Biomedical Engineering Reference
In-Depth Information
the physical input data used,
i.e
., the cross sections implemented into the code for
describing at the finest scale the charged particle induced collisions. Thus, in view
of their potential applications in radioprotection, radiobiology, medical imaging and
even in radiotherapy for treatment planning, most of the existing MC codes are
based on cross sections in water, this molecule being considered as a good surrogate
for biological targets. Furthermore, to overcome the lack of existing data in liquid
water, they have traditionally relied on theoretical descriptions of condensed-phase
interaction probabilities for model systems combined with parameters extrapolated
from gas phase studies, or entirely on gas phase data. However, let us mention
that significant advances have been achieved in the last few years for considering
water in liquid phase and then evaluating the expected differences in terms of spatial
patterns of energy deposition between liquid and vapor water [
1
].
Besides, MC track structure simulations play an important role to provide a
quantitative understanding of the mechanisms of radio-induced damages. On this
subject, numerous Monte Carlo codes have been developed among which we dis-
tinguish the specialized Monte Carlo codes - usually called “track structure codes”
- which have been developed for microdosimetry simulations (see for example [
2
]
and references therein). These codes are able to simulate precisely particle-matter
interactions, the so-called “physical stage”, some of them including also additional
features, e.g. taking into account the “physico-chemical” and “chemical” stages
which take place after the “physical” stage and allow in particular the simulation
of oxidative radical species. With the use of sophisticated geometry models, some
of the available MC codes are even able to predict - with a reasonable precision -
direct and non-direct biological damages to the DNA molecule. This is the case
of the PARTRAC software which is nowadays the most advanced Monte Carlo
simulation package for modeling the biological effects of radiation. On the other
hand, several general-purpose MC codes are already accessible to scientists for the
simulation of particle transport. Among them we can cite the most commonly used,
namely, EGS, FLUKA and MCNP with their different available versions. Some of
them are limited to the simulation of electron and photon interactions, while others
include a comprehensive description of hadronic interactions for a large variety
of ions. However, in the major part these codes limit their lower energy range
applicability down to 1 keV, which is not compatible with functionalities specific
to microdosimetry. Such codes should indeed be able to simulate particle track
structures (incident particles and the full consequent shower of secondary particles)
over lengths at the nanometer scale, compatible with the DNA molecular size and
sub-cellular scale. However, most of them are based on a semi-empirical description
of the main ionizing process by means of least-squares fittings of experimental
measurements of differential as well as total cross sections.
In this context, in the past we have developed a Monte Carlo code called TILDA
for tracking heavy charged particles in liquid water [
3
] in which all the ion- and
electron-induced interactions are described in details [
4
], liquid water being first
used for modelling the biological medium. However, DNA lesions - and more
particularly those involved in clustered damages - are nowadays considered of
prime importance for understanding the radio-induced cellular death process (see
