Human malignant glioma cell lines are sensitive to low radiation doses

Malignant gliomas display aggressive local behavior and are not cured by existing therapy. Some cell lines that are considered radioresistant respond to low radiation doses (<1 Gy) with increased cell killing (low-dose hypersensitivity). In our study, 4 of 5 human glioma cell lines exhibited significant X-ray sensitivity at doses below 1 Gy. The surviving fractions (SFs) obtained at 0.7 and/or 0.8 Gy were comparable to those at 1.5 Gy. Low-dose hypersensitivity was evident when irradiation was combined with etoposide treatment. Repeated irradiation with low doses was markedly more effective than irradiation with single, biologically equivalent doses in decreasing SFs, inhibiting xenograft tumor growth in mice. All experiments were conducted with an accelerator used in clinics, establishing that low-dose hypersensitivity was present following megavoltage X-irradiation. Thus, repeated low-dose irradiation (ultrafractionation) could greatly improve the effectiveness of radiotherapy of gliomas and could allow safe treatment of patients with cumulative doses greater than 60 Gy.