HETEROGENEITY IN THE FRACTIONATION SENSITIVITIES OF HUMAN TUMOR-CELL LINES - STUDIES IN A 3-DIMENSIONAL MODEL SYSTEM

Purpose: Current concepts to optimize the therapeutic gain of radiothe rapy by hyperfractionation assume that human tumors are less sensitive to fractionation than late reacting normal tissues. The aim of this s tudy was to investigate the extent of the intercell line heterogeneity of fractionation sensitivity of a wide variety of human tumor cell li nes in a three-dimensional model system under fully oxic conditions us ing schedules with one to eight fractions. Biological characteristics of the tumors that correlate with fractionation sensitivity should be identified. Methods and Materials: A total of 21 cell lines from human tumors maintained as multicellular spheroids consisting of 1000-1500 cells were given fractionated irradiation within a total treatment tim e of maximally 50 h. Complete dose-spheroid control curves were determ ined for each fractionation scheme. The spheroid control data were ade quately described by the linear quadratic model assuming Poisson stati stics. In addition, the induction of a G2 block by a fractionated test dose of seven 3 Gy fractions given at 6-h intervals was determined in spheroid cells using how cytometry of propidium bromide stained cell nuclei. Results: The fractionation sensitivities of human turner cells in multicellular spheroids could be characterized by alpha/beta value s, ranging from 2.8-37 Gy in dependence on the cell line. The log norm ally distributed alpha/beta values were positively correlated with the percentage increase in G2/M phase after the fractionated test dose co mpared to the controls (r = 0.72, p < 0.01), and were associated with the degree of tumor differentiation (p = 0.01, ANOVA F-test). No signi ficant correlation between the log (alpha/beta) values and the survivi ng fractions at 2 Gy (SF2) or the total doses with 2 Gy per fraction n ecessary to control 50% of the spheroids (SCD50) was observed. Despite the intercell line variability of the alpha/beta values, the SCD50 va lues of the different cell lines, given with one and eight fractions o r one fraction and 2 Gy per fraction, were closely associated (Spearma n rank correlation coefficients: r = 0.89 or r = 0.90, p < 0.0001). Co nclusion: Human tumor cell lines showed a marked heterogeneity in the fractionation sensitivity when irradiated as multicellular spheroids a nd assayed lit situ using the spheroid control end point. Therefore, t he therapeutic gain of altered fractionation also depends on those bio logical characteristics of each individual tumor that affects its frac tionation sensitivity. Parameters that correlate with fractionation se nsitivity of the tumor lines in the spheroid system were identified as grade of tumor differentiation and percentage increase in G2/M cells at the end of an eight-fraction schedule.