title: RBE variation as a function of depth in the 200-MeV proton beam produced at the National Accelerator Centre in Faure (South Africa).
abstract: OBJECTIVE
  Thorough knowledge of the RBE of clinical proton beams is indispensable for exploiting their full ballistic advantage. Therefore, the RBE of the 200-MeV clinical proton beam produced at the National Accelerator Centre of Faure (South Africa) was measured at different critical points of the depth-dose distribution.
  
  
  METHODS
  RBEs were determined at the initial plateau of the unmodulated and modulated beam (depth in Perspex = 43.5 mm), and at the beginning, middle and end of a 7-cm spread-out Bragg peak (SOBP) (depths in Perspex = 144.5, 165.5 and 191.5 mm, respectively). The biological system was the regeneration of intestinal crypts in mice after irradiation with a single fraction.
  
  
  RESULTS
  Using 60Co gamma-rays as the reference, the RBE values (for a gamma-dose of 14.38 Gy corresponding to 10 regenerated crypts) were found equal to 1.16 +/- 0.04, 1.10 +/- 0.03, 1.18 +/- 0.04, 1.12 +/- 0.03 and 1.23 +/- 0.03, respectively. At all depths, RBEs were found to increase slightly (about 4%) with decreasing dose, in the investigated dose range (12-17 Gy). No significant RBE variation with depth was observed, although RBEs in the SOBP were found to average a higher value (1.18 +/- 0.06) than in the entrance plateau (1.13 +/- 0.04).
  
  
  CONCLUSIONS
  An RBE value slightly larger than the current value of 1.10 should be adopted for clinical application with a 200-MeV proton beam.
authors.affiliation: Université Catholique de Louvain, Cliniques Universitaires St-Luc, RBNT 5469, Brussels, Belgium.
authors.forename: J
authors.initials: J
authors.lastname: Gueulette
authors.forename: L
authors.initials: L
authors.lastname: Böhm
authors.forename: B M
authors.initials: BM
authors.lastname: De Coster
authors.forename: S
authors.initials: S
authors.lastname: Vynckier
authors.forename: M
authors.initials: M
authors.lastname: Octave-Prignot
authors.forename: A N
authors.initials: AN
authors.lastname: Schreuder
authors.forename: J E
authors.initials: JE
authors.lastname: Symons
authors.forename: D T
authors.initials: DT
authors.lastname: Jones
authors.forename: A
authors.initials: A
authors.lastname: Wambersie
authors.forename: P
authors.initials: P
authors.lastname: Scalliet
mesh_terms: D000818:Animals; D003037:Cobalt Radioisotopes; D004307:Dose-Response Relationship, Radiation; D005260:Female; D005720:Gamma Rays; D007414:Intestinal Neoplasms / Q000532:radiotherapy; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009502:Neutrons; D010315:Particle Accelerators; D011230:Precancerous Conditions / Q000532:radiotherapy; D011522:Protons; D011882:Radiotherapy, High-Energy / Q000379:methods*; D012062:Relative Biological Effectiveness; D013019:South Africa
chemical_list: D003037:Cobalt Radioisotopes; D011522:Protons