Effects of fractionated radiation therapy on human brain tumor multicellular spheroids

We investigated the cytotoxic effects of fractionated radiation therapy on multicellular spheroids of human malignant glioma cell lines U-87 MG, U-251 MG, and U-373 MG. Graded doses of x-rays were administered in 1, 3, 8, 15, and 30 fractions over 15 days. The isoeffect dose for a 1 log cell kill ranged from 4-4.5 Gy for a single fraction to 7-8 Gy for an 8-fraction protocol; no additional dose-sparing was achieved with more fractions. Therefore, the effects of individual doses (1.56 Gy) of the 8-fraction protocol were studied in U-251 MG spheroids. A cell survival assay showed that the first dose of radiation killed 30-50% of the cells; subsequent doses usually killed fewer cells. The cell kill after all 8 doses was about 1.0 log. No consistent relationship between the intracellular glutathione level and fraction number was observed. The 24-hour labeling index of the spheroids did not decrease until after the second fraction. Thus, the higher cell kill of the first dose does not seem to be related to cell cycle synchrony. Multinuclear and mononuclear giant cells were limited almost entirely to the periphery of the spheroids and increased with the number of radiation fractions. We conclude that multicellular spheroids can be used to study the biological effects of fractionated radiation therapy on human brain tumor cells. Although this model cannot be used to evaluate the effect of radiation on normal tissue, it may be useful in developing more effective radiation therapy protocols for human brain tumors.