Implications of thymidine labeling index in the growth kinetics of human solid tumors

The biological implication of the thymidine labeling index (LI) in the growth kinetics of human solid tumors were investigated by regression analysis of paired individual data in single tumors from the literature. The results indicate that LI correlated best with the corresponding growth fraction (GF) or ratio of duration of the S phase to the median cell cycle time (Tc). The LI also significantly correlated, but to a lesser degree, with Tc, the volume doubling time (TD) and/or the duration of the G1 phase (Tg1), and the correlation coefficients decreased in that order. LI did ot correlate with Ts, duration of the G2 phase or cell loss factor (CL). Although the regression coefficients and constants in the equations are significantly different in tumors of different histological types, the difference seem to be small. Further, the LI highly correlated with the calculated cell loss rate in terms of the number of cells lost per 109 cells per day, although it did not correlate with CL. This trend was seen not only with individual tumors, but also among tumors of various histological types. It is thus likely that individual tumors with higher LI tend to have higher GF, shorter Tg1 and Tc, and therefore shorter TD, which results in higher rates of cell production and cell loss.