet, it must be recognized that these are statistical estimates that are good guidelines but not exact projections. Nonetheless, the projections are useful for comparing treatments and estimating prognosis. Finally, as time goes on, results of treatment tend to get better due to earlier diagnosis, better therapy and improved supportive care.
Doctors often ask their patients with cancer to participate in clinical trials.
A properly designed clinical trial, one that is rigorously reviewed by experts in the field and by an institutional review board composed of professionals and lay people, will explain the risks and benefits of participation. So the most rigorously designed studies tend to be those done in National Cancer Institute Cancer Centers, university hospitals or through the national cooperative groups. One must also recognize that there are an increasing number of trials supported by the pharmaceutical industry which involve assessing drug toxicity and efficacy. A proper study will have an informed consent document specific for the study. This is usually in addition to a routine hospital or clinic consent for treatment.