Effect of Dose-Per-Pulse and Pulse Repetition Frequency in X-Ray Beam Irradiation of In-Vitro Cell Cultures
G Lasio1*, M Guerrero1, W Goetz2, F Lima2, J Baulch2, (1) Division of Medical Physics and (2) Division of Radiobiology, University of Maryland School of Medicine, Department of Radiation Oncology, Baltimore, MDSU-D-137-6 Sunday 2:05PM - 3:00PM Room: 137
Purpose: It has been reported that flattening-filter-free (FFF) beams may have a different biological effect than flattened beams, possibly due to the increased dose- per-pulse in FFF beams. The purpose of this work is to determine if in-vitro cell survival is affected by the higher dose-per-pulse and/or repetition rates generated by flattening-filter-free beams in comparison to flattened beams. Methods: three cell lines, T98G (p53 mutant human glioblastoma), U87-MG (p53 wild type human glioblastoma) and V79-4 (Chinese hamster lung fibroblast) were irradiated to 5 Gy and 10 Gy with Linac X-ray photon energies of 6 MV, 6 MV FFF and 10 MV FFF, using dose rates of 400, 1400 and 2400 MU/min. The relative variation of the average dose-per-pulse among the beams was measured as 1.0: 2.8: 4.6 for 6X: 6XFFF: 10XFFF. The dose delivered to each sample was verified with a parallel-plate ionization chamber. Gafchromic film measurements were made to confirm the dosimetry methodology. Clonogenic cell survival assays were used to evaluate biological effect of each beam. Three or more biological replicates were performed for each experiment. Results: For T98G cells, surviving fractions at 5 Gy for 6MV at 400 MU/min, 6MVFFF for 400MU/min and 1400MU/min, 10XFFF 400MU/min and 2400MU/min, were 0.59(0.09), 0.53(0.07), 0.55(0.08), 0.58(0.08), and 0.60(0.07) respectively. For U87-MG also at 5 Gy surviving fractions were 0.27(0.02), 0.29(0.05), 0.29(0.05), 0.32(0.05), and 0.31(0.06). For V79-4 survival at 5 Gy was 0.59(0.12), 0.56(0.06), 0.50(0.08), 0.51(0.03), and 0.54(0.04). Increased cell killing was observed using 10 Gy doses but no difference in biological effect among beams was observed. Conclusion: No effects due to dose-per-pulse or pulse repetition frequency variation were observed on cell killing at either 5 Gy or 10 Gy for any of the cell lines tested.
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