Simulation

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Simulation Acquisition Todd Pawlicki,
Ph.D. Professor Vice-Chair Director,
Division of Medical Physics
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Objectives

• Discuss the CT simulation process
• Discuss applications of imaging to radiotherapy

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Radiation Oncology Workflow
Tx Plan
Consultation
CT Scan
Virtual Sim (Image Fusion)
Plan QA
Imaging
Tx Delivery
Follow-up
Slide courtesy of Jack Yang, PhD
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Acquisition of Patient Data

• 2 Dimensional Process
• Solder wire
• Plaster cast
• Contour plotter
• 3 Dimensional Process
• CT of treatment area
• CT, MR, PET Registration / Fusion

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The CT Simulator

• Identical to regular CT
• Scanner, couch, computer console, etc.
• Flat hard top couch
• Same as linac
• Important for position reproducibility
• Laser marking system
• Virtual simulation workstation
• 3D image
• Larger aperture size
• 70 to 90 cm

Slide courtesy of Jack Yang, PhD
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Computed Tomography

• The use of CT data in radiation therapy
• Diagnosis
• Tumor / Normal tissue delineation
• Treatment planning / beam orientations
• Dose calculation

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Types of CT Scanners
http//www.asnt.org/publications/Materialseval/bas
ics/may00basics/may00basics.htm
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Measured Linear Attn Coefficients
Hounsfield Unit (HU) -1000 lt HU lt 1000
12 bit Internal Computer Number (C)
0 lt HU lt 4096
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Tissue Types
Sternum (1030)
Soft Tissue 1 (1050)
Soft Tissue 2 (875)
Lung (285)
Soft Bone (1220)
Hard Bone (1810)
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Window Width and Level

• Window Width
• The range of gray shades in the image
• Window Level
• The middle HU of the window
• Window adjustments are set to highlight the
  structures to be viewed

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DRR Image Quality
Slide courtesy of Jack Yang, PhD

• Scanning slice thickness creates different
  quality of DRR, thinner slices produce better
  images, but need more processing power
• Need balance between large amounts of data and
  image quality (scanning protocols need to be
  established)

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DRR and Image Quality
RTCT
CBCT
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CT Simulator Pros and Cons

• Small Bore vs. Large Bore
• Patient population with different setup technique
• Conventional simulator availability
• Larger bore creates more scattering dose
• Single Slice vs. Multi Slice
• DRR quality (rendering technique and speed)
• Image quality (Scanning slice thickness)
• 4D CT (respiratory gating)

Slide courtesy of Jack Yang, PhD
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CT Simulator Large Bore (FOV)
70 cm Bore Opening
85 cm Bore Opening
Better for Breast Simulation
Slide courtesy of Jack Yang, PhD
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Isocenter Marking

• Isocenter placement could affect the patient
  treatment accuracy since it serves as the base
  line information

Slide courtesy of Jack Yang, PhD
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General Immobilization Equipment
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Immbolization Options(Active)
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Immbolization Options(Passive)

• shoulder constraints

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CT Sim Head Immobilization
Achievable variability in position1-3 mm
Slide courtesy of Jack Yang, PhD
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Reproducibility

• Difficult to reproduce a misaligned patient
• Other artifacts can also create setup problems

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Head Position

• Optimal position depends on the treatment volume
  and delivery type

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Example CT Simulation Checklist
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CT Simulation Policy Procedure
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CT Simulation Contrast Issues

• Contrast can be used to help differentiate
  between tumors and surrounding healthy tissue
• Using contrast is risky, nursing required to be
  present
• For heterogeneity-based CT planning, contrast may
  create dose distribution errors due to large CT
  numbers
• Contrast density override should be carefully
  examined

Slide adapted from Jack Yang, PhD
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Simulation General Issues
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CT Used for Dose Calculation
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CT/MR Anatomy

• Primarily used for target delineation

CT
MR
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Bushburg et al. The Essential Physics of Medical
Imaging. 2nd Ed, 2002.
T1 Weighted
T2 Weighted
FLAIR
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MRSI

• MR spectroscopy and MR imaging methods
• Produce a spectrum identifying different chemical
  compounds (metabolites) in various tissues
• Metabolite ratios differentiate between active
  tumor, normal tissue, and necrosis

Hunjan et al. IJROBP, 2003.
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MRSI
Delorme and Weber. Applications of MRS in the
evaluation of focal malignant brain lesions.
Cancer Imaging, 2006.
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FDG PET
(fluorodeoxyglucose PET Imaging)

• Malignant cells compared to healthy cells
• Divide rapidly
• Metabolize glucose at a higher rate
• Attach a positron emitter to a glucose analogue
• Example, fluorine-18
• FDG PET studies show utilization of the glucose
  analogue
• Tumor metabolism

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PET/CT Images
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Registration / Fusion

• Choose fusion algorithm
• Transform source image into coordinate system of
  destination image
• Evaluation and interpretation of the registered
  images

Hutton and Braun 2003
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Choose Fusion Algorithm
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Use Pixel Data
Hutton and Braun 2003
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Manual Match

• Interactive tools

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Fusion Evaluation
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Summary

• CT Simulation is replacing conventional
  simulation
• Multi-modality imaging is increasingly important
• CT Simulation can improve target and normal
  tissue contouring and treatment planning
• TG-66 report provides a very thorough QA process
  for CT simulation