Prognostic and treatment predictive factors - is there a difference?

The terms risk factor, prognostic factor and predictive factor are often im precisely used. We defined a prognostic factor as a patient characteristic that identifies subgroups of untreated patients having different outcomes, and a factor predictive of treatment effect as a patient characteristic tha t identifies subgroups of treated patients having different (as a consequen ce of treatment) outcomes. To illustrate this, theoretical graphical exampl es were constructed and data from the literature on prostate cancer were us ed to substantiate the theoretical examples. In most situations, but not ne cessarily always, a prognostic factor is also predictive of the effect of s pecific treatments. Whether a prognostic factor is also predictive of treat ment effect or not can only be assessed in a valid comparative setting such as in a randomized trial. A factor that is predictive for treatment effect may not be predictive for another treatment.