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 general indicator of the clinical aggressiveness of that tumor. Actually, the S-phase fraction of a cancer should be interpreted as no more than an indicator of the mean duration of its mitotic cycle. The S-phase fraction of a solid tumor is difficult to measure accurately, but in principle it is a powerful predictor of the duration of the recurrence-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 decisions because the duration of the mitotic cycle is not a predictor of invasion and metastasis.