Prognostic factors can be tumor related or patient related. Tumor-related factors reflect the type of the tumor, the extent of the disease, and the characteristics related to the tumor growth. These characteristics can be measured on the tumor itself or may be surrogate measures such as serum markers. Patient-related factors more often correspond to demographic characteristics or patient physiologic reserve, such as performance status. Generally speaking, tumor as well as patient-related factors are important for outcome although one may keep them separate, especially when they are used in the selection of treatment.

Usually, prognostic factors are grouped according to the point in time at which they are recorded.6 Their values should be known at the point from which prognosis (i.e., time to response or more generally time to event) is measured. Such prognostic factors for which a single value is available (measured at diagnosis, before the start of treatment) are called fixed covari-ates. Other prognostic factors may be available only during treatment (e.g., received dose intensity, acute toxicity), or after the treatment is completed, or the study is closed (e.g., response to treatment, time to response). These prognostic factors are called time-dependent covariates.5 Their use is generally a source of difficulties because time-dependent covariates may be affected by the treatment and such variables should never be used in statistical adjusted comparisons between treatments.

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