definition of igrt

Definition of IGRT

Eugene Lief, Ph.D.

Christ Hospital

Jersey City, New Jersey

USA

DISCLAIMER:

I am not affiliated with any vendor and did not receive any financial support from any vendor.

I am not recommending any particular product.

• D. Michael Lovelock, Ph.D. – MSKCC

• Professor Ben Mijnheer

• University of California at San Diego

• Accuray, Elekta, Siemens, Tomotherapy, Varian

Acknowledgments

Image-guided radiation therapy (IGRT)

Dirk Verellen et al., “Innovations in image-guided radiotherapy”, Nature Reviews Cancer, 7, 949-960, 2007

“Frequent imaging in the treatment room during a course of radiotherapy to guide

the treatment process”

Definitions

Image-guided radiation therapy (IGRT)

Palta et al., Workshop April, 2007

“Any use of planar imaging, volumetric imaging or volumetric cine imaging, marker localization,

marker tracking, patient surface imaging or patient surface tracking to improve the precision of

radiation-therapy delivery”.

Definitions

Purposes

• Precise patient positioning

• Automatic localization of bony structures or implanted fiducial markers

• Imaging and positioning of soft tissue tumors and critical structures

• Margin reduction and healthy organ sparing

• Delivering the desired dose distribution

Negative Side• Physics staffing at treatment delivery

• Longer treatment times

– I min. arm deployment, setup for scan

– 1 min. gantry rotation - volume image acquisition,

– 30 sec. reconstruction (depends on resolution)

– 30 sec. load CT and CB into registration

– Registration: from few sec. to few min.

• Feasible if the number of treatments is low

10

Evolution of the radiation therapy workflow towards an image-based process

(from David Jaffray, PMH, Toronto)

Head and Neck

Abdomen - Liver

Abdomen - Kidneys

Pelvis - Prostate

Prostate

Reduced Artifacts on daily fan beam MVCT

MVMV kVkV

kVkV

MVMV

More sample MVCT imagesMore sample MVCT images

Varian OBI Hardware

kV source

kV aSi imager

MV source

MV aSi imager

Position of Critical StructuresLiver PTV

Stomach

CT

CB

• Stomach is a critical structure in liver treatment

• Changing stomach contents and gas can cause noticeable changes in stomach position

• Cone beam scan useful to verify position of soft tissue critical structures such as stomach and bowel

• Bowel prep. now used for all liver treatments

Measures taken to mitigate effects of respiration

Abdominal Compression• Patient wears a tight belt, effectively

reducing the excursion of the diaphragm

• Generally motion can be reduced to less than 10 mm

CT CB

CT and Cone Beam scans of a patient wearing a compression belt

Respiratory Motion in Treatment

Registration

Markers aligned in CT and cone beam scan

Pre-treatment – Post Treatment Comparison

• Post-treatment cone beam scan (blue) superposed on pre-treatment scan (red)

• Isocenters were aligned

• Alignment of markers and liver contour indicate target sufficiently ‘immobilized’

Pre-treatment Post-treatment

0.80.80.7Std D

-1.1-0.20.1Mean

S/IP/AL/R(mm)

Pre-post marker shifts (5 patients)

Ways to Mitigate Respiration Effects

Breath Hold• Three

20-sec. breath holds

• CB scan acquired during these intervals

• High quality images

• Treatment delivery also done using breath hold.

• Difficult for patients, approximately 60 % of patients tolerate the breath hold procedure

Hawkins, Brock, Eccles, et al, “Assessment of residual error in liver position using kV cone beam CT for liver cancer high precision radiation therapy” IJROBP 66(2) p. 610 – 619, 2006.

Measures to Mitigate Effects of RespirationDigital Tomosynthesis (DTS)

• Instead of a full 360 degree rotation, gantry rotates only through an arc of 20 – 40 degrees.

• Using a gated technique, it may be possible to get a reconstructed slice through the patient showing soft tissue contrast resolution

Plane of reconstructed image

Setup Errors• Random

– Associated with each registration.

– Vary with each imaging and registration session

• Systematic– Arise from the residual mechanical errors and misalignments

– Generally remain the same throughout the course of treatment

Both random and systematic errors in patient setup can be significantly reduced

using CB guidance

Systematic Errors• Most important contributor is the displacement of the

origin of the imaging system from the radiation isocenter

if a small BB could be placed exactly at the radiation isocenter, how far from the origin would it appear in the cone beam scan?

• Measurement technique:

– Place a small (5 mm dia.) tungsten sphere approximately at the radiation isocenter using the room lasers, then precisely measure it position using

• MV system (Winston - Lutz tests)

• Cone beam imaging system

• The difference is a measure of the systematic error

Clinical Process

Single Fraction Paraspinal Sites

CT CB

• Prescription dose 24 Gy delivered in a single fraction

• Equivalent cord dose, in 2 Gy fractions, would be > 150 Gy

• Accurate positioning, immobilization, and patient monitoring during dose delivery is critical for these treatments

All treatments delivered using cone beam guidance.

Cone beam imaging of targets that move with respiration

• Cone beam acquisition takes one minute

• Ribs and soft tissues move, creating image artifacts

• Image quality severely degraded by respiratory motion

CT CB

Respiratory motion approximately 1.5 cm S/I

Real time monitoring of patient position

Time (seconds)

Mar

ker D

ispl

acem

ent (

cm)

Stereoscopic infra-red camera

Marker Locations

- Left chest- Right chest- Belly

• Infra-red reflectors taped to patient’s skin

• Positions monitored in real time to check patient still and breathing regularly

Immobilization

• Developed at MSKCC for spinal treatments.

• Designed for patient comfort over the long (2 hour) total treatment times

• Arms at patient’s sides

• Position monitoring using infra-red tracking of patient in the reference frame attached to the immobilization cradle

Immobilization ‘QA’• Take pre and post treatment images of all

single fraction patients for the first 3 years

• Require patient movement <= 2 mm

Single Fraction Treatment to a Lymph NodeCT

TargetBowel

CB

Soft Tissue Targeting

• Compared with a conventional CT, a cone beam CT suffers from increased scatter, resulting in poorer low contrast resolution.

• However, in sites that are not affected by respiratory motion, image quality is sufficient to permit very accurate setup of soft tissue targets

• Node and bowel clearly seen in cone beam scan

• Position of bowel checked at time of treatment

Change in bowel during treatment

Overlay of the pre-treatment cone beam with the post-treatment cone beam

Significant change in bowel position.

No change in bony anatomy, target node

Target

Pre- treatment + Post treatment

Cone beam – tumor growth

Random errorsarise in the registration process

Pre-treatment Cone Beam

Planning CT

Process: • Accurate (sub mm)

• Robust registration

• Automatic registration works well

• Rotational setup errors easily detected

Imaging system origin• Perform Winston-Lutz test

from 4 different gantry angles using the MV imager

• Vendor software used to determine position of sphere with respect to radiation isocenter

• Rad. Isocenter taken to be intersection of beam central axes for all 4 gantry angles

• Orientation is that of a head first supine patient

Imaging system origin - continued

Position of Sphere with respect to Radiation Isocenter

Gantry Angle

0.16-0.23-0.17Mean (P)0.17-0.072700.25-0.39900.58-0.1180-0.36-0.240

Sup / I. (mm)

Ant / P. (mm)

Lft / Rgt(mm)

• Position of sphere with respect to the average position of the radiation isocenter can be determined.

• Observe the ~1mmSup-Inf shift due to gravity acting on gantry

• Radiation isocentertaken to be mean position

Cone beam scan of the sphere

Treatments with Imaging:Treatments with Imaging:

•• SiteSite Time in RoomTime in Room Treatment Time (beam onTreatment Time (beam on) ) CT TimeCT Time Target LengthTarget Length

•• Esophagus Esophagus 17 min17 min 4 min 35 sec 4 min 35 sec 2min 17 sec 13.8 cm2min 17 sec 13.8 cm

•• ChestChest 21 min21 min 6 min 29 sec 6 min 29 sec 1min 41 sec 19.6 cm1min 41 sec 19.6 cm

•• Head and NeckHead and Neck 13 min13 min 4 min 13 sec 4 min 13 sec 1min1min 12.7 cm12.7 cm

•• Head and NeckHead and Neck 19 min19 min 4 min 36 sec 4 min 36 sec 4 min 37 sec 13.8 cm4 min 37 sec 13.8 cm

•• ChestChest 22 min22 min 4 min 47 sec4 min 47 sec 2 min 17 sec 14.2 cm2 min 17 sec 14.2 cm

•• Head and NeckHead and Neck 17 min17 min 4 min 50 sec4 min 50 sec 1 min 23 sec 14.3 cm1 min 23 sec 14.3 cm

•• RectumRectum 15 min15 min 4 min 32 sec4 min 32 sec 1 min 42 sec 13.7 cm1 min 42 sec 13.7 cm

•• LiverLiver 17 min17 min 7 min 26 sec7 min 26 sec 1 min 47 sec 22.3 cm1 min 47 sec 22.3 cm

•• BladderBladder 15 min15 min 4 min 18 sec4 min 18 sec 1 min 47 sec 12.8 cm1 min 47 sec 12.8 cm

Treatments without Imaging:Treatments without Imaging:

•• Head and Neck Head and Neck 13 min13 min 5 min 11 sec5 min 11 sec 00 16.1 cm16.1 cm

•• Head and Neck Head and Neck 12 min12 min 6 min 5 sec6 min 5 sec 00 18.3 cm18.3 cm

•• Head and Neck Head and Neck 10 min10 min 6 min 6 min 00 18.2 cm18.2 cm

Courtesy of Southeast Regional Cancer CenterTallahassee, FL

min

max

min

max

Normal Treatment Times for Tomotherapy

Cone beam image guidance is a powerful new tool. It has

• Facilitated the delivery of highly conformal dose distributions to sites outside the brain with a much greater degree precision than previously possible

• The dose delivery to the target and surrounding normal structures, including soft tissue, can be verified

• This has lead to such new types of treatments as:– 20Gy x 3 Fractions to lung

– 24 Gy x 1 fraction to spine, lymph nodes, for example

that can be delivered accurately and safely

• Many challenges remain– Respiratory motion

Conclusions