Biomedical Engineering Reference
In-Depth Information
Fig. 15.7
Experimental,
calculated and parameterized
curves of the stopping power
of liquid water, for proton
beams. The corresponding
sources are indicated in the
inset. Notice that only the
experiments by Shimizu
et al
.
[
70
,
71
] correspond to liquid
water, the rest being done in
ice water. See the text for
more details
In Fig.
15.7
we observe that for energies larger than 300 keV, both the MELF-
GOS and the ICRU49 curves are rather similar; however discrepancies appear for
energies around and lower than the maximum stopping. The discrepancies between
the results by Dingfelder
et al.
[
9
] and the MELF-GOS [
19
,
22
] model could be
attributed to the different OELF used as input by these models.
Given the discrepancies predicted by the different models at low proton energies,
new experimental data of the stopping power of liquid water for protons are
necessary to elucidate its behaviour for projectile energies around and lower the
maximum stopping.
15.4.2
Depth-dose distributions
In what follows we use the simulation code SEICS (Simulation of Energetic Ions
and Clusters through Solids) [
73
-
75
] to calculate the depth dose distribution of
a proton beam travelling through a liquid water target. This code is based on
a combination of the Monte Carlo and Molecular Dynamics methods to follow
dynamically the trajectories of the protons incident on the target until they are
stopped. Thus, knowing the coordinates, velocities and charges of the projectiles
at each time it is possible to find the deposited energy by the projectile as a function
of the depth into the irradiated target. The SEICS code includes the electronic force
on the projectile, which is mainly responsible for the energy loss in the energy range
of keV-MeV; this force is given by the stopping power S
Q
, but taking into account
the statistical fluctuation around that mean value, which is provided through the
energy-loss straggling
2
Q
. The SEICS code also includes the interaction of the
