1. Representing Range Compensators with Computational Geometry in TOPAS Forrest Iandola 1,2 and Joseph Perl 1 1 SLAC National Accelerator Laboratory 2 University of Illinois at Urbana-Champaign

2. 2 Forrest Iandola Computational Geometry Compensators Overview Medical Physics in 30 Seconds Introduction to TOPAS What is a range compensator? Subtraction Solid geometry Modeling compensators with Subtraction Solids Approximation with polyhedrons for (potentially) faster performance

3. 3 Forrest Iandola Computational Geometry Compensators Medical Physics in 30 Seconds Goal: kill cancer with radiation Deliver radiation with protons, photons, other particles, or ions Monte Carlo simulation of proton therapy beams is an up-and-coming field

4. 4 Forrest Iandola Computational Geometry Compensators Introduction to TOPAS TOPAS (Tool for Particle Simulation) TOPAS = Monte Carlo simulation of radiation therapy beamlines User can easily customize beamline for specific treatment facilities Uses Geant4 for the “real” physics

5. 5 Forrest Iandola Computational Geometry Compensators What is a Range Compensator? A radiation therapy beamline collimates the beam and produces a specific energy spread Range compensator produces an energy spread Construction: drill a number of holes out of a cylinder of lucite Each drill hole may have a unique depth “The thickness of the Lucite [plastic] will proportionally reduce the depth [energy] of the protons” 1 1 http://neurosurgery.mgh.harvard.edu/protonbeam/

6. 6 Forrest Iandola Computational Geometry Compensators What is a Range Compensator?

7. 7 Forrest Iandola Computational Geometry Compensators Subtraction Solids Geant4 supports boolean solid combinatorial geometry –Subtraction solids –Union solids It’s as simple as newSolid = Solid1 - Solid2 Overlap among subtracted solids is acceptable Solids can be recursively subtracted

8. 8 Forrest Iandola Computational Geometry Compensators Compensator with Subtraction Solids Compensator comprised of a bigCylinder with n holes subtracted: newSolid1 = bigCylinder - smallCylinder1 newSolid2 = newSolid1 - smallCylinder2 … Compensator = newSolid(n-1) - smallCylinder(n)

9. 9 Forrest Iandola Computational Geometry Compensators Compensator with Subtraction Solids

10. 10 Forrest Iandola Computational Geometry Compensators Approximation for Performance Gains Approximate the drill holes with a collection of hexagons Lack of overlap among hexagons allows us to model all hexagons as a single polyhedron Future work: evaluate performance benefits (and accuracy reduction) with polyhedron method

11. 11 Forrest Iandola Computational Geometry Compensators Approximation for Performance Gains Subtraction SolidPolyhedron

12. 12 Forrest Iandola Computational Geometry Compensators Approximation for Performance Gains Future work: evaluate performance benefits (and accuracy reduction) with polyhedron method

13. 13 Forrest Iandola Computational Geometry Compensators Acknowledgements Harald Paganetti (Massachusetts General Hospital and Harvard University) Jan Schuemann (Massachusetts General Hospital and Harvard University) Jungwook Shin (UC San Francisco) Bruce Faddegon (UC San Francisco) DOE and NIH for generous support Contact: forrest@slac.stanford.edu