GEANT4 simulation of an ocular proton beam & benchmark against MC codes

1. GEANT4 simulation of an ocular proton beam & benchmark against MC codes D. R. Shipley, H. Palmans, C. Baker, A. Kacperek Monte Carlo 2005 Topical Meeting Chattanooga, Tennessee, USA 17 - 21 April 2005

2. Overview of talk • Introduction and advantages of proton therapy • Typical low-energy clinical proton therapy facility • 62 MeV ocular proton beam at the Clatterbridge Centre for Oncology (CCO), UK • GEANT4 simulations • Dose distributions for full-energy and modulated CCO beam line and comparison with measurement • Key physical processes and dose distributions for 50, 150, 250 MeV monoenergetic beams • Conclusions and future work

3. Introduction • Proton therapy is on the increase due to commercial availability of turnkey facilities and reduced cost • Main treatment site is eye (melanoma) but more and more deep seated tumours • Dosimetry is not as well established as for x-ray therapy; NPL has research projects for improved proton dosimetry • Monte Carlo is essential for studying dosimetry, detector perturbation factors, stopping powers, etc.

4. modulated 100 X - 10 MV 80 60 pdd 40 p - 100 MeV non-modulated 20 Tumour 0 0 2 4 6 8 z (cm) Background: Improved therapy using protons versus x-rays

5. Principle of range modulation Dw z

6. CCO proton beam line – treatment room

7. CCO proton beam line – GEANT4 (VRML)

8. GEANT4 simulations: general • GEANT4 6.2.p02 with Low Energy EM Physics package (G4EMLOW 2.3). • Physics processes: • ICRU 49 parameterisation: proton energy loss (EM interactions) • LHEP and HEP models: hadronic processes (non-elastic nuclear interactions) • recommended for use in medical applications (?) • Cuts • 0.02 mm in phantom (<< dose scoring region), 10 mm elsewhere • minimize any dependencies of particle transport on geometry

9. GEANT4: CCO beam line • HadronTherapy (GEANT4): • calculates dose distributions in a PMMA phantom for the CCO beam line • derived from the HadronTherapy advanced example distributed with GEANT4 • depth dose and lateral dose distributions • modulated and full-energy (no mod wheel) beams • Comparison with: • McPTRAN.RZ: Palmans, NPL (2005) • and MCNPX • Film and diode measurements

10. Depth and lateral dose distributions in PMMA: CCO full-energy beam Lateral dose: front face Depth dose Lateral dose: 0.5 x r0

11. Depth and lateral dose distributions in PMMA: CCO modulated beam Lateral dose: front face Depth dose Lateral dose: 0.5 x r0

12. GEANT4: monoenergetic beams • WaterPhantom (GEANT4) • calculates dose distributions in a water phantom for monoenergetic pencil beams • depth dose : 200 cylindrical slabs on beam axis • radial dose : 100 annular rings (0.1 mm thick) at 0.5, 0.9 x r0 • energy : scoring phase space data • at a plane – analysed with MATLAB • 50, 150, 250 MeV pencil proton beams • Comparison with: • PTRAN: Berger, NIST (1993) • MCNPX v2.5d (beta): LANL (2004) • SRIM v2003.12: Ziegler (2003)

13. 180 180 40 CSDA 160 40 CSDA 160 CSDA 40 35 + energy straggling 140 CSDA 40 ) + energy straggling 35 CSDA 140 2 ) + energy straggling 35 CSDA cm 2 120 + energy straggling ) 30 35 cm 2 + multiple scattering 120 + energy straggling ) 30 -1 + multiple scattering cm 2 ) 30 -1 100 + multiple scattering cm 2 25 + nuclear interactions ) 30 -1 100 cm 2 25 + nuclear interactions -1 cm 80 25 dE/dx (MeV g -1 20 80 25 dE/dx (MeV g -1 20 dE/dx (MeV g 60 20 dE/dx (MeV g 60 15 20 dE/dx (MeV g 15 40 dE/dx (MeV g 15 40 10 15 10 20 10 20 5 10 0 5 0 5 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 5 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 z/r 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 0 z/r 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 0 z/r 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 0 z/r 0 0.2 0.4 0.6 0.8 1 0 z/r 0 z/r 0 Influence of key processes on depth dose distributions

14. 1E+04 1E+04 CSDA 1E+04 CSDA 1E+04 1E+03 CSDA 1E+03 + multiple scattering CSDA 1E+03 + multiple scattering 1E+03 1E+02 + multiple scattering + energy straggling 1E+02 + energy straggling 1E+02 1E+02 1E+01 + nuclear interactions 1E+01 Dose (a.u.) 1E+01 Dose (a.u.) 1E+01 1E+00 Dose (a.u.) 1E+00 Dose (arb. units) 1E+00 1E+00 1E-01 1E-01 1E-01 1E-01 1E-02 1E-02 1E-02 1E-02 1E-03 1E-03 0.0 0.5 1.0 1.5 2.0 1E-03 0.0 0.5 1.0 1.5 2.0 1E-03 r (cm) 0.0 0.5 1.0 1.5 2.0 r (cm) 0.0 0.5 1.0 1.5 2.0 r (cm) r (cm) Influence of key processes on lateral dose distributions

15. Depth dose distributions in water:monoenergetic beams 250 MeV 150 MeV 50 MeV

16. Lateral dose distributions in water:monoenergetic beams 0.9 x r0 0.5 x r0

17. Summary and conclusions • GEANT4: low-energy clinical proton beam (CCO): • full-energy beam: • overestimates Bragg peak by ~20% • modulated beam: • increasing depth dose profile with distinctive peak • sharper penumbra and ‘horns’ at edge of lateral dose profile • some dependence on GEANT4 models, daily beam variations and type of detector • GEANT4: monoenergetic proton beams (clinical energies) • different physical models (or their implementation) in the MC codes give distinct differences in dose distributions at these energies • improvements in GEANT4 model used is still needed (or a better physical model chosen)

18. Future work • Proton stopping powers for clinical beams: • directly using Monte Carlo • from fluence spectra at depth • Perturbation factors for detectors • Graphite-to-water conversion factors • proton calorimetry (NPL)