title: Ion recombination correction in carbon ion beams.
abstract: OBJECTIVE
  In this work, ion recombination is studied as a function of energy and depth in carbon ion beams.
  
  
  METHODS
  Measurements were performed in three different passively scattered carbon ion beams with energies of 62 MeV/n, 135 MeV/n, and 290 MeV/n using various types of plane-parallel ionization chambers. Experimental results were compared with two analytical models for initial recombination. One model is generally used for photon beams and the other model, developed by Jaffé, takes into account the ionization density along the ion track. An investigation was carried out to ascertain the effect on the ion recombination correction with varying ionization chamber orientation with respect to the direction of the ion tracks. The variation of the ion recombination correction factors as a function of depth was studied for a Markus ionization chamber in the 62 MeV/n nonmodulated carbon ion beam. This variation can be related to the depth distribution of linear energy transfer.
  
  
  RESULTS
  Results show that the theory for photon beams is not applicable to carbon ion beams. On the other hand, by optimizing the value of the ionization density and the initial mean-square radius, good agreement is found between Jaffé's theory and the experimental results. As predicted by Jaffé's theory, the results confirm that ion recombination corrections strongly decrease with an increasing angle between the ion tracks and the electric field lines. For the Markus ionization chamber, the variation of the ion recombination correction factor with depth was modeled adequately by a sigmoid function, which is approximately constant in the plateau and strongly increasing in the Bragg peak region to values of up to 1.06. Except in the distal edge region, all experimental results are accurately described by Jaffé's theory.
  
  
  CONCLUSIONS
  Experimental results confirm that ion recombination in the investigated carbon ion beams is dominated by initial recombination. Ion recombination corrections are found to be significant and cannot be neglected for reference dosimetry and for the determination of depth dose curves in carbon ion beams.
authors.affiliation: Molecular Imaging, Radiotherapy and Oncology, Institute for Experimental and Clinical Research, Université Catholique de Louvain, Brussels B-1200, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Rossomme
authors.affiliation: EBG MedAustron GmbH, Wiener Neustadt A-2700, Austria.
authors.forename: J
authors.initials: J
authors.lastname: Hopfgartner
authors.affiliation: Acoustics and Ionising Radiation Division, National Physical Laboratory, Teddington TW11 0LW, United Kingdom.
authors.forename: N D
authors.initials: ND
authors.lastname: Lee
authors.affiliation: Radiotherapy and Oncology Department, Cliniques Universitaires Saint-Luc, Brussels B-1200, Belgium.
authors.forename: A
authors.initials: A
authors.lastname: Delor
authors.affiliation: Acoustics and Ionising Radiation Division, National Physical Laboratory, Teddington TW11 0LW, United Kingdom.
authors.forename: R A S
authors.initials: RA
authors.lastname: Thomas
authors.affiliation: Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, Catania, IT-95125, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Romano
authors.affiliation: Division of Radiation Research, National Institute of Radiological Sciences, Chiba-shi 263-8555, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Fukumura
authors.affiliation: Molecular Imaging, Radiotherapy and Oncology, Institute for Experimental and Clinical Research, Université catholique de Louvain, Brussels B-1200, Belgium and Radiotherapy and Oncology Department, Cliniques Universitaires Saint-Luc, Brussels B-1200, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Vynckier
authors.affiliation: EBG MedAustron GmbH, Wiener Neustadt A-2700, Austria and Acoustics and Ionising Radiation Division, National Physical Laboratory, Teddington TW11 0LW, United Kingdom.
authors.forename: H
authors.initials: H
authors.lastname: Palmans
mesh_terms: D000465:Algorithms; D002244:Carbon / Q000627:therapeutic use*; D003198:Computer Simulation; D017357:Cyclotrons; D007477:Ions / Q000627:therapeutic use*; D008962:Models, Theoretical; D009010:Monte Carlo Method; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D017356:Synchrotrons
chemical_list: D007477:Ions; D002244:Carbon