Download - Aapm tg 179
Samir Laoui, Ph.D. 04/12/2016
IGRT: Definition
In general, most tumors are radioresistant if
they are not in the treatment beam
It is a process of frequent imaging in the
treatment room during a course of radiotherapy
that allows treatment decisions made on the
basis of imaging
PTV Design with IGRT
IGRT reduces, but does not eliminate,
geometric uncertainties
Reduced geometric uncertainties may allow
reduced PTV margins
IGRT may also empower adaptive
radiotherapy because clinicians can assess
anatomical changes seen during a course of
radiation and rationally respond to those
changes
Workflow
Initial • Patient is set up by laser on external markers
IGRT • IG is done to retrieve coordinates du jour (xyz)
• (xyz) are compared to planning coordinates (x’y’z’)
Rx • PTV is brought from (xyz) to (x’y’z’)
• Tx Begins
IGRT
Target is typically not imaged during IGRT
Usually image something that is a surrogate,
Xs, of the target position, Xt
It is often assumed that the target and
surrogate move in tandem
Good Surrogate Bad Surrogate
Xs Xs
Xt Xt
Need for image guidance
3-D Conformal
IMRT
2-D Planning
4 field Box
Intent
Historical perspective of IGRT
History
1958- Holloway et.al reported portable x-ray
machine mounted on the counter weight to
TheratronCo-60 machine
1958-Weissbluth et. al introduced the
concept of in-room imaging with
integrated diagnostic x-ray unit in the
linac head. -Stanford Linear accelerator
1961, the Netherlands
The base part of the construction is a sturdy ring (460mm) partly
sunk in the floor enabling rotation of 540 degrees
Orthogonal to the Co-60 beam, a 120 kV x-ray unit was mounted
1970, first patient setup error
analysis In 1970, Haus et al. study is considered as one of the earlier
reports on assessment of intra-fraction uncertainty describing
reduction in error rates (an error was recorded as the shift
exceeded 1 cm) from 36% to 15% with more frequent
verification films
First KV source
mounted at 45
degree to TX beam
KV and MV share
the same detector
Time consuming/
Patient may move, or
be moved, between
the 2 exposures
True verification is
prevented
Only AP views
The solution was to
have the imager
move with the
gantry, but was not
done at the time
1985, MGH on-board KV imager
Available CT-based IGRT systems
Available CT-based IGRT systems
EPID
Pros
Initiated the IGRT ‘culture’
Image created with treatment beam
Direct verification of alignment target-beam
Verification of field, MLC, dose, …
Cons
Only 2D information (requires multiple gantry positions for
3D info)
Requires surrogate to localize target
Existing technologies: CT-on-rails
CT-on-rails: Uematsu et al. (1996), National Defense
Medical College, Saitama, Japan
CT-on-rails
Pros
Simplest form of IGRT
Familiarity of the diagnostic quality
CT images.
Cons
A couch correction is used to realign the patient
Prone to isocenters misalignments
KV-CBCT
• Elekta and Varian
KV-CBCT
On Board Imager CBCT acquires projections of a
patient
Send them to a reconstruction application, and then
returns a 3D image
In the 3D/3D Match workspace, the CBCT images
can be registered to the reference images, either
manually or automatically, using a 3D mutual
information algorithm
KV-CBCT
Enables high dose to targets while sparing normal tissue
CBCT helps reduce interfractional motion and assess patient
status
Results in “tighter” margins during planning
KV-CBCT: Clinical applications
Distinct advantage over projection imaging in that
some soft tissue structures can be directly imaged and
thus targeted
Prostate (fiducials), lung, H&N, breast, esophagus, liver
and bladder
Simplification of SBRT
Eliminated the need for body frames
Low dose
Make adaptive planning possible, margin reduction
Existing technologies: Fan beam
MVCT Tomotherapy systems can be used to obtain fan-beam
MVCT images of the patient in the treatment position
MV-CBCT
Existing technologies: MV-CBCT
Similar to fan-beam MVCT, the imaging beam
is in the megavoltage range, thus rendering the
images immune to typical high-Z artifacts
3D patient anatomy volume in the actual
treatment position that can be aligned to the
planning CT moments before the dose
delivery, enabling the IGRT process
Dose: 6-10 cGy
MV-CBCT: Clinical applications
Prostate, H&N, and lung alignment
Monitoring of tumor growth or shrinkage
Improves delineation of structures in CT
images that suffer from metal artifacts
Exactrac BainLab
X-Ray based monitoring system that
detects intra-fractional tumor motion
during treatment delivery
Deviations or unintended shifts from
the prescribed treatment position are
automatically detected
Beam is shut off when target is out of
range
ViewRay
MRI guided adaptive radiotherapy
Patient treatment can be customized on a daily
basis
UCI: Trilogy/TrueBeam
Portal Vision
Electronic radiation field placement
verification
Uses the treatment beam (MV
imaging)
KV Imaging
Diagnostic quality imaging for
accuracy of patient setup verification
Uses kV x-ray source
CBCT
Volumetric imaging for matching
planned and delivered treatment
fields
Uses kV x-ray source
QA
QA Test for CT based IGRT
Geometric Accuracy KV CBCT- Winston Lutz Test
place a metal ball bearing at radiation isocenter portal
images are acquired at the four cardinal angles to compare
the ball bearing image centroid to the field edges
Fan-beam MVCT
Imaging beam and treatment beams are generated
by the same source Robust geometric accuracy
TrueBeam March 2016
Geometric Accuracy
CT-on-rails: The treatment and imaging beam
do not share the same gantry The clinical accuracy is expected to be worse because the
process of rotating the patient introduces more errors
External fiducials are necessary (placed on couch,
immobilization devices and patient surface)
Limited by room laser alignment and couch readout
Image quality tests
Scale and distance accuracy Objects of known dimensions
Low contrast resolution Low contrast detectability is tested by scanning
a phantom containing objects with a variety of
linear attenuation coefficients
KV-CBCT: visibility of 1% contrast objects that
are 7 mm in diameter.
MVCT: visibility of 2% for 13 mm diameter
objects
MV-CBCT: visibility of 1% contrast for 2 cm
diameter objects
Spatial resolution
In IGRT, spatial resolution is usually
compromised for the benefit on low contrast
detection. Routine QA is necessary
nevertheless
KV-CBCT: 6-9 line-pairs/cm
MV-CBCT: Up to 4 line-pairs/cm (resolving 1-2.5
mm objects)
MVCT: 1.6 mm objects should be resolved
Image dose
KV-CBCT: 0.1 to 2 cGy /scan
MV-CBCT: 0.7 to 10.8 cGy /scan
Fan MVCT: 0.7 to 4 cGy /scan
Dose amount from 3 to 370 cGy over a course
of treatment which above the threshold doses
for secondary malignancy occurrence
Accuracy of CT numbers
This test is only recommended if images are
used for dose calculations
Densities should be within 30-50 HU
Accuracy of remote-controlled
couch
Tolerances suggested by AAPM TG-142
+/- 2mm translational; 1 degree rotational
The report failed to specify test frequency
TG-179 recommends TG-142 tolerances but
the test should be performed daily
Daily QA
To identify any sudden performance changes
Isocenter accuracy: CBCT a cubic phantom
with a marker at the center
Image orientation can be verified with a
phantom with multiple markers
Tolerance of +/- 2mm
Daily QA Couch motion accuracy: The “residual
correction error” is a useful measure of the
targeting and couch correction accuracy
Placing a phantom at iso and then displacing it in
the 3 directions (within 2 cm)
Acquire an image to assess the
displacement/required shift
Apply shifts and acquire another image
TrueBeam 4/11/2016
TrueBeam April 2016
MV/KV iso verification before
couch shift
MV/KV iso verification after
couch shift
IGRT Commisioning
IGRT Commissioning
Acceptance testing
End-to-end tests where a phantom is treated exactly
like a patient, from CT simulation to treatment
delivery
assessing performance under a clinical load;
defining an appropriate frequency of site-specific image
guidance protocols;
Defining the roles, responsibilities, and involvement of
team members in the image-guidance process
Establish QA
Commissioning process
Clinical needs
Assess imaging techniques for anatomical sites
ALARA
Identify appropriate immobilization devices
Documentation