Intrinsic radiation sensitivity may not be the major determinant of the poor clinical outcome of glioblastoma multiforme.

PURPOSE: Many radiobiologic mechanisms may contribute to the clinical radiation resistance of Glioblastoma Multiforme. One of them is considered to be an unusually low intrinsic radiation sensitivity. This is a collaborative study between three laboratories to evaluate the intrinsic radiation sensitivity of 85 cell lines derived from human malignant gliomas as the major cause of the poor clinical results of radiation treatment to these tumors. METHODS AND MATERIALS: Fifty-one cell lines were early passage. The distribution by histologic type was: 58 glioblastoma, 17 anaplastic astrocytoma, six oligodendroglioma and four astrocytoma grade 2. The intrinsic radiation sensitivity will be expressed by the surviving fraction at 2 Gy (SF2). The SF2 has been determined for single dose irradiation for cell lines on exponential phase, under aerobic conditions, growing on plastic. The patient age, Karnofski Status, histological grade, survival, dose of irradiation for 50 patients are investigated for correlation with SF2 of the corresponding newly established cell lines.
RESULTS: The mean SF2 of the 85 cell lines was 0.46 (0.12-0.87). The mean SF2 by histologic type was 0.50, 0.34, 0.54 and 0.38 for glioblastoma, anaplastic astrocytoma, oligodendroglioma and astrocytoma grade 2 cell lines, respectively. No correlation was found between SF2 and the patient age or Karnofski status. The difference in SF2 between the 58 glioblastoma and 17 anaplastic astrocytoma cell lines was significant p = 0.002. The difference in actuarial survival between glioblastoma and anaplastic astrocytoma patients was borderline of significance (p = 0.08). The difference in SF2 of cell lines derived from these two groups of patients was of borderline significance (p = 0.08). The difference in radiation sensitivity for anaplastic astrocytoma and glioblastoma cell lines was clearly reflected in the difference in survival for the two groups of patients from where the cell lines were derived. However, no correlation was found between SF2 and survival within each grade. In a multivariate analysis the age, grade and Karnofski status were found to be significant prognostic values for survival with a p values of 0.032, 0.03 and 0.038, respectively, however, the ln SF2 was not significant (p = 0.40). The mean SF2 of the 6 oligodendroglioma cell lines (0.54) was comparable to that of glioblastoma multiforme (0.50). The high SF2 for oligodendroglioma does not accord with the much better clinical outcome of these tumors.
CONCLUSIONS: These data on 85 malignant glioma cell lines show a very broad distribution of SF2 values for irradiation in vitro. SF2 reflected the difference in sensitivity between AA (Grade 3) and GBM (Grade 4). This may suggest that the parameter SF2 is useful to discriminate between the sensitivity of different grades or types of histology in vitro. However, SF2 was not a predictor of the clinical outcome on individual basis for malignant gliomas. The in vitro studies will need to be supplemented by physiologic characterization of the tumors in vivo. Such conclusions would limit the predictive value of current radiation sensitivity assays based on in vitro dose-survival measurement for at least high grade malignant gliomas.