  Relative biological effectiveness (RBE) plays an important role in designing
a uniform dose response for ion beam therapy. In this study the biological
effectiveness of a carbon ion beam delivery system was investigated using Monte
Carlo simulation. A carbon ion beam delivery line was designed for the Korea
Heavy Ion Medical Accelerator (KHIMA) project. The GEANT4 simulation tool kit
was used to simulate carbon beam transporting into media. An incident energy
carbon ion beam in the range between 220 MeV/u and 290 MeV/u was chosen to
generate secondary particles. The microdosimetric-kinetic (MK) model is applied
to describe the RBE of 10% survival in human salivary gland (HSG) cells. The
RBE weighted dose was estimated as a function of the penetrating depth of the
water phantom along the incident beam direction. A biologically
photon-equivalent Spread Out Bragg Peak (SOBP) was designed using the RBE
weighted absorbed dose. Finally, the RBE of mixed beams was predicted as a
function of the water phantom depth.
