Radioresponsiveness of human glioma, sarcoma, and breast cancer spheroids depends on tumor differentiation.

PURPOSE: Differences in the intrinsic radiosensitivity within and between different tumor classes have been noticed for human tumor cell lines using the clonogenic assay. By far, most of the cell lines studied up to now were derived from poorly differentiated tumors. In this study, the influence of tumor differentiation on the radiation doses necessary to control 50% of small oxic spheroids (SCD50) was determined. Evidence of a distinct dependence of radioresponsiveness on tumor progression provides a background for an investigation of the underlying mechanisms. METHODS AND MATERIALS: Spheroids were aggregated from 1000-1500 cells in agarose coated 24 multi-well plates. Their diameters ranged from 156 to 405 microns, depending on the cell line. Spheroids were irradiated with graded 60Co single doses using spheroid control as end point and a minimum follow-up period of 3 months.
RESULTS: Cell lines from three low grade gliomas and 10 malignant gliomas were studied in the spheroid control assay. The group mean SCD50 values were 6.1 +/- 1.6 Gy and 13.1 +/- 3.3 Gy, respectively. Four cell lines from grade 2 soft tissue sarcomas had a mean SCD50 value of 6.2 +/- 0.5 Gy and one undifferentiated sarcoma line of 11.0 Gy. Three well-differentiated breast cancer lines expressed the cell adhesion molecule E-cadherin, had an epithelioid morphology in monolayer culture, were estrogen receptor positive or contact inhibited in multicellular spheroids. Two undifferentiated breast cancer lines had a fibroblastoid morphology and were marker negative. The mean SCD50 value of the former was 10.5 +/- 1.0 Gy while that of the undifferentiated lines was 14.8 +/- 2.8 Gy. Analysis of variance revealed a significant effect of the tumor type as well as the grade of dedifferentiation on the SCD50 after irradiation with one fraction or 2Gy/fraction. The surviving fractions at 2 Gy (SF2), obtained from the spheroid control rates after different fractionation schedules by approximation of the linear quadratic model assuming Poisson statistics were significantly dependent on tumor type (p = 0.001, ANOVA F-test) but not on tumor differentiation (p = 0.27). The alpha/beta ratios did not depend on tumor type (p = 0.08, ANOVA F-Test) but significantly increased with the grade of tumor cell dedifferentiation (p = 0.03).
CONCLUSION: The spheroid model is suitable for measuring the radioresponsiveness of differentiated cell lines with very low colony forming efficiencies. Tumor cell differentiation is an important factor for the radioresponsiveness and recovery capacity of human tumor cells.