Monte Carlo Methods to Model Radiation Interactions and Induced Damage - Radiation Damage in Biomolecular Systems - page 205

“condensed history” technique [ 9 ] (an approximation for multiple scattering which
is the base of many modern MC codes) and the application of variance-reduction
techniques [ 10 , 11 ] which reduce statistical uncertainty. Concerning radiation effects
in biomolecular systems, the adaptation of the interaction model for use at lower
energies [ 12 , 13 , later 14 , 15 ] and modifications in electron physics in order to
distinguish different inelastic processes [ 16 , 17 ] have constituted important advances
in MC modelling.
Today, a variety of MC codes are available for simulating radiation-matter
interactions. Amongst the most important ones, there are Geant4 [ 18 ] which origins
are at the European Organization for Nuclear Research (CERN) and is being
developed and maintained by an international collaboration, EGS4 [ 19 ], developed
at Stanford Linear Accelerator Centre and now maintained as the EGSnrc [ 20 ]
version by the National Research Council of Canada, ITS [ 21 ], MCNP [ 22 ] from
Los Alamos National Laboratory, PENELOPE [ 15 ], PARTRAC [ 23 ], EPOTRAN
[ 24 ], and LEPTS (Low-Energy Particle Track Simulation, [ 16 , 25 ]). Principal
differences can be found in the interaction models utilized, the origin and nature
of the input databases, and the kind of output data and representation. Their
applications currently range from high-energy physics (e.g. detector responses and
shielding requirements) over electron microscopy to medical physics.
There, rapid developments - especially in the last decade - have led from initially
general programmes used only for comparative studies to user-friendly, specialized
codes that are clinically applicable to a growing range of situations. Additionally
to treatment verification or planning systems, MC calculation is extending to uses
including diagnostic imaging and linac beam simulation (Chap. 19). However, many
efforts in the biomedical context still focus on modelling radiation transport and
damage induced in patient tissue.
A complete (from the physics point of view) simulation tool for modelling radi-
ation damage in biomaterials should be able to model all possible combinations of:
- different tissues such as muscle, bone, tumour, lung, etc.
- different radiation types (photons, electrons, heavy charged particles)
- different energies/spectra, including that of secondary particles
- incident radiation geometries
Ultimately, it would be interesting to include the effects of reactive molecular
species produced and biological response factors. This would provide a detailed
view of the molecular damage induced and the related bio-functional impact at
different scales.
In the following sections, we describe the current versions of the MC codes
Geant4 (GEometry ANd Tracking 4), PENELOPE (PENetration and Energy LOss
of Positrons and Electrons), EPOTRAN (Electron and POsitron TRANsport),
and LEPTS (Low-Energy Particle Track Simulation). Special emphasis lies on
recent developments that, together with (still emerging) new databases that include
adequate data for biologically relevant materials, bring us continuously closer to
a realistic, physically meaningful description of radiation damage in biological
tissues.