title: Optimizing normal tissue sparing in ion therapy using calculated isoeffective dose for ion selection.
abstract: OBJECTIVE
  To investigate how the selection of ion type affects the calculated isoeffective dose to the surrounding normal tissue as a function of both normal tissue and target tissue α/β ratios.
  
  
  METHODS
  A microdosimetric biologic dose model was incorporated into a Geant4 simulation of parallel opposed beams of protons, helium, lithium, beryllium, carbon, and neon ions. The beams were constructed to give a homogeneous isoeffective dose to a volume in the center of a water phantom for target tissues covering a range of cobalt equivalent α/β ratios of 1-20 Gy. Concomitant normal tissue isoeffective doses in the plateau of the ion beam were then compared for different ions across the range of normal tissue and target tissue radiosensitivities for a fixed isoeffective dose to the target tissue.
  
  
  RESULTS
  The ion type yielding the optimal normal tissue sparing was highly dependent on the α/β ratio of both the normal and the target tissue. For carbon ions, the calculated isoeffective dose to normal tissue at a 5-cm depth varied by almost a factor of 5, depending on the α/β ratios of the normal and target tissue. This ranges from a factor of 2 less than the isoeffective dose of a similar proton treatment to a factor of 2 greater.
  
  
  CONCLUSIONS
  No single ion is optimal for all treatment scenarios. The heavier ions are superior in cases in which the α/β ratio of the target tissue is low and the α/β ratio of normal tissue is high, and protons are superior in the opposite circumstances. Lithium and beryllium appear to offer dose advantages similar to carbon, with a considerably lower normal tissue dose when the α/β ratio in the target tissue is high and the α/β ratio in the normal tissue is low.
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, MN 55905, USA.
authors.forename: Nicholas B
authors.initials: NB
authors.lastname: Remmes
authors.forename: Michael G
authors.initials: MG
authors.lastname: Herman
authors.forename: Jon J
authors.initials: JJ
authors.lastname: Kruse
mesh_terms: D001608:Beryllium / Q000627:therapeutic use; D002244:Carbon / Q000627:therapeutic use; D063193:Heavy Ion Radiotherapy; D006371:Helium / Q000627:therapeutic use; D007477:Ions / Q000627:therapeutic use*; D008094:Lithium / Q000627:therapeutic use; D008954:Models, Biological; D009010:Monte Carlo Method; D009356:Neon / Q000627:therapeutic use; D059351:Organ Sparing Treatments / Q000379:methods / Q000592:standards*; D058958:Organs at Risk / Q000528:radiation effects*; D019047:Phantoms, Imaging; D061766:Proton Therapy; D011836:Radiation Tolerance; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness
chemical_list: D007477:Ions; D006371:Helium; D009356:Neon; D002244:Carbon; D008094:Lithium; D001608:Beryllium