Effects of radiation on a three-dimensional model of malignant glioma invasion.

An experimental model of malignant glioma growth involving implantation of spheroids into a gel matrix of collagen type I has been developed. This model has been used to characterize changes in glioma cell invasion in response to single dose and fractionated radiation treatment. Suspensions of C6 astrocytoma cells were grown in spinner culture flasks to yield spheroids of varying size (300-1000 microm). Implantation of spheroids into a gel matrix of collagen type I was associated with measurable invasion of the surrounding gel by individual tumor cells. Changes in the distance of invasion in response to single dose and fractionated radiation were measured. Changes in apoptosis and proliferative indices in different regions of the spheroids in response to radiation were also assessed. In unirradiated gels, maximum depth of invasion, 1300-1750 microm, was achieved by 5 days after implantation. A radiation dose-dependent inhibition of invasion was noted and was most profound for larger spheroids. Fractionation of the radiation dose was associated with a partial recovery of invasion. Changes in apoptotic and proliferative indices in response to radiation depended on the region of the spheroid examined. Increases in apoptosis were noted for cells at the surface of the spheroid and invading cells while cells at the centre of the spheroid demonstrated virtually no increase in apoptosis. Likewise, a dose-dependent decrease in proliferative indices following radiation was noted among the invading cells and cells at the surface of the spheroid but not at the centre of the spheroid. We have described a model of malignant glioma invasion which possesses many of the qualities of in vivo malignant gliomas. Within this model, invasion appeared to be inhibited by radiation in a dose- and fractionation-dependent fashion. Measurement of apoptotic and cell proliferation indices favour a direct cytotoxic effect on the invading cells as the most likely mechanism for this phenomenon.