Monte Carlo Simulations in Proton Dosimetry with Geant4

uniklinikum.regensburg.de

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

( particle, process )

Methods 3 / 11


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

Energy deposited due to nuclear

processes and their offsprings

Results 6 / 11


Klinik und Poliklinik für Strahlentherapie der Universität Regensburg

Medizinische Physik

Results – energy per process, etc.:

Total energy due to

a process

Deflection angle

due to a processes

Results 7 / 11


Klinik und Poliklinik für Strahlentherapie der Universität Regensburg

Medizinische Physik

Results – process calls / escaping energy:

Number of routine calls

for a processes

Escaping energy from nuclear and

related processes

Results 8 / 11


Klinik und Poliklinik für Strahlentherapie der Universität Regensburg

Medizinische Physik

Results – neutron properties

– 0.13 neutrons per 160MeV proton (E 2 dependence) with ¼ of energy taken away

is a small effect, but

may become important at the peak tail

Neutrons and

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

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