title: Microdosimetry of proton and carbon ions.
abstract: OBJECTIVE
  To investigate microdosimetry properties of 160 MeV/u protons and 290 MeV/u(12)C ion beams in small volumes of diameters 10-100 nm.
  
  
  METHODS
  Energy distributions of primary particles and nuclear fragments in the beams were calculated from simulations with the general purpose code SHIELD-HIT, while energy depositions by monoenergetic ions in nanometer volumes were obtained from the event-by-event Monte Carlo track structure ion code PITS99 coupled with the electron track structure code KURBUC.
  
  
  RESULTS
  The results are presented for frequencies of energy depositions in cylindrical targets of diameters 10-100 nm, dose distributions yd(y) in lineal energy y, and dose-mean lineal energies yD. For monoenergetic ions, the yD was found to increase with an increasing target size for high-linear energy transfer (LET) ions, but decrease with an increasing target size for low-LET ions. Compared to the depth dose profile of the ion beams, the maximum of the yD depth profile for the 160 MeV proton beam was located at ∼ 0.5 cm behind the Bragg peak maximum, while the yD peak of the 290 MeV/u (12)C beam coincided well with the peak of the absorbed dose profile. Differences between the yD and dose-averaged linear energy transfer (LETD) were large in the proton beam for both target volumes studied, and in the (12)C beam for the 10 nm diameter cylindrical volumes. The yD determined for 100 nm diameter cylindrical volumes in the (12)C beam was approximately equal to the LETD. The contributions from secondary particles to the yD of the beams are presented, including the contributions from secondary protons in the proton beam and from fragments with atomic number Z = 1-6 in the (12)C beam.
  
  
  CONCLUSIONS
  The present investigation provides an insight into differences in energy depositions in subcellular-size volumes when irradiated by proton and carbon ion beams. The results are useful for characterizing ion beams of practical importance for biophysical modeling of radiation-induced DNA damage response and repair in the depth profiles of protons and carbon ions used in radiotherapy.
authors.affiliation: Thailand Institute of Nuclear Technology, Ongkharak, Nakhon Nayok 26120, Thailand.
authors.forename: Thiansin
authors.initials: T
authors.lastname: Liamsuwan
authors.affiliation: Medical Radiation Physics, Department of Physics, Stockholm University, SE-10691, Sweden.
authors.forename: Martha
authors.initials: M
authors.lastname: Hultqvist
authors.affiliation: Radiation Biophysics Group, Department of Oncology-Pathology, Karolinska Institutet, Box 260 SE-17176, Stockholm, Sweden.
authors.forename: Lennart
authors.initials: L
authors.lastname: Lindborg
authors.affiliation: School of Health Sciences, Kyushu University, Fukuoka 812-8581, Japan.
authors.forename: Shuzo
authors.initials: S
authors.lastname: Uehara
authors.affiliation: Radiation Biophysics Group, Department of Oncology-Pathology, Karolinska Institutet, Box 260 SE-17176, Stockholm, Sweden.
authors.forename: Hooshang
authors.initials: H
authors.lastname: Nikjoo
mesh_terms: D002244:Carbon; D003198:Computer Simulation; D007477:Ions; D018499:Linear Energy Transfer; D055616:Microtechnology / Q000379:methods*; D009010:Monte Carlo Method; D011522:Protons; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D014867:Water
chemical_list: D007477:Ions; D011522:Protons; D014867:Water; D002244:Carbon