title: RBE of 50 MV scanned bremsstrahlung beams determined using clonogenic assay.
abstract: OBJECTIVE
  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.
  
  
  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.
authors.affiliation: Medical Radiation Physics, Department of Oncology-Pathology, Karolinska Institutet and Stockholm University, Box 260, SE 17176, Stockholm, Sweden. linda@radfys.ks.se
authors.forename: L M
authors.initials: LM
authors.lastname: Persson
authors.forename: B K
authors.initials: BK
authors.lastname: Lind
authors.forename: M R
authors.initials: MR
authors.lastname: Edgren
authors.forename: I
authors.initials: I
authors.lastname: Hedlöf
authors.forename: A
authors.initials: A
authors.lastname: Brahme
mesh_terms: D000818:Animals; D018288:Carcinoma, Small Cell / Q000532:radiotherapy; D002460:Cell Line; D002470:Cell Survival / Q000528:radiation effects*; D003037:Cobalt Radioisotopes; D003114:Colony-Forming Units Assay; D006224:Cricetinae; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy; D008954:Models, Biological; D019047:Phantoms, Imaging; D017785:Photons; D011882:Radiotherapy, High-Energy; D012062:Relative Biological Effectiveness; D014407:Tumor Cells, Cultured
chemical_list: D003037:Cobalt Radioisotopes