ON THE CLINICAL-SIGNIFICANCE OF THE S-PHASE FRACTIONS OF TUMORS

The S-phase fraction of a tumor cell population is often viewed as a g eneral indicator of the clinical aggressiveness of that tumor. Actuall y, the S-phase fraction of a cancer should be interpreted as no more t han an indicator of the mean duration of its mitotic cycle. The S-phas e fraction of a solid tumor is difficult to measure accurately, but in principle it is a powerful predictor of the duration of the recurrenc e-free interval. However, since the duration of the mitotic cycle does not correlate with malignancy, the S-phase fraction cannot be used to diagnose malignancy. It is of little value in making therapeutic deci sions because the duration of the mitotic cycle is not a predictor of invasion and metastasis.