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Management of Moving
Targets in Radiotherapy
Steve B. Jiang, Ph.D.
Department of Radiation Oncology
jiang.steve@mgh.harvard.edu
http://gray.mgh.harvard.edu/
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A Simplified View of the RT Process
¾ Treatment preparation
− Treatment simulation
‰Build a virtual patient model (reference geometry)
− Treatment planning
‰Perform virtual treatment using virtual machine on
virtual patient
¾ Treatment execution
− Patient positioning
‰Reproduce the reference geometry acquired through
treatment simulation and used for treatment planning
− Treatment delivery
‰Maintain the reference geometry
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Image Guided Radiation Therapy
¾ There should be a consistency in the
patient geometry throughout the whole
treatment course.
¾ How to keep the patient geometry close to
the reference model in the machine
coordinate system, or how to update the
model to accommodate the changing
patient geometry, are exactly the research
topics of IGRT.
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Two types of patient/organ motions: Inter- and intra-fractional motions
Figure Courtesy of Di Yan
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Tumor Home Position and Instant Position
Respiratory motion is both intra- and inter-fraction motion
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Treatment Simulation of Mobile Tumors
¾ To build the reference patient/tumor model
at the reference home position
¾ Errors in treatment simulation will
influence all treatment fractions and
should be handled carefully
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Three Types of Motion Artifacts
¾ If CT scanning speed << tumor motion
speed, → smeared tumor image
¾ If CT scanning speed >> tumor motion
speed, → tumor position and shape
captured at an arbitrary breathing phase
¾ If CT scanning speed ~ tumor motion
speed, → tumor position and shape
heavily distorted
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Move Objects during CT
Courtesy of George T.Y. Chen
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Motion Artifacts
Photo
Static
Moving / HS mode
Courtesy of George T.Y. Chen
HQ
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CT Artifacts in Patients
Helical light breathing scan
4DCT – 1 phase
Courtesy of Eike Rietzel and George T.Y. Chen
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4D CT Scan
GE Lightspeed Scanner
Record amplitude
and breathing phase
for each slice
Varian RPM system
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Resorting 500-1500 CT Slices
Courtesy of Eike Rietzel and George T.Y. Chen
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4DCT Scan of A Phantom
Courtesy of Eike Rietzel and George T.Y. Chen
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4DCT Scan of A Lung Tumor
Courtesy of Eike Rietzel and George T.Y. Chen
Defining the gating window
ungated
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gated
5 mm residual clip motion in SI direction
Patient 5618
Chen et al., MGH 2004
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Some Notes on 4D CT
¾ 4D CT scan not only reduces motion
artifacts, but also gives the tumor/organ
motion information.
¾ 4D CT scan is NOT really 4D.
− Temporal information is mapped into one
breathing cycle.
− Irregular respiration will cause artifacts in 4D
CT images.
− Breath coaching is always needed.
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Techniques for Treating Moving Tumor
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Techniques for Treating Moving Tumor
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“Margin” Techniques for Mobile Tumors
¾ Patient population-based CTV-PTV margin
¾ Patient specific CTV-PTV margin
− measured for a specific patient through
fluoroscopy or 4D CT scan
¾ ITV based on 4D CT or slow CT scan
¾ IMRT optimization using motion PDF
− The more patient specific motion information
is used, the more accurate motion “margin”
results. However such info can vary during the
treatment course.
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Current Status of MLC Tracking
¾ Various algorithm development work
− MCV, MGH, Indiana Univ, Steve Webb, etc
¾ MCV implementation
− Leaf motion based on real time tumor position
¾ MGH implementation
− Synchronized Moving Aperture Radiation
Therapy (SMART)
¾ May have a role in SBRT (efficiency)
¾ Precise localization of tumor position in
real time is required
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SMART: Planning/Simulation
Measure average
tumor trajectory (ATT)
IMRT inverse planning
using CT data at a
particular breathing phase
SMART: Include tumor motion into IMRT
MLC leaf sequence
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SMART: Treatment Delivery
‰Monitor tumor motion
‰Synchronize the MLC motion with tumor motion
‰Interrupt and restart the treatment when target motion
differs from the simulated pattern
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Two Types of Gating
¾ Internal gating (fluoroscopic gating)
− Use internal tumor motion surrogates such as
implanted fiducial markers to indicate tumor
position
− Mitsubishi/Hokkaido RTRT system
¾ External gating (optical gating)
− Use external respiratory surrogates such as
abdominal surface to derive tumor position
− Varian RPM system
Real-Time Tumor Tracking System
Image intensifier
X-ray tube
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ƒ 4 sets diagnostic
X-ray systems
ƒ Motion tracking
software
ƒ Carbon fiber
couch
ƒ 2 mm gold marker
inside/near tumor
ƒ 1 mm detection
accuracy
ƒ 0.033 s time delay
Shirato et al IJROBP 48:435-442, 2000
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Tumor Tracking System at Hokkaido University
Courtesy of H Shirato
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Problems with Internal Gating
¾ Invasiveness of the marker implantation
procedure
− Might be acceptable for liver and other
abdominal sites
− Not for lung due to the risk of pneumothorax
¾ Significant imaging dose may be required
for fluoroscopic tracking
Varian RPM System
camera
Marker block
Photos courtesy of P Keall
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Problems with External Gating
¾ Uncertainties in deriving tumor position
from external surrogates
− The correlation between tumor position and
external marker position can vary inter- and
intra-fractionally.
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Accurate External Gating
¾ During treatment simulation, the reference home
position should be accurately measured, using
techniques such as 4D CT.
¾ During treatment planning, the patient and tumor
geometry at the reference home position should
be used.
¾ During patient setup, the tumor daily home
position should be matched to the reference
home position.
¾ During the treatment delivery, the tumor home
position should be maintained the same.
Key Components for Accurate
External Gating
¾Patient breath coaching
¾4D CT simulation
¾Patient setup based on gated
tumor image
¾Treatment verification
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Hardware Platform for MGH Gating Procedure
X-ray
Tubes
Camera
Marker
Block
Video
Goggles
Flat
Panels
EPID
Breath Coaching Using RPM System
¾ Audio instruction
¾ Visual feedback
¾ 5 volunteers with 6
sessions (free breathing
vs coached breathing)
¾ ~ 30 patients with one
session (free breathing
vs coached breathing)
T Neicu, R Berbeco, J Wolfgang, and SB Jiang, Phys Med Biol. 51(3):617-636.
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A New Breath Coaching Protocol
Free breathing
Coached breathing
Patient 5618
Patient compliance is crucial!
T Neicu, R Berbeco,
J Wolfgang, and SB Jiang,
Phys Med Biol. 51(3):617-636.
Gated Radiograph for Patient Setup
LAT
AP
Each radiograph requires ~1/5 dose of film
Patient 5618
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Gated Radiograph for Patient Setup
DRR
Radiograph
XL Tang, GC Sharp, and SB Jiang, 2006.
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EPID Images versus DRRs
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~12 cm
t = 4.8 s
t = 11.2 s
t = 17.6 s
t = 24 s
t = 30.4 s
t = 35.2 s
t = 44.8 s
t = 48 s
RI Berbeco, T Neicu, E Rietzel, GT Chen, and SB Jiang, Phys Med Biol 50(16):3669-3679, 2005.
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Future Development of Gated Radiotherapy
¾Combine external and internal
signals
− More accurate than external gating
− Less imaging dose than internal gating
¾Lung treatment without implanted
markers
¾Issues with gated IMRT
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Combine External/Internal Signals
¾ External gating, internal verification
− Dynamically display tumor positions in the
gating window, detected in x-ray images
¾ Double gating
− External signal → imaging → tumor position →
linac
¾ Hybrid gating
− Drive tumor positions from external surrogates
− Low frequency updating of external/internal
correlation
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Gating Directly Based on Tumor Mass
Patient
Setup:
Treatment:
Training
Fluoroscopic
Images
Motion
Enhanced
Images
Reference
Templates
Incoming
New Images
Motion
Enhanced
Images
Template
Matching and
Scoring
RI Berbeco, H Mostafavi, GC Sharp, and SB
Jiang, Phys Med Biol. 50(19): 4481-4490, 2005.
Gating
Signal
Multiple-Template Based Tracking
Y Cui, J Dy, GC Sharp, and SB Jiang, 2006.
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Tracking with Deformation
QY Xu, RJ Hamilton, and SB Jiang, 2006.
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Issues with Gated IMRT
¾ Treatment verification
¾ Treatment time
− May not be a problem
for Step and Shoot
¾ Biased beam-on time
in the gating window
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Acknowledgement
¾ MGH
− George Chen, Ross Berbeco,
Greg Sharp, Toni Neicu, Lily
Tang, John Wolfgang, Noah
Choi, et al
¾ Varian
− Hassan Mostafavi, Andrew
Jeung, et al