RBE of 50 MV scanned bremsstrahlung beams determined using clonogenic assay

Abstract

Purpose : In order to compare the biological effectiveness of a 50 MV scanned bremsstrahlung beam to 60 Co and 6 MV photons, the survival of Chinese hamster cells (V79-379A), human normal fibroblasts cells (GSH +/+) and human small cell lung cancer cells (U-1690) were analysed. Materials and methods : Cells were irradiated in vitro under aerobic conditions in a plastic phantom. Dose to the cells was determined using ferrous sulphate and ionization chamber dosimetry. A number of cell survival models were fitted to the experimental data, including the standard LQ model with and without the induced repair. In particular, a new model treating damage and repair separately was used in combination with a new technique for accurate RBE determination. Results : The measured RBE for the three cell lines were 0.988 (0.984-0.992), 0.999 (0.996-1.002) and 1.013 (1.009-1.016) for V79-379A, GSH +/+ and U-1690 respectively and thus 50 MV scanned beams did not differ more than a fraction of a per cent from conventional therapy beams. Conclusions : The present study gives RBE consistent with previously calculated RBEs based on photonuclear reaction cross-sections of high-energy photons.