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