SCIPP Towards Proton Computed Tomography L. R. Johnson, B. Keeney, G. Ross, H. F.-W. Sadrozinski, A. Seiden, D.C. Williams, L. Zhang Santa Cruz Institute for Particle Physics, UC Santa Cruz, CA 95064 V. Bashkirov, R. W. M. Schulte, K. Shahnazi Loma Linda University Medical Center, Loma Linda, CA 92354 Proton Energy Loss in Matter Proton Tomography / Proton Transmission Radiography Proton Transmission Radiography Data Proton Transmission Radiography MC Study
Computed Tomography (CT) CT: Based on X-ray absorption Faithful reconstruction of patient s anatomy Stacked 2D maps of linear X-ray attenuation Coupled linear equations Invert matrices and reconstruct z- dependent features Proton CT: replaces X-ray absorption with proton energy loss reconstruct mass density (ρ) distribution instead of electron distribution X-ray tube Detector array
Radiography: X-rays vs. Protons Attenuation of Photons, Z N(x) = N o e - µ x Energy Loss of Protons, ρ E = de dx dx de ρ dx l µ 10 4 100 [1/cm] 1 X-Ray Absorption Coefficient Bone Muscle H2O Fat 0.01 1 1 10 100 1000 10 100 1000 X-Ray Energy [kev] NIST Data Proton Energy E [MeV] Measure statistical process of X-ray removal 100 de/dl [MeV/cm] 10 Stopping Power for Protons de dl de = ρ dx Bethe-Bloch Low Contrast: ρ = 0.1 for tissue, 0.5 for bone Bone Muscle H2O Fat Measure energy loss on individual protons
Negative Slope in the Bethe-Bloch Formula 300 Proton Energy Loss in H 2 O 10 Relatively low entrance dose (plateau) Maximum dose at depth (Bragg peak) Rapid distal dose fall-off 200 Proton Energy [MeV] 100 E = 130 MeV E = 250 MeV Energy Deposit in 1mm [MeV/mm] 5 RBE close to unity 0 0 0 10 20 30 40 50 Water Depth [cm] Imaging Treatment
Protons vs. X-Rays in Therapy Protons: Energy modulation spreads the Bragg peak across the malignancy X-rays: High entrance dose Reduced dose at depth Slow distal dose fall-off leads to increased dose in non-target tissue
Milestones of Proton Computed Tomography R. R. Wilson (1946) Points out the Bragg peak, defined range of protons A. M. Cormack (1963) M. Goitein (1972) Tomography 2-D to 3-D, Simulations A. M. Cormack & Koehler (1976) K. M.Hanson et al. (1982) U. Schneider et al. (1996) Tomography, ρ 0.5 % Human tissue, Dose advantage Calibration of CT values, Stoichiometric method T. J. Satogata et al. (Poster M10-204) Reduced Dose of Proton CT compared to X-Ray CT
What is new in pct? SCIPP Increased # of Facilities with gantries etc. See the following talk by Stephen G. Peggs) 2 Ph.D. Theses at PSI and Harvard Cyclotron (U. Schneider & P. Zygmanski) Existence of high bandwidth detector systems for protons semiconductors high rate data acquisition ( > MHz) large-scale (6 wafers) fine-grained (100 s um pitch) Concerted simulation effort Exploitation of angular and energy correlations Support of data analysis Optimization of pct set-up (detector, energy,..) Dose calculation
Potential of Proton CT: Treatment Planning X-ray CT use in proton cancer therapy can lead to large uncertainties in range determination Range Uncertainties (measured with PTR) > 5 mm > 10 mm > 15 mm Schneider U. & Pedroni E. (1995), Proton radiography as a tool for quality control in proton therapy, Med Phys. 22, 353. Alderson Head Phantom Proton CT can measure the density distribution needed for range calculation. There is an expectation (hope?) that with pct the required dose can be reduced.
Low Contrast in Proton CT Sensitivity Study: Inclusion of 1cm thickness and density ρ at midpoint of 20cm H 2 O 1 0.8 Energy Loss in Water ρ l [g/cm 2 ] 1.0 1.1 ρ Energy [MeV] 164.1 163.6 Range [cm] 38.2 38.1 TOF [ps] 1309 1311 0.6 Energy Loss 0.4 [MeV/mm] 0.2 2500 200 Proton Energy [MeV] de (250) de (250)+1.1 de (250)+1.5 de (250)+2 Proton Energy ρ 1.0 1.1 1.5 2.0 1.5 2.0 161.5 158.9 37.7 37.2 1317 1325 150 0 5 10 15 20 25 Depth in H 2 O [cm]
Requirements for pct Measurements Tracking of individual Protons requires Measurement of: Proton location to few hundred um Proton angle to much better than a degree Multiple Coulomb Scattering Θ MCS 1 o θ CMS 13.6MeV β p z l / X 0 Average Proton Energy <E> to better than % Improve energy determination with statistics σ = < E> σ E N Issue: Dose D = Absorbed Energy / Mass N/A = Fluence ( for Voxel with diameter d = 1mm 10 5 protons of 200 MeV = 7 [mgy]) D = A N de dx In order to minimize the dose, the final system needs the best energy resolution! Energy straggling is 1-2 %. σ E 1%
Dose vs. Voxel Size for pct Measurements Trade-off between Voxel size and Contrast ( ρ ρ) to minimize the Dose Define voxel of volume d 3 Dose in voxel = D v Take n =20cm/d settings Total dose D = n* D v Dose [mgy] 1 pct: Contrast - Voxel Size - Dose 200 MeV Protons, 3Stdv. 1 0.01 0.01 Require 3σ Significance D ~ 2 σ E 2 ρ d 5 0.0001 10-6 Voxel Diameter d=5mm, Si d=2mm, 2% d=5mm, 2% d=1cm, 2% 0.01 0.1 1 10 Density Difference [g/cm 3 ] 0.0001 10-6
Studies in Proton Computed Tomography Collaboration Loma Linda University Medical Center UC Santa Cruz Exploratory Study in Proton Transmission Radiography Silicon detector telescope Simple phantom in front Understand influence of multiple scattering and energy resolution on image Theoretical Study (GEANT4 MC simulation) Evaluation of MCS, range straggling, and need for angular measurements Optimization of energy
Exploratory Proton Radiography Set-up Proton Beam from Loma Linda University Medical Ctr @ 250 MeV Degraded down to 130 MeV by 10 wax block Object is aluminum annulus 5 cm long, 3 cm OD, 0.67 cm ID Very large effects expected, x = ρ*l = 13.5 g/cm 2 Traversing protons have 50 MeV, by-passing protons have 130 MeV Silicon detector telescope with 2 x-y modules: measure energy and location of exiting protons Wax block Air Air Object Beam from Synchrotron 30 cm 27.3 cm y x Si Modules x y 250MeV 130MeV 50 + 130MeV
Silicon Detector Telescope Simple 2D Silicon Strip Detector (SSD) Telescope of 2 x-y modules built for Nanodosimetry 2 single-sided SSD / module measure x-y coordinates GLAST Space Mission developed SSD 194 µm pitch, 400 µm thickness 6.4 cm GLAST Readout 1.3 µs shaping time Binary readout Time-over-Threshold TOT Large dynamic range 6.4 cm 5 x 64 channels
Time-Over-Threshold ~ Energy Transfer Digitization of position (hit channel) and energy deposit (TOT) Time-over-Threshold TOT TOT charge LET TOT Measurement vs Charge 120 Pulse Threshold 100 80 TOT [us] 60 40 20 0 0 50 100 150 200 Input Charge [fc]
300 250 200 150 100 50 TOT Spectra for Low-energy Protons Calibration of Proton Energy vs. TOT 250MeV 40MeV 24MeV 17MeV 0 0 50 100 150 TOT [us] Good agreement between measurement and MC simulations TOT [us] 100 TOT vs Proton Energy Measurement and Expectation TOT Saturation ToT measured TOT expected σ /Ε Ε 0.4 0.3 0.2 Proton Energy Resolution 0.4 0.3 0.2 10 LLUMC Synchrotron P Beam 10 100 1000 10 4 Proton Energy [MeV] GLAST SLAC Test Beam 0.1 0.1 0 0 10 100 1000 Proton Energy [MeV] Derive energy resolution from TOT vs. E plot
Image of Al Annulus Subdivide SSD area into pixels 1. Strip x strip 194um x 194um 2. 4 x 4 strips (0.8mm x 0.8mm) Image corresponds to average energy in pixel
Energy Resolution => Position Resolution Slice of average pixel energy in 4x4 pixels (need to apply off-line calibration!) Hole filled in Fuzzy Edges Clear profile of pipe, but interfaces are blurred
Image Features: Multiple Scattering: Migration Washed out image in 2 nd plane (30cm downstream) Energy diluted at interfaces (Fuzzy edges, Large RMS, Hole filled in partially) Migration of events are explained by Multiple Coulomb Scattering MCS Protons scatter OUT OF target (not INTO). Scatters have larger energy loss, larger angles, fill hole, dilute energy
GEANT4 MC: Use of Angular Information Si Telescope allows reconstruction of beam divergence and scattering angles Select 2 Areas in both MC and Data A = inside annulus : Wax + Al B = outside annulus : Wax only Angular distributions well understood
GEANT4 MC: Migration Beam profile in slice Migration out of object Energy of protons entering front face Protons entering the object in front face but leaving it before the rear face
GEANT4 MC: Use of Angular Information Angular Cut at Θ MCS of the Wax Less Migration Energy Profile before (after ) Angle Cut Sharp Edges (Energy Average) Sharp Edges (Energy RMS) Angular cut improves the contrast at the interfaces Energy RMS before (after ) Angle Cut
Present status of pct: Conclusions Long tradition, increased interest with many new proton accelerators (see next talk by Stephen G. Peggs) pct will be useful for treatment planning (reconstruction of true density distribution) Potential dose advantage wrt X-rays ( see Poster M10-204 by Satogata et al. ) Use of GEANT4 simulation program aids in planning of experiments Our future plans: Optimization of beam energy (correlation of energy and angle, migration ) (see Poster M6-2, L. R. Johnson et al.) Investigation of optimal energy measurement method Dose contrast - resolution relationship on realistic phantoms