Microdosimetry of proton and carbon ions

Purpose: To investigate microdosimetry properties of 160 MeV/u protons and 290 MeV/u¹²C 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 distributionsyd(y) in lineal energy y, and dose-mean lineal energies y^¯_D. For monoenergetic ions, the y^¯_D 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 y^¯_D depth profile for the 160 MeV proton beam was located at ∼0.5 cm behind the Bragg peak maximum, while the y^¯_D peak of the 290 MeV/u ¹²C beam coincided well with the peak of the absorbed dose profile. Differences between the y^¯_D and dose-averaged linear energy transfer (LET_D) were large in the proton beam for both target volumes studied, and in the ¹²C beam for the 10 nm diameter cylindrical volumes. The y^¯_D determined for 100 nm diameter cylindrical volumes in the ¹²C beam was approximately equal to the LET_D. The contributions from secondary particles to the y^¯_D 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 ¹²C 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