Monte Carlo Dose Calculation for Radiotherapy Treatment Planning

Monte Carlo Dose Calculation for Radiotherapy Treatment Planning Monte Carlo Dose Calculation for Radiotherapy Treatment Planning C-M M Charlie Ma, Ph.D. Department of Radiation Oncology Fox Chase Cancer Center Philadelphia, PA 19111, USA Chetty I, Curran B, Cygler J, DeMarco J, Ezzell G, Faddegon B, Kawrakow I, Keall P, Liu H, Ma C-M, Sheikh-Baghery D, Rogers D, Seuntjens S, Siebers S. The AAPM Task Group 105 Report Med Phys (2007) 34: 4818-53 What is Monte Carlo? (a mathematical method using random sampling) The originators : Von Neumann and Ulam 1949 The method : Random sampling from pdf s to construct solutions to problems. Why call it Monte Carlo? John Von Neumann (1903-1957) Stanislow M. Ulam (1909-1984) After the city in the Monaco principality... 1

What is Monte Carlo Radiation Transport? Photons Random sampling of particle interactions a good supply of random numbers probability distributions governing the physics processes Information obtained by simulating large number of histories e- e+ CPU time required 1 Gy => 1 billion electrons 1% uncertainty for 0.3 cm cubes requires a few million electrons (hours on a PC) 1 Gy => 1000 billion photons 1% uncertainty for 0.3 cm cubes requires up to a few billion photons (days on a PC) Applications of M-C in radiotherapy Fluence and spectrum calculations Dosimetric parameters (stopping powers, etc.) Correction factors (BSF, HS, PS, P/S ratio...) Dosimeter response simulations Treatment head simulations Treatment planning dose calculations 2

Correction factor for bone for 6 MV photons 1.20 1.15 1.10 1.05 1.00 water bone Correction factor for bone Cl2300 C/D 6MV calculated by FOCUS measured with ion chamber calculated by EGS4/DOSXYZ water Comparisons of Pencil Beam and Monte Carlo Mohan et al (1997) Conventional Method (Pencil Beam) PEREGRINE 70 Gy 0.95 0.90 Tumor 70 Gy 0.85 0.80 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 cm Depth (cm) Dose Verification for IMRT (Pawlicki et al 2000) 100 DVH Comparison 80 MC Corvus GTV Monte Carlo Gy 16.2 14.4 12.6 9.0 5.4 1.8 Corvus Volume (%) 60 40 20 RT Lung Cord 0 0 5 10 15 20 Dose (Gy) 3

Implementation of MCTP Simulation of Clinical Accelerators Accelerator simulation Source modeling Beam commissioning CT data conversion and phantom setup Dose calculation algorithms Data processing and plan evaluation Plan optimization Tissue Type/Mass Density Determination Frequency 0.05 0.04 0.03 0.02 0.01 0.00 (Pawlicki 1998) 512 x 512 128 x 128 64 x 64 0 200 400 600 800 1000 1200 1400 CT Number CT Number to Medium Conversion 2000~2.088g/cm3 125~1.101g/cm3-700~0.302g/cm3-950~0.044g/cm3-1000~0.001g/cm3 Mass density (g/cm^3) 2.5 2.0 1.5 1.0 0.5 0.0 air 0 500 1000 1500 2000 2500 CT Number teflon Skeleton - Ribs polycarbonate Vertebral body water polyethylene Alderson Muscle Alderson Lung 4

M-C Simulation Geometry Source plane contours Density map Medium map Beam modifier CT phantom Monte Carlo Algorithms Electron beams reported VMC DOSXYZ MCDOSE/MCSIM PENELOPE MCRTP DPM Photon beams reported XVMC PEREGRINE MCNP DOSXYZ MCDOSE/MCSIM/MCRS Current Status of MCTP BrainLab NOMOS Varian Accuray CMS Nucletron AAPM TG157 Survey 5

Measured vs MC Reconstructed Dose Distributions Beam Commissioning for MC 6 MV 40cmx40cm 18 MV 40cmx40cm Yang et al, Phys Med Biol (2004) 49: 2657-73 Combined Dose Distribution: Film vs Monte Carlo Patient plan (Corvus) Delivery in film phantom Monte Carlo dose calculation Calculations vs. Measurements superior inferior 3 1 2 cm from isocenter Energy M-C Meas CORVUS 4 MV 2.177 Gy 2.177 Gy 2.201 Gy 15 MV 2.146 Gy 2.161 Gy 2.276 Gy Lee et al (2001) 6

Treatment Plan Comparison Prostate Treatment Plan Comparison Monte Carlo Gy 77.2 70.0 56.1 48.9 35.0 27.8 21.1 13.9 Corvus Prostate T11-L1 Vertebra Volume (%) 100 80 60 40 20 Rectum MC Corvus Bladder Prostate Monte Carlo Gy 17.6 15.6 13.7 11.7 9.8 7.8 5.9 3.9 2.0 Corvus 0 0 15 30 45 60 75 Dose (Gy) 90 7

100 T11-L1 Vertebra CyberKnife Case Study Volume (%) 80 60 40 20 Cord MC Corvus GTV 0 0 5 10 15 20 Dose (Gy) Comparison of Dose Distribution Variation of Dose Kernels with Density 12.5 cm depth 5 cm depth High-density medium Low-density medium 8

d=3.3cm calculated d=3.3cm measured d=10cm calculated d=10cm measured Variation of Dose Kernels with Density Beamlet Dose Distributions High-density medium Low-density medium 90 70 50 5 FSPB 90 70 50 5 Monte Carlo High-density medium The accuracy depends on how it is implemented. Pawlicki and Ma (2001) Effect of Couch Bar on IMRT Dose Distribution Attenuation for 18 MV photons Other Applications for Advanced RT 40 30 Relative Dose (%) 20 Without bar 10 0-20 -10 0 10 20 Off-axis Distance (cm) MC vs film measurement Yang et al 2005 With bar 9

100 Effect of Couch Bar on IMRT Dose Distribution IMRT using 18 MV photons 18MV DVH 100 IMRT using 6 MV photons DVH Data TPS Monte Carlo Volume (%) 80 60 40 bladder-no bar rectum-no bar CTV-no bar bladder-with bar rectum-with bar CTV-with bar Volume (%) 80 60 40 bladder - no bar rectum - no bar CTV - no bar bladder - with bar rectum - with bar CTV - with bar 20 20 0 0 50 100 150 200 250 300 Dose (cgy) 0 0 50 100 150 200 Dose Yang et al 2005 Dose Volume Histogram (H&N) 100 Pencil beam Volume (%) 80 60 40 LT Eye Target Monte Carlo MC for Modulated Electron RT Optic chiasm 20 0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Dose (cgy) 10

MU real patient vs.water tank (MC / Water tank= 292 / 256=1.14) MU real patient vs.water tank Target 1,2 MC based MU Target 1,2 water tank based MU Lt eye water tank based MU Rt eye water tank based MU Rt eye MC based MU Lt eye MC based MU Courtesy of Joanna Cygler Internal mammary nodes (MC / Water tank= 210 / 206=1.019) Isodose Distributions of MBRT for Hypofractionated Breast Treatment Courtesy of Joanna Cygler A. Photon IMRT 58.0Gy 56.0Gy 47.3Gy 45.0Gy 42.8Gy 40.5Gy 36.0Gy 31.5Gy 22.5Gy 13.5Gy 4.5Gy B. Electron beam C. Combined dose 11

Mixed Beam Breast Plan Statistics and Denoising Optimize IMRT based on a fixed e- field Optimize IMRT and the e- field weight Xiong et al (PMB 2004) Optimize IMRT and MERT Statistics and Smoothing Techniques Dose Prescription Smoothing or denoising is safe at 3-4% statistical level 12

CTV or PTV Dose Prescription Dose to what medium? 4% stats 2mm σ 4mm σ 9% 5% 3% 1% Dose prescription based on DVH and isodose distributions Dose to Water or Dose to Medium? Summary MC dose calculation is becoming a practical tool for advanced radiotherapy treatments The accuracy of MC dose calculation depends on the implementation The potential of MC dose calculation remains to be explored 13

Acknowledgments The FCCC/Stanford Monte Carlo Team Charlie Ma Bob Price Lili Chen Eugene Fourkal Jinsheng Li Lu Wang Yan Chen James Fan Teh Lin Max Jin Steve Jiang Todd Pawlicki Jun Deng Bilal Shahine Ajay Kapur Michael Lee Qianyi Xu Iavor Veltchev Alain Tafo Ahmed ElDib Francis Tang Sotirios Stathakis Jay Chen Wei Luo Jie Yang Lihong Qin Meisong Ding Omar Chibani Grisel Mora Thai Bing Nguyen William Xiong Antonio Leal Freek Du Plessis Thank You FCCC Monte Carlo course, April 2010 14