  The carbon beams show more advantages on the biological properties compared
with proton beams in radiation therapy. The carbon beam shows high linear
energy transfer (LET) to medium and it increases the relative biological
effectiveness (RBE). To design spread-out Bragg peak (SOBP) of biological dose
using carbon beam, a practical method was purposed by using the
linear-quadratic (LQ) model and Geant4 based Monte Carlo simulation code. The
various Bragg peak profiles and LET was calculated for each slice at the target
region. To generate appropriate biological SOBP, a set of weighting factor,
which is a power function in terms of energy step, was applied to the obtained
each physical dose. The designed biological SOBP showed 1.34 % of uniformity.
