SU-E-T-281: Secondary Light-Ions in Carbon-Ion Therapy: A GEANT4 Simulation of LET and Dose Contributions.

Purpose: The purpose of this investigation was to reveal the dosimetric characteristics and contributions of the secondary light ions produced from carbon-ion beams in water via a GEANT4 simulation.

Methods: GEANT4 low energy electromagnetic physics processes were used for all ions. The hadronic interactions of protons and neutrons are simulated using low energy elastic interactions while inelastic scattering is simulated with a binary cascade (BIC) model. Other inelastic light-ion interactions are being simulated through the application of a quasi-molecular dynamics (QMD) model. A 27 liter cubic water phantom consisting of 3000 packed rectangular detector sheets (30 cm × 30 cm × 0.1mm) was used in this simulation. A 1 mm diameter pencil beam of 1 million incident carbon-ions was used in all simulations presented. The incident energies per nucleon of the carbon beams studied were selected 155 MeV, 262 MeV and 369 MeV. The energy deposition, total kinetic energy, fluence, and dose averaged LET of each secondary particle produced within each detection sheet was determined to study their variations with depth. The particles investigated were gamma, neutron, electron, positron, proton, deuteron, triton, alpha, He- 3, Li-6, Li-7, Be-7, Be-9, Be-10, B-10, B-1 1, C-1 1, C-12, C-13, C-14, N-13, N-14, N-15, O-16.

Results: The relative dose contributions of four secondary ions (B-11, C-1 1, Alpha and Proton) were found to be each greater than 1%. The highest dose averaged LET value was from N-13, though its averaged dose contribution was less than 5×10-4 %.

Conclusions: The secondary particles contribute small portion to the total dose deposited from primary carbon beam. Though heavier ions (N, O) show significant higher LET values, their relative dose contributions were negligible.