Purpose: Human glioblastoma (GBM) is one of the most resistant tumors to radiation. In previous reports, we have demonstrated a wide range of radiation sensitivity of GBM in vitro; that is, SF2 values of 0.2 to 0.8. The great sensitivity of some of the cell lines is not in accord with the almost invariably fatal clinical outcome of patients with GBM. The sensitivity of cells in vitro pertains to cells cultured in optimal nutritional conditions. The TCD50 (the radiation dose necessary to control 50% of the tumors locally) determined in lab animals is analogous to the use of radiation with curative intent in clinical radiation oncology. The aim of the present study was (a) to evaluate the sensitivity of GBM in vivo relative to that of other tumor types and (b) assess the relationship between the single dose TCD50 of the xenografts and the sensitivity of the corresponding cell lines in vitro.
Methods and materials: The TCD50 assay was used to study twelve human tumor lines. Four previously published values were added. A total of 10 GBM, 4 squamous cell carcinoma (SCC), 1 soft tissue sarcoma (STS), and 1 cancer colon (CC) are included in the analysis. For further suppression of the residual immune system, all the animals received 6 Gy whole-body irradiation 1 day before transplantation. Local tumor irradiations were given as a single dose, under conditions of clamp hypoxia using a Cs irradiator.
Results: The TCD50 values for the 10 GBM xenografts varied between 32.5 and 75.2 Gy, with an average of 47.2 +/- 13.1 Gy. The TCD50 values for the SCC were similar to those of the GBM and ranged from 40.7 and 54.4 Gy, with a mean of 46.8 +/- 6.4. The difference between the average TCD50 of GBM and SCC was not significant. The STS and CC xenografts had TCD50 values of 46.0 and 49.2 Gy, respectively. No correlation was found between the TCD50 in vivo and the SF2 or D0 in vitro.
Conclusions: Our data on GBM xenografts showed a wide range of sensitivities to single dose irradiation in vivo, which does not correlate with the almost invariably fatal clinical outcome of these patients. No correlation was observed between the TCD50 in vivo and the in vitro SF2/D0 of the corresponding cell lines. Our in vivo and in vitro data on GBM suggest that radiation sensitivity alone does not explain the cause of the poor clinical response of GBM to radiation, and other factors could contribute to this response.