Image Guided Radiotherapy

Image Guided Radiation
Therapy
Moderator: Prof. Bhavana Rai
Dr. Namrata Das
1st year Junior Resident
12-10-2018

OUTLINE
• Introduction
- Evolution of radiotherapy techniques
- ICRU definitions
• Types of Error
• Timeline when errors are addressed by image guidance
- Positioning and Immobilization
- Image Acquisition and Registration: 4DCT
- Target Delineation: advanced imaging modalities
- Patient Setup confirmation: Types of in room imaging
• Offline, online correction strategies
- No Action Level (NAL), Shrinking Action Level (SAL)
• Clinical application of IGRT:
- Head and neck cancer
- Lung cancer
- Prostate cancer
- Upper GI malignancy: Liver
• Summary
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Introduction
The purpose of radiotherapy is to maximize tumour kill and minimize normal
tissue damage by delivering a precisely measured dose uniformly to the target
tissue.

• Radiotherapy treatment planning has always used imaging.
• History of development of radiotherapy has closely followed the development of new
imaging techniques.
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Evolution of Radiotherapy Treatment Techniques
2D Planning
3D Planning/
Conformal
Radiotherapy
X Ray Based:
Discovery of X Ray and
subsequent boom during the
World War
Volumetric Imaging:
1970s: Computed
Tomography
1980s: Magnetic
Resonance Imaging
3D CRT: geometrical field shaping
IMRT: geometrical field shaping+
intensity modulation
IGRT: geometrical field shaping +
intensity modulation + image
guidance

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2D/ Conventional
Planning
3D/
Conformal
Planning
Drawbacks
• Inability to visualize tumor or normal
tissue
• Only square/rectangle fields
• Large safety margins resulting in high dose
to normal tissue

ICRU 29 (1978): Dose specification for the reporting of external beam
radiotherapy with photons and electrons
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Evolution of the concept of treatment volumes
Target
volume
Treated Volume
Irradiated Volume
Irradiated Volume: volume which receives an absorbed
dose considered to be significant in relation to tissue
tolerance
Target Volume: Tissues that are to be irradiated to a
specific dose
Treatment volume: Volume enclosed by a relevant
isodose surface that is selected and specified by the
radiation oncologist as being appropriate to achieve the
purpose of treatment

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ICRU 50 (1993): Prescribing, Recording, Reporting Photon Beam
Therapy
Treated Volume
Irradiated Volume
PTV
CTV
GTV
Planning Treatment Volume: Geometrical concept defined to
select appropriate beam sizes, beam arrangements taking
into consideration the net effect of all possible geometrical
variations, inaccuracies in order to ensure prescription done
is actually absorbed in the CTV.
Clinical Target Volume: Tissue volume that contains GTV and/or
microscopic malignant disease which is to be eliminated – has
to be treated respecting anatomical boundaries.
Gross Tumour Volume: Gross palpable or visible or
demonstratable extent and localization of tumour

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ICRU 62 (1999): Supplement to Rep. 50
Planning Treatment Volume (PTV) = Internal Target Volume (ITV) + Setup Margin
(SM)
ITV = CTV+ Internal Margin (IM)
Treated Volume
Irradiated Volume
PTV
CTV
GTV
ITV
Internal Margin (IM): A margin that must be added to the
CTV to compensate for the expected physiological
movements and the variations in size, shape and position of
the CTV during therapy in relation to the internal reference
point.
Internal Target Volume (ITV): CTV +IM
Setup Margin (SM): It is the margin that must be added to
account specifically for uncertainties (inaccuracies and lack of
reproducibility) in patient positioning and alignment of the
therapeutic beams during treatment planning and through all
treatment sessions.

Conformal Radiotherapy: 3D CRT
3 Dimensional Conformal Radiotherapy
(3D CRT)
It is the use of 3 dimensional
anatomical information to plan and
deliver treatment so that the
resultant dose distribution conforms
as closely as possible to the target
volume in 3 dimensions with
minimum dose to the surrounding
normal tissue.
Advantage over 2D:
• increased tumour dose, reduced
normal tissue dose
• Reduced safety margin
Drawback:
• Forward planning: trial and error
method
• Increased dose spillage
• Ability to sculpt tumour limited
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Transition from 2-D Radiotherapy to 3-D Conformal and Intensity Modulated Radiotherapy. IAEA-TECDOC-1588. May 2008.

Intensity Modulated Radiotherapy is
a special form of 3D conformal dose
delivery, enhanced by generating a
non-uniform photon fluence within
each beam, calculated by an inverse
treatment planning process
designed to meet specified
dosimetric objectives.
Advantage over 3D CRT:
• Able to achieve concave dose
distribution
• Allows dose painting:
inhomogeneous dose prescription
to different targets
• Reduced margins
• Inverse planning
Drawback: Chance of missing target
higher due to reduced margins at the
time of treatment
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Conformal Radiotherapy: IMRT

• Uses conformal uniform fields
leading to convex distribution
• By varying the fluence of beams,
more fluence can be delivered to
rays reaching target as compared
to normal tissue. Allows concave
dose distribution
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Need for Image Guided Radiotherapy
The uncertainty in target localization while delivering highly conformal
radiotherapy is specially important for:
• Tumours prone to intrafraction motion: lung cancer
• Tumours prone to interfraction motion: prostate cancer
• Tumours prone to deformation: head and neck cancers
• Treatment delivery by IMRT, SBRT (high dose per fraction with narrow
margins)
Thus there is a need for image guidance at various stages of the treatment
process.
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Definition
• Multiple definitions - some too extensive, others too narrow
• ACR, ASTRO practice guidelines,
“a procedure that refines the delivery of therapeutic radiation by
applying image based targeted re-localization to allow proper patient
repositioning for the purpose of ensuring accurate treatment and
minimizing the volume of normal tissue exposed to radiation.”
• IGRT Committee, RTOG, 2001,
“IGRT refers broadly to treatment delivery using modern imaging
methods such as CT, MRI, PET and USG in target and non target
structures for RT definition, design and delivery”.
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IGRT can be defined as the use of:
• Advanced imaging modalities, especially those incorporating
functional and/or biological information, to augment target and
normal tissue delineation
• In-room imaging to adjust for target motion or positional
uncertainty (interfractional and intrafractional), and, potentially,
to adapt treatment to tumour response
Image Guided Radiation Therapy. A clinical perspective. Arno Mundt, John Roeske.
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Use of imaging for:
• Target and normal tissue delineation
• Margin design and treatment planning
• Setup adjustments and delivery
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Use of advanced imaging
modality for accurate
delineation of target
Use of imaging for setup
verification and adjustment

Errors
• Error is any mismatch between the true anatomy and pathology
during treatment and planning
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Types of Errors
• Deviation that varies in direction
and magnitude for each fraction
• Cumulative dose distribution gets
blurred with random errors
• Direction and magnitude
remains same for each fraction
RANDOM ERRORS SYSTEMATIC ERRORS

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Planned
Isocentre
Y
X
Systematic
error
Random
errors

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Systematic Error
External
Wrong isocentre shift;
discordance between planning
CT and LINAC laser systems
Solution:
On Board Imaging
Internal
Physical changes since planning
CT: drainage of pyometra
Shrinkage of tumour
Solution:
Adaptive replanning

Systematic errors
A. CHOICE OF IMAGING
All tumours and organs cannot be
identified using a planning CT
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B. IMAGE FUSION
Fusing two or more image sets for
delineation can lead to some mismatch –
leading to suboptimal delineation

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C. IMAGE DISTORTION
Planning images can have artefacts due to
motion, foreign objects or matching
related imaging artefacts
1. Movement induced artefact: Fast free
breathing CT Scans produce these errors
C. IMAGE DISTORTION
Planning images can have artefacts due to motion, foreign objects
or matching related imaging artefacts
Prosthetics, foreign metallic/plastic objects can
distort images
Systematic errors

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Systematic errors
Anatomical changes due to:
• Tumour regression: the shrink in tumour will lead
to a change in the dose distribution of OARs (lung,
head and neck)
• Weight loss: changes the anatomic contour of
patients
• Resolving atelectasis: Opening up of blocked
airways

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Random Error
External
Daily setup variation
On board imaging
Internal
Daily position change due to
bowel, bladder filling
Respiratory Motion
OBI
Generation of individualized
ITV margin
Respiratory gating/tracking

Random Error: organ motion and deformation
Seconds:
respiration,
heartbeat
Minutes: Peristaltic
motion, rectal
motion
Days: Radiation
induced changes in
mucosa, bladder,
bowel distension
Weeks: changes in organ
shape and position
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Random Error: organ motion in the pelvis
• Rectal filling and bowel filling changes: inter-fraction variability of
position and rotation
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• Sites affected by respiratory motion:
Lung and mediastinum, breast and
chest wall, upper abdomen (liver,
biliary tract, stomach, pancreas)
• Motion of lung tumours:
- Supero-inferior (longitudinal)
direction: maximum movement
- Lower lobe tumours: greater
movement in SI direction
- Anterior tumours: greater AP motion
- Central tumours have less motion
than peripheral
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Random Error: respiratory motion
Highly variable motion range:
• Lung: SI = 3.9 cm, AP = 2.4cm, LR = 2.4cm
• Liver: Normal breathing: 10-25 mm, deep
breathing = 37-55 mm

Timeline when errors can be addressed
• Positioning and Immobilization
• Image Acquisition and Registration
• Target Delineation
• Treatment Planning
• Patient specific dosimetric verification of treatment plan
• Patient Setup confirmation
• Treatment Delivery
• Quality assurance
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Immobilization in IGRT
Rationale:
• Reduces interfraction and intrafraction variation in position
• Reduces intrafraction motion of anatomic region or specific targets
• Reduces dependence on frequent imaging
• Reduce setup and imaging time
• External or internal
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Immobilization aids
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BRAIN HEAD AND NECK
- 3D CRT – thermoplastic mask
- Stereotactic treatment: Rigid frames
(<=1mm)/non-invasive and frameless systems with image
guidance (<=2mm)
Thermoplastic mask – 3-5mm
setup errors in the region of the
head and face

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Immobilization aids
PROSTATE THORAX AND
UPPER ABDOMEN

Timeline when errors can be addressed
•Positioning and Immobilization
•Image Acquisition and Registration
•Target Delineation
•Treatment Planning
•Setup confirmation
•Treatment Delivery
•Quality assurance
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4D CT Simulation
In 4D CT, a series of CT scans is reconstructed at different phases of the
breathing cycle.
The three components of a normal CT simulator are:
(1) A large bore (75-85cm) to accommodate various treatment positions along
with treatment accessories with a flat couch insert to simulate treatment
machine couch.
(2) An integrated laser marking system
(3) Virtual simulation and visualization
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INFRA RED CAMERA PLASTIC MARKER
WITH INFRA RED
REFLECTORS

4D CT: Real Time Position Management (RPM)
Steps:
• Patient trained to breath regularly
• Setup/immobilize the patient in the
treatment position
• Infrared red reflector placed over
diaphragm
• Images acquired for multiple phases
of respiration, about 8-25 datasets at
each couch position.
• System sorts the images into different
3D image sets (binned) with the help
of respiratory signal
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CT Scan
Axial scan trigger,
1st couch position
Axial scan trigger,
2nd couch position
Exhalation
Inhalation
Scan Scan Scan
Axial scan trigger,
3rd couch position
Prospective CT Image Acquisition

X-ray on
Exhalation
Inhalation
1st couch
position
2nd couch
position
3rd couch
position
“Image acquired”
signal to RPM
system
(Ford 2003, Vedam 2003)
Retrospective 4D CT Image Acquisition

Limitations of 4D CT
• Depends upon uniform patient respiratory pattern
• The range of motion may not be similar during treatment so image
guidance during treatment delivery is important
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Derivation of ITV
• Population Based: may over or underestimate motion.
Not ideal.
• Individual Based:
1. Combining Breath hold scans
2. Combining 4DCT images
3. MIP: Maximum intensity projection
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PTV
CTV
GTV
ITV
ITV = CTV + IM
PTV = ITV + SM

1. ITV: Combining 4DCT images
Target is contoured in every phase
of respiration and then fused to
generate the ITV.
Advantage: Captures the entire
trajectory of motion
Disadvantage: Time consuming
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2. ITV: MIP
Maximum Intensity Projection (MIP):
MIP is an image set generated which
includes all the possible locations of a
tumour.
Maximum range of motion of the
hyperdense tumour is considered and
images are stacked together to yield
the MIP.
Advantage: accurate and automatic
method of generating the ITV
Disadvantage: beneficial only for lung
tumours surrounded by normal
parenchyma (not those in contact with
mediastinum, chest wall, diaphragm)
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Timeline
•Positioning and Immobilization
•Image Acquisition and Registration
•Target Delineation
•Treatment Planning
•Setup confirmation
•Treatment Delivery
•Quality assurance
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Tissue delineation using advanced imaging
• Image fusion for radiotherapy is very important to correctly define
tumour and normal tissue
• Necessary to wrap high doses around tumour
• CT image essential for dose calculation
• However, target delineation on CT not accurate for all sites
• To avoid systematic errors from inaccurate delineation, fusion of
other modalities important
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Different imaging modalities
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3 steps of image fusion
• Registration: Computes the difference between images
• Fusion: Fuses the images with the calculated differences
• Validation: Check if images are matching well
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PET-CT fusion
In lung cancers, PET-CT fusion helps
in:
• Primary tumour delineation
• Nodal volume
Incorporation of FDG PET in CT based
planning:
• Increase or decrease in target
volume (usually decrease)
• Reduces interobserver variability
• Distinguish between collapse and
consolidation
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Setup Error
It is the difference in position of the target (or surrogate bony
anatomy) between planning and treatment after a treatment setup
based on external setup markers.
• Skin marks/tattoos are the set of coordinates based on which
patients are initially et up.
• Treatment verification imaging using internal boy anatomy
(2D verification) or soft tissue target (3D verification) does not
exactly match planning images (DRRs or CT).
• This difference in position is called setup error
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Causes of setup error
• Technical/QA
- Laser misalignment, isocentre alignment not coinciding
• Initial setup
-isocentre shifts
• Positioning and Immobilization
- Imperfect arms and legs can change position of external skin marks
- Suboptimal immobilization may lead to changes in between setup and
verification/treatment
• Daily Setup
-positioning
-suboptimal marking of external markers
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6 axes of motion
Motion and errors are described in 6 axes, 3 translational and 3
rotational
THREE TRANSLATIONAL:
lateral, longitudinal,
vertical
THREE ROTATIONAL:
pitch, roll, yaw
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Imaging Techniques
USG Based
BAT
- SonArray
- I-Beam
- Restitu
Video Based
- AlignRT
Planar Imaging
Based
KV Xray OBI
(On Board
Imaging)
Gantry
Mounted
- Varian OBI
- Elekta
Synergy
-IRIS
Stereoscopic/
Room
Mounted
- Cyberknife
- RTRT
-Extractrac
System
(BrainLab)
MV X Ray
Imaging
EPID
(Electronic
Portal Imaging
Device)
Volumetric
imaging based
Fan Beam
- Helical
Tomotherapy
- In room CT
Cone Beam
KVCBCT
- Varian OBI
- Elekta
MVCBCT
Siemens
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2D imaging: Gantry mounted
Portal Imaging
• Portal imaging is a method to verify that the area planned by a
radiation field (or portal) matches with the area treated on a patient.
• Aim: To match bony anatomy covered in plan with that of the covered
by portal curing treatment
• Earliest form of image guidance
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Steps of obtaining a portal radiograph
Film in a cassette holder (with phosphor screen) placed
across patient
MV treatment field (portal) exposed
Film manually taken to a developing room and
developed image compared with planning radiograph
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Portal imaging
Advantages
• Inexpensive
• First method of verification
Limitations
• Time consuming, cumbersome
• Poor quality (megavoltage)
• Setup data could not be stored
Digital (electronic) imaging solved most of these drawbacks.
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Electronic portal Imaging (EPI)
EPI uses a detector, usually attached to the gantry opposite the
treatment beam, to produce high quality digital images rapidly.
- MV treatment beam used
- Allows fast identification of setup errors and online correction
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EPIDs on Linear Accelerators
• IGRT EPID is a flat panel detector
mounted on retractable arms
opposite the treatment head
• Arms often capable of
movement to allow them to
place detector at variable
distance from source
EPID
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Amorphous silicon based EPIDs
• Tiles of photodiodes arranged on
glass substrate on a flat panel
array
• Incident photons pass through a
phosphor and emit light
• Photodiode captures light, emits
electronic signal
Three types of commercial electronic portal imaging devices:
1. Camera based
2. Liquid Ionization Chamber Matrix Based
3. Amorphous Silicon Based
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Advantages
• Simple addition to linear
accelerators
• By identifying bony anatomy and
implanted fiducials, maybe
sufficient for several sites
• Less mechanical calibration since
treatment and imaging beam
same
• Non IGRT use: Portal dosimetry
Disadvantage
• Relatively poor contrast in 2D
images
• Lack of soft tissue detail
• Relatively high dose
• Images need to be taken in two
lanes for translational errors
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Gantry Mounted KV 2D Imaging
• There is additional kilovoltage 2D imaging system (source and detector) mounted
on the gantry
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Advantages
• Allows KV imaging of superior
quality, less imaging dose
• Allows orthogonal image pair
(KV and MV) without turning
gantry
Disadvantages
• Separate attachment
• Calibration of treatment and kV
imaging isocentres required
• Cannot be used when the couch
is rotated (to avoid collisions)
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Summary: 2D imaging - Gantry mounted
• 2D imaging using MV EPIDs and kV imagers have applicability in sites
where matching of bony surrogates or implanted fiducials suffice
- brain tumours
- gynaecological tumours
-prostate cancer (implanted fiducials)
• Good reproducibility, ease of use
• Limited when soft tissue details need to be addressed
• Unable to account for intrafraction variability
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2D imaging: Floor ceiling mounted
stereoscopic imaging
Setup of a floor-ceiling mounted
system:
• Pair of KV source of imager –
one mounted on ceiling, another
on floor
• Two such orthogonal pairs with
imaging planes intersecting at
treatment isocenter
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Advantages of floor-ceiling mounted systems
• Fast imaging
• Intrafraction motion imaging possible since imaging while treatment beam on is
possible
• Imaging dose is low as two KV images are used instead of a KV/MV pair
• Therefore more frequent imaging can be done
Frameless stereotaxy
For stereotactic treatment requiring highly
conformal treatment either small or no
margins
Respiratory Motion Gating/ Tracking
External surrogates (chest wall motion) can be used
to determine phase and amplitude of respiration
Lung/GI lesion motion can be predicted/tracked
INDICATIONS
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Platforms for floor ceiling imaging
1. Extractac System (BrainLab)
(1) Two KV X Ray generators
placed in recesses in the floor
(2) Two amorphous silicon flat
panel imagers on ceiling
(3) Robotic couch capable of 6
dimensional correction
(4) Optical tracking camera
monitoring surface markers
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2. Cyberknife System
Parts:
1. LINAC: X-band linac on a
robotic arm
2. KV 2D Imager Pairs
3. Robotic couch
4. Optical tracking system
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Cyberknife – robotic workspace
• Robotic manipulator designed to
move the LINAC in a fixed,
predetermined workspace.
• Nodes: Preassigned points in
workspace where LINAC can
stop and deliver RT
• At each node, beams can be
delivered form different angles
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Difference in how Cyberknife responds to
shifts
Traditional Linear accelerators
Setup errors/ intrafraction change
determined by image guidance
Corrections made by shifting
patient/couch to bring back target
to planning position
Cyberknife
Setup errors/ intrafraction change
determined by image guidance
Corrections made by continuously
changing the LINAC position/angle
to adjust the treatment beam to
match the planning position
before treatment is delivered
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3. RTRT System
• Works by fluoroscopic tracking of
implanted fiducials
• Gold fiducials, 1.5-2mm diameter
• Placement methods:
bronchoscopic for peripheral lung
tumours, operative for spinal
tumours
• Fluroscopic cameras with moving
object recognition software
capture intrafraction motion
(gating)
• Disadvantage: exposure high
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Summary: 2D imaging - Ceiling mounted
• Floor ceiling mounted imaging fast as not related to gantry motion
• Intrafraction imaging possible – useful in stereotactic
hypofractionated treatments with long treatment times and in sites
with high organ motion uncertainty
• Frameless intracranial and body stereotactic platforms rely on
frequent stereoscopic imaging
• Limitation: no soft tissue visualization
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3D imaging: KV CT based IGRT
• Involves use of kilovoltage X Rays from a source separate from the
LINAC beam but within the treatment room to generate a 3D
(volumetric) CT image
EPIDS
Use a pair of MV/KV images
to obtain setup information
in 3 axes (X, Y, Z) based on
bony anatomy
In room CT
Many KV/MV images
acquired as the gantry
rotates and reconstructing
them to obtain volume
images – setup information
in 3 axes (X,Y,Z) and
anatomical changes based in
bony and soft tissue anatomy
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Types of KV CT
CT on rails
Separate diagnostic CT unit is
places in the treatment room with
rails connecting the CT with the
couch
Cone beam CT (CBCT)
Imaging beam is shaped like a
cone and is captured by a flat
panel detector
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3D imaging: MV CT based IGRT
MV imaging involves use of megavoltage beams from the same LINAC
that delivers treatment to take verification images for patient setup and
position
Evolution from EPIDs
EPIDs
Use a pair of MV images to
obtain setup information in
3 axes based on bony
anatomy
MV CT
Use many MV images to
obtain setup information in 3
axes and deformation based
on bony and soft-tissue
anatomy
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MVCBCT: Imaging beam
• Imaging beam is different form treatment beam
• Treatment beam: 6 MV, 300 MU/min
• Imaging beam: 6 MV, 50 MU/min
• Aim is to minimize imaging dose while acquiring good quality images
• New Imaging Beam Line (IBL) uses:
- 4 MV beam
- graphite/diamond target (produce low energy photons)
- no flattening filter (prevent beam hardening)
Purpose is to develop a photon beam with optimal energy
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Tomotherapy: Fan beam based MVCT
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USG Based Image Guidance
• Relatively inexpensive modality
that allows soft tissue visualization
without use of ionizing radiation
• Commonly used for prostate
cancer
• Diminished use now due to fiducial
based 2D X ray or CBCT based
verification
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Electromagnetic tracking
• Method of position verification of target with implanted beacons
before and during treatment using electromagnetic signals instead of
ionizing radiation
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Commercially available systems
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Extractac (Brainlab)
kV X ray
CBCT
Fluoroscopy
EPID

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Uncertainties in planning and delivery due to respiratory motion

Breath hold techniques
1. Voluntary Breath Hold
- Patient holds breath without mechanical
assistance
- Deep Inspiratory Breath hold:
• Deep inspiration breath hold (DIBH) :- Free-
breathing interval followed by a breath hold
at approximately 100% vital capacity.
• The patient is trained to take a deep breath
and exhale slowly and then to take another
deep breath and hold it for as long as
he/she can.
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2. Assisted Breath hold
Assisted techniques
mechanically block the air
passage of the spirometer for
short periods of time to maintain
a static lung volume (and tumour
position)
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Breath hold techniques

• Gating is delivery of radiation during a
particular part of the breathing cycle
• Breathing cycle is monitored with the help
of external signals or fiducials
• Beam on-off synchronised with motion
surrogate
• Real Time Radiotherapy (RTRT) system:
gating using internal signals
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Respiratory Gating

• Tumour tracking involves the
radiation beam which is designed
to treat the tumour with small
margins by following the tumour
• Achieved by:
- Movement of the linear
accelerator assembly
- Movement of MLCs
• Machines: Cyberknife, Vero
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Tumour tracking

Offline and online correction strategies
• Offline verification: When in room imaging before treatment is
compared to the reference (planning) image for setup errors at a time
after treatment has been delivered, it is called offline verification.
• Setup errors detected for that set of fractions are only used for setup
corrections for future treatment.
• Offline verification protocols are used when daily imaging is not
essential.
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Image for first few
fractions
Match after
treatment
Use setup errors from
first few treatments
to change setup for
future treatment

• Online verification: When in room imaging before treatment is
compared to the reference (planning) image for setup errors/organ
motion before treatment has been delivered with a view to base setup
corrections for the same day, it is called online verification.
• Setup errors detected for that fraction are used for setup corrections
the same day’s treatment.
Image daily
Match before
treatment
Use match for
correction before
each day’s
treatment
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Imaging protocol
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OFFLINE ONLINE
Corrects systematic errors only Corrects systematic and random errors
Less resource intensive More resource intensive
Less imaging dose More imaging dose
Conventionally fractionated treatments
with good immobilization and less target
motion
Stereotactic, hypofractionated treatments
(with small PTV margins). Sites with
significant variations in target motion,
deformation

Offline protocol
Done one of several established correction protocols:
1) Shrinking Action Level (SAL) protocol
2) No Action Level (NAL) protocol
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No Action Level Protocol
• Measure the errors in the first 3-5 fractions
• Calculate the mean
• Implement it as an isocentre shift for the remaining fractions
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Example of NAL implementation
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X Y Z
Fraction 1 0.7 0.3 0.6
Fraction 2 0.3 0.4 0.5
Fraction 3 0.5 0.5 0.4
Mean 0.5 0.4 0.5
The mean value after 3-5 fractions
is implemented as an isocentre
adjustment during setup for all
subsequent fractions no matter
how small its value. There is no
threshold or action level.
Setup errors are weekly
checked to verify if there
are no residual errors. If
yes, they are corrected.

Example of Shrinking
Action Level Protocol
• SAL: for every fraction, the mean
error is compared with the
action level.
• If mean error remains lower
than action level for 5 fractions,
no need to correct setup and
further imaging is stopped after
5 fractions.
• Advantage of SAL: Less need for
setup correction
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X Mean
error
Action
Level, A0
Action
Fraction 1 0.7 0.7 0.7 Continue
Fraction 2 0.3 0.5 0.5 Continue
Fraction 3 0.5 0.5 0.4 Correct by
0.4
An = A0/ 𝑁,
An = Action level at fraction N
A0 = Initial action level
Practically, NAL is widely used for its
simplicity and less imaging requirement.

Derivation of setup PTV margins
• Planning Target Volume: geometrical concept that takes into account the net
effect of all possible geometrical variations, in order to ensure that the
prescribed dose is actually absorbed in the CTV.
• PTV = ITV + setup margin
• Setup margins are derived from the systematic and random errors using
margin recipes or formulas.
Van Herk (2000):
Set up margin
PTV = 2.5ƹ + 0.7σ where ƹ = systematic error, σ = random error
To ensure minimum CTV dose of 95% to 90% of the patients
• Margin recipes give more importance to systematic errors. Random errors
cause dose blurring but random errors cause persistent underdosage.
12-10-2018

Head and neck
• Immobilization is easy and effective – S clamp orfit cast.
• Target delineation: MRI, PET.
• Role of adaptive planning: Tumour shrinkage/weight loss
12-10-2018

Lung cancer
• Image acquisition: Special strategies like breath hold or 4D CT
• Advanced imaging modality: PET CT for identification of primary and
nodal volume
• Individualized margins
• Motion management strategies for reducing requirement of large PTV
margins: motion restriction, breath hold, gating, tracking
12-10-2018

Prostate cancer
• Treatment uncertainties arise:
rectal distension, rotation of
seminal vesicle position,
intrafraction motion
• Interfraction motion: 2D imaging
with implanted fiducials, X based
volumetric imaging and USG based
IGRT
• Daily online image guidance with
soft tissue matching
12-10-2018

Upper GI malignancy: Liver, pancreas
• Appropriate imaging: Triphasic CT +/- fusion with MRI
• Consider fiducial placement: important SBRT
• Respirator motion management strategy: 4D CT, motion guided
delivery
• Imaging verification for motion and setup: Online verification for SBRT
12-10-2018

Summary
The use of modern imaging modalities incorporating functional or
biological information to augment target delineation and the use of
imaging to adjust for target motion or positional uncertainty and
potentially to adapt treatment to tumor response
12-10-2018

Image Guided Radiotherapy

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