a comparison of x-ray, proton & alpha beam track structures geant4 very low energy models

7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models

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A comparison of X-ray, proton and alpha beamtrack structures Geant4 very low energy models

Aimee McNamara1, Susanna Guatelli2, Dale Prokopovich1,Mark Reinhard1, Anatoly Rosenfeld2

1. Australian Nuclear Science and Technology Organisation(ANSTO)

2. Centre for Medical Radiation Physics (CMRP), Universityof Wollongong, Australia


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Motivation for new dosimetric concepts

Lethal damage to cells by ionising radiation isinitiated by damage to the DNA molecule in theform of single and double strand breaks (DSBs).

Particle track structure and the ionisation cluster

distribution are important factors in assessing thebiological effects of different radiation fields.

Low-energy secondary electrons (< 1 keV)produce ~ 50% of all ionisations

Doublestrand break(DSB)

Singlestrand break(SSB)


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Motivation for new dosimetric concepts

Absorbed dose is insufficient atsatisfactory describing radiationdamage on nanometer scalesand as our knowledge of cellularfunction grows we need toevaluate the effect of radiationon the DNA level.

Experimentally measuringionisation cluster-size formationin nanometric targets is verychallenging and experimentalnanodosimetry can benefit fromcomputational simulations.


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Aim of this investigation

Investigate the track structure (down to ~ eV) of ionising

radiation, particularly: Low energy X-rays, as those found in microbeam radiation therapy

(MRT) and

10 50 MeV protons found in abundance in the Bragg peak ofproton therapy.

Alpha particles e.g. targeted alpha therapy

Better understanding of the relative biological effectiveness (RBE).

Possible to substitute or supplement different therapies.


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Microbeam Radiation Therapy (MRT)

Experimental treatment consisting of an array of microscopic thin x-ray beams with E ~ 20 100 keV

Microbeams are aimed at the tumoral volume, killing cells directly inbeam path while sparing cells in between the peaks

http://www.esrf.eu

Spiga et al. (2007) Med. Phys. 34 4322Brauer-Krisch et al. (2005) Phys. Med. Biol. 50 3103

Spiga et al. (2007) Med. Phys. 34 4322

Lateral dose profile


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Proton Therapy

Proton beams produce distinct depth dose distributions in matter

Appropriate selection of a distribution of proton energies can produce

a modulated or "spread-out Bragg peak" (SOBP), which can becalculated to coincide with tumors.

Proton therapy cost needs to be reduced.http://www.nytimes.com
http://upload.wikimedia.org/wikipedia/en/f/fa/Pdd_sobp_photon.JPG


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Targeted Alpha Therapy

Alpha-emitting radionuclidescould selectively target cancercells.

Alpha particles have a highenergy (3-9 MeV) but a short

path length in tissue ~ 0.1 mm. Surrounding healthy cells are

spared high doses.


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Geant4 Very Low Energy Models(Geant4-DNA)

Allows the detailed modelling of thetrack structure of ionsing particlesdown to ~eV scale

Particles: electron, proton, H,

alpha, He+, He Processes: elastic scattering,

ionisation, excitation, chargeincrease and decrease.

S Chauvie et al. (2007) IEEE Trans. Nucl. Sci. 2 803S Incerti et al. (2010) Med. Phys. 37 4692


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Geant4 Simulation Study

Monoenergetic x-ray, proton and alpha pencil beams incident on a water cube ofdimension 0.4 mm.

The ionisation cluster distribution at different distances from the beam axis and

at different points along the beam trajectory is determined in nanometric voxels(4 x 2 x 2 nm).

Geometric size and shape of

DNA molecule is moreimportant than complexshape (Friedland et al. 1998)


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Photons are tracked down to 250 eV using the Geant4 Low Energypackage (based on the Livermoreevaluated data libraries)

Electrons with energy > 10 keV and protons with energy > 10 MeV aretransported by Geant4 Low Energy Package

Secondary electrons, protons and alphas are transported down to ~eVusing the Geant4 very low energy models

Geant4 Simulation Study

Photons Rayleigh scattering Geant4 Low Energy Physics


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Photons Rayleigh scattering,Photoelectric effect,Compton scattering, pairproduction

Geant4 Low Energy PhysicsPackage (based on Livermoreevaluated data libraries)

Electrons, E> 10 KeV Ionisation, Bremsstrahlung,multiple scattering

Geant4 Low Energy PhysicsPackage (based on Livermoreevaluated data libraries)

Electrons, E < 10 KeV Elastic scattering, excitation,ionisation

Geant4 Very Low Energyextensions

Protons E > 10 MeV Ionisation (Electronic StoppingPower by Bethe Bloch),multiple scattering,Bremsstrahlung, inelastic andelastic scattering (hadronicprocesses)

Geant4 Low Energy PhysicsPackage

Proton E < 10 MeV Ionisation, excitation, chargedecrease Geant4 Very Low Energyextensions

Physics Processes


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Photon pencil beam:

Ionisation distribution

50 keV

100 keV

2 x 2 x 5 nm voxelsPlane 1 Plane 2 Plane 3

Energ deposition Photon beam


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Energy deposition Photon beam

r

2 m

Beam

50 keV

100 keV

P il b


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Proton pencil beam:


50 MeV

20 MeV

Plane 1 Plane 2 Plane 3

E d iti P t b


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Energy deposition Proton beam

20 MeV

50 MeV

Proton pencil beam(104):


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20 MeV

50 MeV

Proton pencil beam (10 ):


Photon and proton beam


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Photon and proton beamcomparison

20 MeV protons

100 keV X-rays

Plane 1 Plane 2 Plane 3

Photon and proton beam


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Photon and proton beamcomparison

Protons: 20 MeV

X-rays: 100 keV

Alpha pencil beam:


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Alpha pencil beam:


4 MeV


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Conclusions

Low energy depositions in living cells needs to be furtherinvestigated.

Nanodosimetric considerations e.g. ionisation clusterdistribution important when considering the RBE of ionisingradiation.

X-ray beams could produce similar ionisation clusterdistributions to MeV protons on the nanometer scale forparticular values of the incident particle energy and depthranges within the target.


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Future work

Further investigation into thenanodosimetric properties of x-rays and

protons over different ranges of energiesand depths.

Include probability of DNA repairmechanisms in analysis.

Models for free radical damage.

Validation of low energy models in liquidwater.


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a comparison of x-ray, proton & alpha beam track structures geant4 very low energy models

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