Fast and Accurate Calculations for Proton Therapy

New Mexico Workshop, May 15, 2011

Pablo P. Yepes, Rice U. 1

Fast and Accurate Calculations for Proton Therapy

Pablo Yepes 1,2, John Eley2, Jessie Huang 1,2, Dragan Mirkovic 2, Wayne Newhauser 2, Sharmalee Randeniya 2, Uwe Titt 2, Phil Taddei 2

1) T.W. Bonner Lab, Rice University2) University of Texas, MD Anderson Cancer Center



Outline

FDC (track repeating algorithm) for dose calculations

Justification: MC vs Analytical Applications

GPU Implementation Applications, examples.

Why the Monte Carlo Approach

Pros: higher accuracy Information about off-field and

background (for example neutrons) radiation

Cons Very high computational demands



Proton TherapyMonte Carlo versus Analytical

MC computational times are very high: >500 cpu hours for 1 beam treatment plan

Dose in Lung

AnalyticalMC (MCNPX)



Proton TherapyMC versus Analytical (Con't)

Along beame



Fast Dose Calculator (FDC)

A track repeating algorithm for proton dose calculations

Uses pre-calculated tracks in water to estimate dose in a variety of other materials by scaling path lengths and scattering angles.

Uses GEANT4 as engine to generate pre-calculated tracks. Same technique could be applied for the MC of your choice (MCNPX, FLUKA, etc).

• J. S. Li, B. Shahine, E. Fourkal and C.M. Ma, “A particle track-repeating algorithm for proton beam dose calculation”, Phys. Med. Biol., 50, 1001–10 (2005).

•P. Yepes, S. Randeniya, P. Taddei, W. Newhauser, A Track Repeating Algorithm for Fast Monte Carlo Dose Calculations of Proton Radiotherapy, Nuclear Technology 168, 334 (2009).

• P.Yepes P, S. Randeniya, P. J. Taddei and W. D. Newhauser, “Monte Carlo fast dose calculator for proton radiotherapy: application to a voxelized geometry representing a patient with prostate cancer”, Phys. Med. Biol., 54, N21-N28 (2009).

• Yepes, T. Brannan, J. Huang, D. Mirkovic, W.D. Newhauser, P.J. Taddei, U. Titt, “Application of a fast proton dosecalculation algorithm to a thorax geometry”, Rad. Meas. 45, 1367 (2010).

• P. Yepes, D. Mirkovic, P.J. Taddei, “A GPU implementation of a track-repeating algorithm for proton radiotherapy dose calculations”, Phys. Med. Biol. 55, 7107 (2010).



Generate Pre-calculated tracks GEANT breaks trajectories in steps. For each step stores deposited energy, energy change, step length and direction. Simulate 250 MeV protons on a 660x660x750 mm3 water phantom. “Pick-up” the trajectory at whatever energy is needed. Average of 460 steps and 3.5Kbytes per proton Generate 10 million protons in water for database in ~15 min on 100 CPUs



“Water history”

Re-trackinga “water history” inheterogeneousmedia


FDC Scaling Parameters

Analytical Approach: Length scaling from Proton Stopping Power

(Bethe-Block Formula) Angle scaling from Multiple Scattering Theory.

Numerical Approach:

Obtain the scaling parameters by minimizing the difference in dose distributions in various homogeneous phantoms for various materials for FDC and GEANT4.



Proton Simulation Setup

Aperture conforms the beam/field to the transverse shape of the tumor

Range compensator conforms the beam/field to the longitudinal shape of the tumor

Aperture

Range Compensator

Phantom

p



GEANT4 versus FDCProtons Prostate

Realistic Beam and Shaping Elements (Aperture and Range Compensator



Proton Prostate SimulationAccuracy/Timing

GEANT: 0.3 s/proton; FDC: 0.6 ms/proton

Accuracy relative to full Monte Carlo (GEANT) better than 2%

FDC Timing is ~ 500 faster than GEANT


Protons Lung• 140 MeV proton circular beam• Lung is a highly inhomogeneous medium, a tough test for FDC

FDCGEANT4

GEANT4-FDC


FDC on GPUs


FDC on GPUs (GFDC) Graphic Cards with multiple (GPU cores)

processors.

NVIDIA developed tools to allow running “regular” code on such cards.

Miniature computer cluster on a card for a fraction of the cost and maintenance.

Code divided on CPU and GPU sections

GPU code is regular (C and quasi C++) with some size and process limitations.

Each GPU core works on the re-tracking of one particular history.

Substantial reorganization of the code

GEFORCE GTX 295, NVIDIA


GFDC Logic Flow


GPU FDC for Protons

Circular Mono-energetic 121 MeV beam with 4 cm radius

Thorax phantom with 6 million voxels GFDC speed up of 75.5 with respect to FDC

CPU version

Protons/s

GEANT4 ~1

FDC 2,444

GFDC 184,525


Energy Deposited in Thorax for ProtonsAlong beam axis


Energy Deposited in Thorax for ProtonsCross Section: Axis Perpendicular to Beam


Lung Cross Section for ProtonsWhat about tails?


Accuracy


Statistical Errors/Timing

Batch

History-by-history


FDC Website(Part of code from dicompyler: Adit Panchal, Roy Keyes)



Conclusions Track-repeating algorithm for proton and photon

implemented on a GPU architecture. Computational times reduced by four orders of

magnitude compared to full Monte Carlo. GPU results reproduce full Monte Carlo within

2%. A system with multiple GPU cards should allow

real-time Monte-Carlo-accurate dose calculations for radio-therapy.

February 23, 2009 Pablo P. Yepes, Rice U. 25

Keep it simple!Keep it simple!



Fast Dose Calculator in a Nutshell

Water to Material M scaling

For each step, length (L) and angle with respect to previous step (φ) are scaled.Length: LM= α (E) Lh2o

Scattering Angles: φM= β φh20

M=

Fast and Accurate Calculations for Proton Therapy

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