Monte Carlo Simulations in Proton Dosimetry with Geant4

Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Monte Carlo Simulations in Proton Dosimetry with Geant4
Zdenek Moravek, Ludwig Bogner
Klinik und Poliklinik für Strahlentherapie
Medizinische Physik
Universität Regensburg
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Objectives of the Study
what particles and how much they contribute to the depth dose and lateral profiles
what processes and how they constitute the depositions
how often a process is called in the simulation
an amount of energy that escapes the volume and by how means
All these objectives are investigated for a possible dependence on the initial proton energy.
Application of the Results
insight in the physics of the proton – mass interactions
justification for approximations adopted in faster simulation engines
verification of already available simulation engines
Objectives 2 / 11
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Methods
MC simulation with step splitting
Step: position
incident particle
process
released energy
direction change
{ secondaries }
}
Primary
Secondary
Nuclear
Primary_escaping
Secondary_escaping
Nuclear_escaping
Methods 3 / 11
( particle, process )
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Methods
Geant4 Implementation
Physics
– all common particles and physics for proton – water interactions
– standard Geant4 models (EEDL parametrizaion, PreCompound
model for nuclear interaction)
Step information – each deposition is classified by process and particle that produced it
Grouping
– secondaries and all their offsprings are marked by a flag
– nuclear depositions and offsprings are marked by another flag
Data Tables for Evaluation
Matrices in which a contribution is catalogued against a particle (row) and process
(column) that created it.
We define the following tables – primary depositions, secondary depositions,
nuclear depositions
– variants for escaping energy (double filling)
– significant deflection table
– histograms for calls to a process and a process
with significant deflection
Methods 4 / 11
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Parameters & Verification:
Material: water
Beam : width = 7mm
dp/p = 1%
profile = gaussian, symmetric in x, y
E
= 97, 160, 214 MeV ( range [20,214] MeV )
[1] Pedroni E et al 2005. Phys Med Biol 50, 541-561.
[2] Scheib S 1993. Doctoral thesis.
Verification 5 / 11
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Results – energy per particle type:
Total deposited energy
Results 6 / 11
Energy deposited due to nuclear
processes and their offsprings
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Results – energy per process, etc.:
Total energy due to
a process
Results 7 / 11
Deflection angle
due to a processes
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Results – process calls / escaping energy:
Number of routine calls
for a processes
Results 8 / 11
Escaping energy from nuclear and
related processes
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Results – neutron properties
– 0.13 neutrons per 160MeV proton (E2 dependence) with ¼ of energy taken away
is a small effect, but
Neutrons and
may become important at the peak tail
offsprings
Prodiction of secondary protons
Neutron dose beyond the peak
as tail-to-peak ratio for depths
of 2cm (red) and 5cm
Results 9 / 11
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Results – engine test
PETRA is a Monte Carlo simulation engine for proton interactions in water with
transport of secondary protons [3]. Data are available for splitting of nuclear/non-nuclear
terms. A comparison with Geant4 shows the underestimation of nuclear interactions in
the PETRA model.
[3] Medin J, Andeo P 1997. Internal Report MSF 1997-01. Stockholm University
Results 10 / 11
Klinik und Poliklinik für Strahlentherapie der Universität Regensburg
Medizinische Physik
Summary
Deposition needs only protons (including nuclear-scattered ones)
Lateral scattering needs multiple scattering and nuclear interactions
Escaping energy is taken away by gamma particles and neutrons
Processes like electron ionization or transportation are called very often even though
their contribution is small. These may follow some approximations.
Neutron dose beyond the peak can be neglected (more than 10-3 smaller than the
peak value at 2 cm, with further exponential decay). It can be clinically relevant only
when risk organ intrudes into the target.
Summary 11 / 11