Applicability of the linear-quadratic model to hypofractionated radiotherapy.

Abstract

Background: The linear-quadratic (LQ) model is used for evaluating fractionation schedules in radiation oncology. It has been suggested that the biologically effective dose based on the LQ model cannot be applied to hypofractionated radiotherapy. Methods: Glioblastoma cells were transplanted into nude mice. Four equivalent dose fractionation schedules (12 Gy/1 fraction (fr). 15.06 Gy/2 fr, 18.16 Gy/4 fr, and 20.96 Gy/8 fr) were calculated based on the LQ model. The tumor size was measured to evaluate the number of days required for the relative tumor volume to double and become five t imes the initial value (TGD₂ and TGD₅, respectively). A histopathological study was performed after the evaluation of growth delay. Result: The mean TGD₂ of the unirradiated control group and 12 Gy/1 fr, 15.06 Gy/2 fr. 18.16/4 fr, and 20.96 Gy/8 fr groups was 5.69, 22.64, 37.26. 36.81, and 28.96 days, respectively. The mean TGD₅ was 17.83, 48.03. 60.30, 60.40, and 64.94 days, respectively. All TGDs of the irradiated groups were longer than those of the control group. However, no significant differences were observed among the four irradiated groups. Histologically, irregular giant cells were found, but there were no significant differences among the four groups. There was no significant difference in the Ki-67 labeling index and the CD133 and CD44 expression among the groups. Conclusion: The four dose fractionation schedules equivalent to 12 Gy/1 fr yielded comparable responses, suggesting that the LQ model may be applicable to hypofractionated radiotherapy, with a high dose per fraction of up to 12 Gy.