Autoradiographic Analysis of Cell Proliferation in Spontaneous Breast Cancer of C3H Mouse. III. The Growth Fraction2

Abstract

If tumors consist of mixtures of non-growing cells plus cells proliferating at a uniform rate, it is possible to estimate the growth fraction (proliferating cells/total cells) by an autoradiographic method. The method is based on the labeling of a random sample of proliferating cells and requires the parenteral injection of tritiated thymidine several cell generations before the fixation of the tumors. The incidence of labeling in the total population is then determined as labeled cells/100 cells. The corresponding incidence in the proliferating population is given by labeled mitoses/100 mitoses. The ratio of these indexes reduces to a time-dependent estimate of growth fraction. Primary tumors from C3H mice given injections 5 days before killing gave a mean of 0.61 for this estimate. Based on similar data from serially removed autotransplanted tumors, or on doubling times of 1 week for tumor volume, 0.61 at 5 days is equivalent to a growth fraction of 0.40 at the outset. Within the limits of the model, the results suggest that only 40 percent of these tumor cells are actively involved in the mitotic cycle at any one time. Other possible models of tumor growth were considered in order to evaluate the effects of variation in growth rate. In those cases in which the duration of either mitosis or deoxyribonucleic acid (DNA) synthesis are proportional to generation times, the growth-fraction method remains valid in spite of variations in generation time. On the other hand, when the durations of mitosis and DNA synthesis are relatively constant but generation times vary extensively, there may be an aberrant reduction in the measured growth fraction. Models for several types of variation were examined, and hypothetical growth fractions were calculated for several published distributions of cell generation times.