Quality control for evaluation of the S-phase fraction by flow cytometry: a multicentric study. The SICCAB Group for...

Flow cytometric measurement of the S-phase fraction (FCM-S) may be clinically useful in human cancers to identify high-risk patients. However, its clinical application requires a methodologic standardization, a high feasibility, and the reproducibility of results within the same and among different institutions. The interlaboratory quality control program for FCM-S determination (promoted by the Italian Society of Basic and Applied Cell Kinetics) was carried out on DNA diploid and aneuploid cell lines and human breast cancers. FCM-S was quantified by using four mathematical models characterized by different rationales and differences in the degree and shape of background subtraction functions. A satisfactory agreement in FCM-S values among different institutions was observed for cell lines. In human breast cancers, a high reproducibility of FCM-S estimates obtained by the different institutions was observed by using the same model. Conversely, different models gave different FCM-S estimates. Intermodel differences were more evident in aneuploid than in diploid tumors. These findings could explain the contrasting results on the prognostic role of FCM-S in clinically comparable series of breast cancers and should be considered in future efforts to define the optimal and most reproducible mathematical approach to quantify cells in the S-phase.