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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
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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)
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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.
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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.JPG7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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.
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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:
Ionisation distribution
50 MeV
20 MeV
Plane 1 Plane 2 Plane 3
E d iti P t b
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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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 ):
Ionisation distribution
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
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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Photon and proton beamcomparison
Protons: 20 MeV
X-rays: 100 keV
Alpha pencil beam:
7/27/2019 A Comparison of X-Ray, Proton & Alpha Beam Track Structures Geant4 Very Low Energy Models
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Alpha pencil beam:
Ionisation distribution
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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