patient positioning - scott memorial...
TRANSCRIPT
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Patient Set-ups and Tumor Localizations
Amy S. Harrison
Patient Positioning
• Prior to starting any localization or
simulation procedure patients need to
be positioned and immobilized
• Patients disease location and
physical limitations must be taken
into consideration
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Patient Alignment
Reproducible positioning of the entire
patient, not just the treatment region
is imperative
A small angle change of the patient on
the table can represent a significant
change in the delivered treatment
Exaggerated Patient Position Shift
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Patient Positioning-Brain/H+N
• Immobilized by aquaplast masks over
the head alone/ or head and shoulders
• The need for a bite block should be
addressed
• Head holders should be selected for
patient comfort and extension of neck
• Indexing of immobilization improves
reproducibility of set up
Head Immobilization
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Head Immobilization
Full Set Up Photo H+N Case
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Lung
• Alpha cradle
• Arms up? Arms down?
• ABC device
• Abdominal Compression Device
• Leg immobilization-rubber band, plastic
foot holder, angle sponge?
• Will it clear CT/MRI bore
Stereotactic Lung
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Lung
Lung
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Breast Immobilization
• Alpha cradle or wing board
• Opposite arm immobilized how?
• ABC device
• Leg immobilization-rubber band,
plastic foot holder, angle sponge?
• Will it clear CT/MRI bore
Breast
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Breast
Breast
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Breast
Prostate/Pelvis
• Pelvis-full aquaplast, alpha cradle
or nothing
• Legs on angle sponge or flat
• Feet rubber bands or foot holder
• Arms?
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Prostate/Pelvis
Prostate/Pelvis
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Prone Rectum
• Belly board, angle sponge?
• Feet?
• Patient on pillow or not?
Rectum/Prone
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Prone Rectum
Prone Rectum
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Immobilization vs. High Tech
• IGRT- cone beam or fiducials
• Tomotherapy
• Cyber Knife
• ExacTrac
Localization
Procedure where the target and critical
structures are delineated with reference to
the patient’s external surface
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Classical Localization
• The patient’s external surface was attained by
using solder wire or plaster of paris
• This surface was then drawn on a piece of
paper
• Or pantograph
2D Contours
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Classical Localization
• Tumor and critical structures were
transcribed from hard copy CT studies or MD
demarcation on the orthogonal films taken at
the time of localization
2D Contours
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2D Contours
2D Contours
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2D Contours and Coordinates
Simulation
• A procedure where the planned
fields are verified by shooting
diagnostic quality films in the
simulator (a machine which mimics
the geometry of the treatment unit))
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The CT Simulator
• The advent of the CTsim dramatically
modified the simulation and localization
procedures.
• Localization could now be done at the
time of Ctsim
• Target delineation was truly 3D
• Precision and accuracy greatly improved
An integral functionality of the CT Simulator unit is the capability of placing reference marks on the patient to
indicate the isocenter for the treatment fields.
1) Reference marks can be placed near the isocenter of
the patient. This is an estimate of the final isocenter and
does not require extensive contouring. The isocenter is
found through a series of exact moves from the reference
marks.
2) After detailed contouring, the final isocenter position is
marked on the patient.
The Ct Sim For Patient Marking
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XIO FocalSim
3D Contours and Coordinates
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3D Contours and Coordinates
3D CT Images
• Scanned =
Transverse/
Axial
• Generated=
Sagittal
• Generated=
Coronal
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Virtual Simulations
• Ct Sims allowed the verification
simulation process to become
virtual
• Traditional 2D fields could be set in
the 3D dataset
• The patient could be scanned,
marked and sent home given an
appointment time for treatment
XIO Focalsim
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Advantages of the CT Simulator
• Virtual simulation/verification process
• Generating a new/conedown plan can
be accomplished without having to
bring the patient back for another
simulation.
Disadvantages of the CT Simulator
Structure motion cannot easily be
detected with a CT Simulator. Excluding
4D scanners.
• CT doughnut is usually restricted to 70-
80cm in diameter .This can limit the
patient's position for some treatments. For
example, placing the patient's arms up can
be a problem.
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CT Simulators
CT scan process allows 3D volumetric
information to be gathered and carries
out simulation as a digital process.
Relies on construction of digitally
reconstructed radiographs
QA of CT-Simulators
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HISTORICALLY: QA of CT Scanners
• CT scans for treatment planning are often done with a flat top insert on the CT table to reproduce the radiation therapy treatment couch top.
• laser system mimicking that used on the simulation and treatment units
should be mounted in the CT suite and the alignment of the lasers should be checked daily. Such a system is an integral component for relating the patient’s position during CT with that on the simulation and treatment machines.
• The correlation of CT numbers with electron densities and the variation of CT numbers with position and phantom size should be determined. Since this correlation is a function of the quality of the x-ray beam, it should be
checked yearly.
• In addition, the CT scanner should be checked for image quality and other parameters described in the QA protocol provided by the manufacturer.
• QA of CT scanners (AAPM, 1977).
2003
• Quality assurance for computed-tomography
simulators and the computed tomography-
simulation process:
• Report of the AAPM Radiation Therapy
• Committee Task Group No. 66
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AAPM Task Group #66
Mechanicals
Common Sense Applies
• +/- 2mm most items
• Table indexing and motions
are 1mm
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Spatial integrity
QA goals: CT-simulation images should
accurately reproduce true patient anatomy
within 1 mm without spatial distortions in the
entire scan field. This should be verified for both
head and body scan protocols using a phantom
of known dimensions.
Spatial resolution
• Characterizes the imaging system’s
ability to distinguish between two very small
objects placed closely together.
• Spatial resolution is frequently referred to as
high contrast resolution
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High contrast resolution
• most commonly measured using either a resolution pattern ~line pair phantom with a range of spatial frequencies!, or by the modulation transfer function ~MTF! method.
• The line pair pattern in following slide ranges in frequency from 1 lp/cm to 21 lp/cm. Note the Bead in the phantom phantom which is a high-density, tungsten carbide bead which is used to create an impulse, or point source, from which the MTF can be calculated.
• Manufacturers often specify the limiting spatial resolution at the 5% or lower point on the MTF curve. The limiting spatial resolution ~lp/cm measured with MTF, and specified at the 5%value, is typically higher than the resolution that can be observed with a line pair phantom.
CT Scanner Line Pairs
• Slide Images from http://health.siemens.com/ct_applications/somatomsessions/index.php/image-quality-in-computed-tomography-2/
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MTF
• Plots the contrast
against the
resolution
• Completely
characterizes the
high-contrast
resolution of the
scan mode
• Slide Images from http://health.siemens.com/ct_applications/somatomsessions/index.php/image-quality-in-computed-tomography-2/
High
Contrast
Low
Contrast
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Contrast resolution
• Contrast resolution can be defined as the CT-scanner’s ability to distinguish relatively large objects which differ
only slightly in density from background.
• QA goals: Quality assurance should demonstrate that the CT-scanner meets or exceeds manufacturer specifications for
low contrast resolution
Sensitivity and Profiles
• Slide Images from http://health.siemens.com/ct_applications/somatomsessions/index.php/image-quality-in-computed-tomography-2/
Slice sensitivity is a curve
showing the effect of broadening
of the CT slice
Thickness along patient in helical
CT
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Image Performance
When referencing
manufacturers
specifications
tolerances are set to
the acceptance criteria
and
can then be called the
baseline measurement
CT Sim Software:
• Image input test
• Structure delineation (contouring)
• Multimodality image registration
• Machine definition
• Isocenter calculation and movement
• Image reconstruction
• Evaluation of digitally reconstructed
radiographs
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Evaluation of digitally
reconstructed radiographs
• Spatial and contrast resolution: It is generally understood that
smaller slice thickness and spacing produces better spatial resolution DRRs.
• Geometric and spatial accuracy: Magnification should be within 1
mm of expected. Spatial errors ~e.g., collimator, table rotation, incorrect jaw
setting, etc.! can also cause errors which may not be detected from patient
port films. The QA for the CTsimulation process should include evaluation of
DRR geometric errors.
• Hardcopy quality: Printing of standard test patterns and
comparison with baseline data can reveal potential problems
EVALUATION OF THE
CT-SIMULATION PROCESS
• Overall process tests: Patient positioning and
immobilization, Scan limits, Scan protocol,
Contrast, Special considerations and
instructions, Data acquisition,
Localization/marking, Virtual simulation, DRR
and setup documentation
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DRR
• Digitally constructed images from the
3D dataset
• Generated with the same geometry as
a divergent radiograph produced with a
point source of radiation.
• Created by combining the influence
of the CT pixel elements from a CT
dataset along divergent ray lines.
DRR Tools
• Computer generated films allow
selection of the bony anatomy
needing to be imaged
• DRR’s can be adjusted by using a
window/leveling tool
• Drr’s can be generated for any
treatment angle there are no
collision issues in virtual space
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DRR Region of Interest
DRR Quality
• # of Slices
• CT # accuracy
• Slice thickness
• Scan technique used
• Reconstruction algorithm
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DRR Artifacts
• Contrast Agents
• Prosthesis
• Respiratory Motion
• Anatomy (inadequate scan technique)
Scanner Types
• First Generation: Translate/Rotate
• Second Generation:
Translate/Rotate
• Third Generation: Rotate/Rotate
• Fourth Generation: Rotate/Fixed
• Spiral CT (3rd or 4th generation
type)
• Cine Ct
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•Retrieved from http://www.impactscan.org/download/msctdose.pdf
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Single Slice Spiral CT Pitch
Pitch = (table increment
distance (mm) per 360° gantry
rotation) / slice thickness (mm)
slice thickness 5mm, table
motion 7.5mm/rotation , Pitch
= 1.5
Pitch of 1=adjacent rotations
Pitch>1 = gaps between x-ray
beams from adjacent rotations
Multi Slice Spiral CT Beam Pitch
= (table increment distance (mm) per
360° gantry rotation) / slice
thickness (mm) X n (number of
slices acquired)
slice thickness 5mm, on a 4 slice
scanner, table motion 15
mm/rotation
Beam pitch = 15 / 4 x 5 = .75
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Slice Sensitivity
Image Reconstruction
• Iterative Solvers-slower but better when
missing data
• Analytic Methods-Fourier Analysis
• Filtered back projection
•Can begin with first data acquired
•Can be “hard wired” into system
(speed)
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Image
Reconstruction
Ramp
Filters
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Cone Beam CT
• Planar images are acquired with the kV or MV
imaging system.
• Volumetric image reconstruction is performed
Houndsfield Units
The relative attenuation coefficient ()
is usually expressed in HU aka CT
numbers
HU= 1000 x (x- water)/ water where x
is the attenuation coefficient of
material x and water is the
attenuation coefficient of water
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HU vs CT
CT Numbers are based on manufacturer
constant “K”
CT = K x (x- water)/ water where x is the
attenuation coefficient of material x and water
is the attenuation coefficient of water
Houndsfield Units
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Window
and
Level
Lung Windows
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Soft Tissue Windows
Relation of FOV, Matrix
Size and Pixels
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Beam Hardening
Figure 15b. CT images of a patient with metal spine implants, reconstructed without any correction (a) and with metal artifact reduction (b).
Barrett J F , Keat N Radiographics 2004;24:1679-1691
©2004 by Radiological Society of North America
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Partial Volume Effects
Motion
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Typical Doses
Magnetic Resonance Imaging
• Study of the magnetic properties
of the nucleus
• Nuclei under a strong magnetic
field absorb energy which is
then released at a later time
• This time period is unique to the
nuclei and surrounding area
• T1 and T2 are time values
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T1 Images
T2 Images
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MRI: The Pros and Cons
Pros: 1. Better soft tissue imaging
2. Multiplane imaging
3. Data unaffected by bones
Cons: 1. Image distortion
2. No electron density
information-cannot be used for
dose calculation w/o CT fusion
Positron Emission Tomography
• Functional images: provides information
about physiology instead of anatomy
• Generates transverse images depicting the
distribution of positron emitting nuclides
• MUST be fused with CT images for
treatment planning
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PET Continued
When positron annihilates it emits two
511keV photons in nearly opposite
directions; these photons interact with the
annihilation coincidence detectors and
obtain projections of the activity
distributed in the patient
Image Fusion – 4 Techniques
1. Coordinate transformation
Fiducial markers/stereotactic frames
2. Surfaced based registration
The surfaces of one or more structures are matched and used for computation and minimizing mismatch of the data set. Useful with skull or pelvis.
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Image Fusion Continued
3. Image Based Registration
Grayscale data is used directly to measure mismatch or similarity between datasets (Mutual Information-measurement of redundant data)
4. Interactive Techniques
Effective in cases with a limited number of degrees of freedom. Verified visually. Can be used to limit the amount of time needed for calculation based fusion.
Fusion
• Once the datasets are fused structures may
be mapped from one dataset to another
• So target volumes may be delineated on an
MRI or PET and transferred to CT data for
planning
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MRI/CT Mutual Information
MRI/CT Grayscale Visual Check
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PET/CT Pre-Fusion
PET/Ct Post Fusion
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What is the fourth dimension?
• Time and therefore motion
4D CT Scan Measures Lung Cancer Motion
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4D CT scan
•GE Lightspeed with
Varian RPM system
captures repeat CT
images at each couch
position during respiratory
cycle
•CT sample
interval 15-20
images/slice
position
Pan et al, Med Phys 31, 333 (2004); Med Phys 34, 4499 (2007)
4D thoracic CT imaging
Vedam et al PMB 2003
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What use are 4D CT scans?
• Determine tumor motion/screening
• Motion inclusive treatment
• Respiratory gated treatment
• 4D radiotherapy
All video images on 4d treatment techniques are curtesy of Paul Keall
4D CT in radiotherapy
• Scenario 1: No respiratory motion management
devices
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Exhale Inhale
Tumor
Inhale & exhale CT phases
Tumor
Motion
encompassing
volume
Motion inclusive treatment
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Scenario 2: Respiratory gating
• Acquire 4D CT
• Select respiratory phase(s)
• Delineate GTV/CTV on chosen phase(s)
• Create PTV
• Plan and treat with gating
Beam ON Beam ON Beam OFF
tumor
tumor
tumor
Gating
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Respiratory gated treatment
4D Radiotherapy
• Accuray
• Works in Progress
– Dynamic MLC motion to match target motion
– Dynamic table motion
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Things to Chew Over
• Dynamic delivery will require planning
of each phase of respiration
• What will the QA of the delivery devices
look like
• 2010 question- What if the patient
sneezes?!?
• 2011 answer- 4D conebeam
2011 - 4D CBCT – Elekta
XVI 4.5 Symmetry • Slow gantry motions – about 3 minutes for a 200 degree
rotation
• Software auto correlates data by surface or internal motions
• Motion induced blur of structures reduced
• Streaking artifacts common-more visible in axial images
• Have had patients not treated due variations in respirations – usually caused by coughing from illness-returned next day with cough suppressant
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•Sonke et al, Med Phys 32, 1176 (2005)
4D-CBCT Streaking
•Li & Xing, IJROPBP 67, 1211, 2007
Streaking artifacts can be
Reduced with slower gantry
Rotations = increased times
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4D CBCT Pre-Treatment
Verification
• Thank you so much for your time and
consideration
• Good luck on all your future physics endeavors