Reproducibility of measurements of potential doubling time of tumor cells in the multicenter NCI protocol T92-0045

Summary We compared the flow cytometric measurement and analysis of the potential doubling time ( Tpot) between three centres involved in
the National Cancer Institute (NCI) protocol T92-0045. The primary purpose was to understand and minimize the variation within the
measurement. A total of 102 specimens were selected at random from patients entered into the trial. Samples were prepared, stained, run
and analysed in each centre and a single set of data analysed by all three centres. Analysis of the disc data set revealed that the
measurement of labelling index (LI) was robust and reproducible. The estimation of duration of S-phase ( Ts) was subject to errors of profile
interpretation, particularly DNA ploidy status, and analysis. The LI dominated the variation in Tpot such that the level of final agreement, after
removal of outliers and ploidy agreement, reached correlation coefficients of 0.9. The sample data showed poor agreement within each of the
components of the measurement. There was some improvement when ploidy was in agreement, but correlation coefficients failed to exceed
values of 0.5 for Tpot. The data suggest that observer-associated analysis of Ts and tissue processing and tumour heterogeneity were the
major causes of variability in the Tpot measurement. The first two aspects can be standardized and minimized, but heterogeneity will remain a
problem with biopsy techniques.