Geant4 review from the aspect of a GATE developer and user

1. Geant4 reviewfrom the aspect of a GATE developer and user Nicolas Karakatsanis

2. Contents • Tracking in parameterized volumes • Performance of Geant4 with large voxelized phantoms • Profiling of GATE-GEANT4 performance • Variance Reduction Techniques with GEANT4 • GEANT4 implementation of the ion source • Radioactive Decay Module • Dosimetry in GEANT4 • Further suggestions from the medical community

3. Tracking in parameterized volumes • In SPECT a detector is typically made of a scintillator capturing gammas emitted from a patient tracer. • In front of such a detector is a lead block with circa 150.000 air holes • In parallel beam applications all these holes have an identical form and are aligned along a rectangular grid. • We can model such a parallel beam collimator using arrays (repeaters) or using Geant4 replicas. • Both options are available in GATE but for distributed computing only replicas are used since building the geometry goes fast (almost no overhead on a cluster).

4. Tracking in parametrized volumes • However also applying • Fan collimators (oriented towards a focal line) • cone beam collimators (oriented towards a focal spot) • Hence, every individual hole has another form/orientation • which can be pre-calculated and described in a closed analytical expression • To model such collimators • GATE uses G4 parametrised volumes. • Building the geometry goes fast and flawless. • The tracking however is multiple orders slower • probably because every time a particle hits the collimator every form and distance is recalculated.

5. Tracking in parametrized volumes • Temporary solution • Design of a "flying hole array“ • only takes the 20 neighbouring holes around the interaction site into account. • Limitation if the energy of the gammas increases • because then can travel through many of the lead lamella, crossing over multiple holes (not known before how many holes) • Question • How to increase calculation speed in an application with more than 10^4 parametrized inserts in one volume Contact: Steven Staelens : steven.staelens@ugent.be

6. Performance of Geant4 with large voxelized phantoms • Problem: • The simulation time for a voxelized phantom in GATE is prohibitive (dependent on the number of total voxels). • This is a bottleneck in GATE when using voxelized phantom. • Question • Couldn't some implementation in Geant4 reduce this computing time?

7. Performance of Geant4 with large voxelized phantoms • Our analysis: • Geant4 forces one step in material region boundary • This feature could be one of most important (if not the most important) factors that affects simulation speed when using large voxelised phantoms • Because G4 is dealing with each voxel as a different material region • Possible solution • If the material in one voxel is the same as that in its neighboring voxel, • Treat those two voxels as one region (no region boundary between those two voxels) • Check that condition for all voxels

8. Performance of Geant4 with large voxelized phantoms • Possible solution (..continue) • Create an option in stepping function • to test if the material in next region is the same as current one. • If yes • no boundary is applied here and • a normal step is taken • A flag can turn on/off the previous option

9. Performance of Geant4 with large voxelized phantoms • Solution developed in GATE • A compressedMatrix phantom object can be used instead of the parameterizedBoxMatrix • generate a compressed phantom where voxel size is variable • All adjacent voxels of the same material are fused together to form the largest possible rectangular voxel. • A compressed phantom uses • less memory and also • less CPU • It is possible to exclude regions in the phantom from being compressed through the use of an "exclude list" of materials • Question • Using compressedMatrix phantom • reduce the CPU time but only by maybe 30%. • A more efficient way to track particles in a voxelized geometry is welcomed Contact: Richard Taschereau: RTaschereau@mednet.ucla.edu

10. Profiling of GATE-Geant4 performance • Profiling tools • Grpof and Valgrind (GNU profiling tools) • Profiling results indicate • Use of voxelized maps • degrade Geant4 performance • Significant increase of the computation time consumed by the method CLHEP :: Hep3vector G4ThreeVector which is used by the Geant4 class G4ParameterizedNavigation Geant4 navigation follows a voxel-to-voxel approach =>time-consuming approach • Possible Solutions • Need for optimization • Merging of neighboring voxels with similar attributes • Definition of maps using ray-tracing techniques • Abandon discrete maps definition – introduce volume rendering

11. Profiling of GATE-Geant4 performance Contact for profiling results: Nicolas Karakatsanis : knicolas@mail.ntua.gr

12. Profiling of GATE-Geant4 performance • Further questions from the medical community • Profiling results indicate bottleneck caused by the G4 Navigator • Will it be possible for GATE to define its own navigator that would inherit properties from the G4 navigator?

13. Current VRT Implementations • Variance Reduction Techniques (VRTs) • Importance sampling • Photon splitting • Russian roulette • Weight window sampling • Weight roulette • Scoring • A number of VRT implementations are already available in Geant4, but • Some of the Geant4 classes can be optimized more, • Further classes implementing VRTs could be developed

14. Suggested VRTs for Geant4 • Alternative photon splitting technique for G4 • If the first photon of the annihilation pair is detected • => second photon splits into multiple photons with equal weights and total weight sum of 1 • Degree of splitting depends on the probability of detection • Probability is dependent on axial position and emission angle • Therefore geometrical importance sampling is based on axial position and emission angle • Therefore prior knowledge of the geometry of the detector systems is required • High detection probability => photon splits into a small number of secondary photons • Low detection probability => photon splits into a large number of secondary photons • Result • Accuracy is not affected • 3 – 4 times increase in efficiency of the application

15. Suggested VRTs for Geant4 • Alternative photon–splitting technique for G4 (…continue) • Second photon-splitting – avoid adding noise to scatter estimation • If the first photon is detected without having undergone Compton interaction • Second photon splits at the annihilation point • Else if the second photon is detected • First photon splits at the Compton interaction point • Else • No coincidence is recorded • Repeat previous steps for each event • Addition photon transport algorithms could be implemented • Delta scattering including energy discrimination • Flags implementation • user can easily activate or not the various VRT options

16. Suggested actions regarding VRTs • Collaboration between • GATE VRTs workgroup • Geant4 VRTs workgroup • Aim of collaboration • Determination of the existing G4 classes need to be optimized • Determination of further G4 classes possibly needed to be implemented • Implementation of GATE-specific classes within the Geant4 framework • Publication of a GATE-specific patch for Geant4 • Suggested VRTs for Geant4 follow: • Implementation of flags (probably at GATE) for each one of the following VRTs • => VRTs should be activated or deactivated by the user Contact: Nicolas Karakatsanis : knicolas@mail.ntua.gr

17. GEANT4 – VRTs implementation • Further questions regarding VRTs implementation at Geaant4 • Are there any validation data regarding the variance reduction techniques currently available in Geant4? • Are people actually using them? • Who are the persons involved in • the developments of VRT and • the validation of these techniques?

18. G4 implementation of the Ion source • G4 Ion source implementation problem • Too many memory leaks • => leading to abnormal termination of lengthy simulations Contact: Nicolas Karakatsanis : knicolas@mail.ntua.gr

19. RDM (radioactive decay module) • Is it efficient, in terms of computational time, for instance when considering sources such as • Fluorine 18 or • Iodine 124 ? • Computational problems arise when • we have to simulate billions of such decays

20. Dosimetry with GEANT4 • Comparisons between GEANT4 and other codes in the past indicate • always some discrepancies between GEANT and the other MC codes, like EGS4 (more or less the gold standard) • When performing "Rogers' > experiment" using GATE(GEANT4), EGS and MCNP, • observe differences in absorbed dose at boundaries between materials. • Therefore problem seems to be associated with both • the set of low energy processes used and • particle transport

21. Dosimetry with GEANT4 • Problem • Option between two sets of low energy processes viz. "Penelope" and "low > energy“ • A little bewildering • Both sets yield different results • The simple user have to pick the one that is "right" for him • Suggestion • Only one is necessary -> the "right“ one • Question • Which set of low energy processes should be chosen for dosimetry purpose? • Any data regarding the validation of absorbed dose as calculated using Geant4?

22. Using G4 for medical applications – Further suggestions • Build-in Profiling features (time-profiling) • a verbose option could display the average time (and RAM ?) spent for • each physical processes, • for the stepping process, • for the navigation part and • for the initialisation part • Makefile management improvement • Cmake management tool • easier maintenance and smoother linking of third-party libraries • especially important for medical applications, often linked with other software or libraries (e.g. image management)

23. Using G4 for medical applications – Further suggestions • Voxelized scene • a bug with the Nested parameterisation? • David Sarrut exchanged mail with G4 developer community • slight differences with conventional parameterisations • not a problem due to roundoff error or machine precision • probably a problem which occurs in some infrequent situation (particle moving along boundary ...) • A class managing a 3D matrix of objects (voxels) should be useful for • image management, • 3D distribution of measurables for example • deposit dose but also • any other measurable such as Beta+ emitters • different from the way such matrix is represented for navigation (parameterized volume, G4Boxs or other) • 3D managing class characteristics • Fast voxel access • voxel spacing management • capabilities to store any object type • visualization features, storing, retrieving... • David Sarrut is ready to help build such class

24. Using G4 for medical applications – Further suggestions • The documentation on the facrange (facgeom and related) parameters seems important for medical application users • “Wiki” documentation • cooperative community-build documentation in a “wiki” format • interesting way to improve the documentation, particularly in the developer section • “Wiki” technology is now well known and mature • allows any user to modify and improve the on-line documentation • Our experience shows that • is very efficient and • it also encourages information exchange in the community Contact for these questions: David.Sarrut@creatis.insa-lyon.fr buvat@imed.jussieu.fr

25. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Aim • calculate doses from extended source distributions in homogeneous media • averaging doses over finite volumes • Basic Method • Calculate dose-point-kernels in an infinite water medium • Definition: radial distributions of dose around isotropic point sources of electrons or beta emitters.

26. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study L. Maigne, C.O. Thiam Laboratoire de Physique Corpusculaire, 24 avenue des Landais, 63177 AUBIERE cedex

27. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Method • In Monte Carlo simulations, the dose around a point source is obtained by • scoring the energy deposited in thin concentric spherical shells around the source per particle decay • Dose estimation techniques exist for photons • the linear track length kerma estimator of Williamson estimates the kerma by scoring tracks crossing a volume • But no alternatives for electrons

28. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Scoring of energy deposited in spherical shells => • obtain dose point kernels in water for 50keV, 100 keV, 200 keV, 1MeV and 4 MeV monoenergetic electron point sources • The dose distribution is converted into a dimensionless quantity where: • r is the radial distance to the middle of the spherical shells • rE the nominal CSDA range • ρ the density of the medium • D(r,E) the dose per incident particle at distance r

29. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of G4 Versions (50keV, 100keV)

30. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of G4 Versions (1MeV, 2MeV)

31. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of G4 Versions (3MeV, 4MeV)

32. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Conclusion • High discrepancies between G4.5 and G4.6, G4.6 and G4.7 => • Possible cause: Multiple scattering implementation?

33. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of Geant4 with other MC calculations ( kinetic energy = 50keV, 100keV )

34. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of Geant4 with other MC calculations ( kinetic energy = 1MeV, 2MeV )

35. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Comparison of Geant4 with other MC calculations ( kinetic energy = 3MeV, 4MeV )

36. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Conclusion • High discrepancies between Geant4.8 and other MC packages (around 10%) • Possible reason: still undefined

37. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Study of G4example TestEm5 • Transmission of electrons through a thin layer of water (1.02 mm). • Layer at 1.273 cm from the monoenergetic electron source • World filled with air. • Energy of electrons is 4 MeV. • One million of electrons have been generated • Influence of the angular distribution for different G4 versions on electron multi-scattering simulation

38. Beta Ray Point Source Distributions Using GEANT4 – Comparative Study • Plotted • the space angle of transmitted electrons at the exit of the water layer and • the projected angle on the y and z direction of the same scattering angle • (Note: cut-off value in range for electrons is 0.0043 mm, equivalent to 1 keV in air and 2 keV in water medium) Higher discrepancies between G4.8 and G4.6 due to multiple scattering implementation.

39. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study L. Maigne, C.O. Thiam Laboratoire de Physique Corpusculaire, 24 avenue des Landais, 63177 AUBIERE cedex

40. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Source capsule • 0.05 mm thick titanium tube, • density of 4.54 g.cm^-3 • Radioactive seed core • cylindrical ceramic shell • outer and inner diameters of 0.60 and 0.22 mm • length of 3.50 mm • density of 2.88 g.cm^-3 (Alumina Al2O3) • Uniform activity distribution of 10^-22 MBq of 125-I • Gold marker (inside the radioactive seed core) • density of 19.32 g.cm^-3, • 0.17 mm diameter • Length of 3.5 mm long • permits radiographic localisation of the seed

41. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • 2D dose distribution around cylindrically symmetric sources, the dose rate at point (r,θ) can be written as • Line source model • The radial dose function g(r) • accounts for the effects of absorption and scatter in the medium along the transverse axis of the source • 2D anisotropy function F(r, theta) • describes the variation in dose as a function of polar angle relative to the transverse plane

42. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Considering the 125-I brachytherapy sources, the simulations were performed in liquid water • 1 476 000 gamma particles were generated for each simulations • Cut applied on the electrons is 1 meter • High enough because the maximum range of secondary electrons is small comparing to the recovering ring dimensions. • The cut on X-rays is fixed to 1 keV

43. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Comparison of G4 Standard / Low Energy

44. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Comparison of G4 Versions (Low-Energy package)

45. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Comparisons with other MC and measurements (low-energy package)

46. Dosimetric characteristics of an I125 Brachytherapy Source Using Geant4 – A comparative study • Conclusions • Little discrepancies between Standard and Low Energy packages (7%), • Better agreement between Low-energy package and other MC • => to be explained • Good agreement between G4 versions • Good agreement with other Monte Carlo and measurements