Image guided radiation therapy (2011)

Image Guided Radiation Therapy

Basic principles, technologies, and future directions

Parminder S. Basran, PhDDept. Medical Physics

BCCA-Vancouver Island Centre

Introduction

• Some definitions and background

• Descriptions of image guided radiation therapy technologies– 2, 2.5, 3 and 4D– Just-it-time vs. Real-time

• Where are we going?

Definitions

IGRT is not:

In a Golden Room of Tools

Idylic Greeks Rummaged Turkey

I’ll Get a Rounda Tequilas

I’m Guessing its Radiation Therapy

Definitions

IGRT is :

- Image Guided Radiation Therapy

- Not new, but now applied with imaging technologies other than portal imaging

Definitions

IGRT• Radiation therapy has evolved from a 2D

technique to 3D, and in some cases 4D

• 3D morphological and functional imaging techniques change targeting

• Refined conformal delivery techniques, ex: IMRT offers the ability to sculpt the dose more closely to the tumour volume.

Definitions

However, with such dose conformity to the tumour volume and the rapid dose fall off outside the tumour volume, the accuracy of daily delivery becomes crucial.

There will be uncertainties introduced in this process and a ‘‘safety” margin should be accounted for.

ICRU-50 + 62 [2,3] create nomenclatures for facilitating the image guidance task by specifying the geometric constructs and margins.

ICRU standard definitions for margins in radiation therapy

GTV

CTV

ITV

PTV

Background

Sources of uncertainties- Systematically defined before treatment

- Potentially largest source is target volume identification*

- Rx to Tx errors

- During treatment- Systematic errors- Daily non-predicted errors (random?)

*Rasch C, Steenbakkers R, van Herk M. Target definition in prostate, head, and neck. Semin Radiat Oncol 2005;15:136–45.

Displacements with Respect to Isocentre

-15

-12.5

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

12.5

15

-15 -12.5 -10 -7.5 -5 -2.5 0 2.5 5 7.5 10 12.5 15

Left-Right [mm]

Su

p-I

nf

[mm

]Patient 1

Patient 2

Patient 3

Patient 1 AVERAGE

Patient 2 AVERAGE

Patient 3 AVERAGE

Background

During treatment- Systematic errors

- Controllable and correctable

- Daily non-predicted errors (random)- Primarily a function of the “system” itself- Ex: Flipping a balanced coin is unpredictable,

but flipping a weighted coin is somewhat predictable.

Background

Some examples of “weighted” coins in RT- A really bad/good breast-board- A really good/bad couch indexing system- An overzealous/far-sighted Portal Image

analyzer

If you know your coin is weighted, then you will have a better understanding of your margins.

What is IGRT?For purposes here, it is a generalized approach of capturing

and reducing systematic and random errors when delivering radiation therapy.

Involves acquiring data that can capture the position of treatment volumes

Ex: prostate and dose escalation, hypofractionation

avoidance structuresEx: SBRT spinal cord

It doesn’t have to be Image-based (but GRT doesn’t sound sexy). BUT we will assume this data = images

Image Guided RT :Classical approach (Just in Time)

Patient Set-up

Assess Patient Position

AdjustPatient Position

VerifyPatient Position

TreatPatient

Decisions / Processes

On-line

Decisions /

Processes

On-line

Decisions /

Processes

Off-line

Planning CT

TreatmentPlan


Real Time

Image Guided RT :Real-Time (nD +1) approach

Patient Set-up

Assess Patient

Positioning

AdjustPatient Set-up

VerifyPatient Set-up

TreatPatient


On-line Decisions /

Processes

Adaptive

Decisions /

Processes

On-line

Decisions /

Processes

Off-line

Planning CT

TreatmentPlan

Image Guided RT Technologies

Categorized by the

• # of dimensions of

acquisition

• Modality– x-rays: kV / MV– Ultrasound– MRI


2D: MV Portal imaging• Just-in-time• Real-time


2D: kV imaging• Real-time


2D: kV imaging• 2D + 1


2D: Ultrasound• Just-in-time• Real-time


2.5 D Systems- Thin collimated

beam- Coupled with the

treatment machine

T=1 s … 3 s … 5 s … Couch motion

steps in after each gantry rotation

Acquire, translate, and acquire …

Couch motion is constant during continuous gantry rotation

Continuously acquire and translate


2.5 D Systems

Tomotherapy

MV CT

– Just in time

– 2.5D + 1

(research only)


2.5 D Systems

CT on rails

– Just in time

– Real-time

(research)


3D Systems- True acquisition of 3D data, not just

stacking 2D data and calling it 3D- Focus of most clinical research

- Technically challenging- Noise vs. dose vs. accuracy

- Key Modalities- X-ray- MR- Ultrasound


3D kilovoltage X-ray imaging systems


3D X-ray imaging systems


3D MVCT imaging systems


3D Megavoltage imaging systems

Image Guided RT TechnologiesPros / Cons of MV CT (2.5 & 3D)

• the treatment beam is used for imaging, MV-based solutions provide most accurate/direct geometric information concerning alignment of treatment beam and target.

• On the other hand, MV-based solutions will inherently be inferior to kV-based solutions as kV images provide better soft-tissue contrast owing to the prevalence of photoelectric absorption interactions.

Image Guided RT TechnologiesPros / Cons of MV CT (2.5 & 3D)• MV based volumetric imaging features some additional

generic advantages.– The imaging dose can be easily incorporated into the dose

calculation algorithm – Linear relationship between electron density and megavoltage

HUs due to Compton scatter – MV-based CT images can be used to complement or even

replace diagnostic kV CT images when high density objects introduce severe artifacts, due to the fact that these artifacts and beam hardening are less critical for MV sources.

• A potential argument against MV-based solutions could be patient dose, however, it should be noted at this point that extra patient dose due to IGRT is a complex issue (see AAPM TG 75).

Image Guided RT TechnologieskV-CBCT considerations• Field of View

– Determines the length of scan available and possible solutions if longer scan lengths are required

– What size is the reconstruction circle?– Determines the lateral FOV

• Filters required? – Which filters are available?– Involves time to select and insert, and affects image quality– Are filters interlocked?– If not, then risk of poor quality or unusable scans from incorrect

filters selection

• Can panel be positioned remotely? If so, does this the system come with an anti-collision system?– Will involve time to position if not remotely accessed

Image Guided RT TechniqueskV-CBCT considerations• What are the available rotation speeds?

– Determines the acquisition time– What are the possible angles of rotation?– Affects the flexibility of scanning; e.g. the possibility of performing half-

scans for small regions, rotations through 180 degrees (underneath the patient) and using preset or flexible start and stop angles

• How ergonomic is the operation?– One- or two-button operation, foot- or hand-control, several screens

affects the ease of operation and the risk of aborted scans

• Can the scan be stopped and restarted?– Will result in extra dose if the scan is interrupted inadvertently, and

has to be started from the beginning– Also allows the scan to be acquired with the patient in several breath

holds.


• Other 3D / 4D techniques– ViewRay

• 3 Cobalt sources w/ intensity modulation capabilities

• MR scanner• Real-time imaging


• Other 3D / 4D techniques– MR-Linac projects

• Edmonton

• Germany

Image Guided RT techniques

3D +1– Simultaneously collect 3D images

• Prior to treatment

• During treatment

“Symmetry captures image data during the breathing phase and performs sophisticated computations to provide 4D data. This data helps to visualize the tumor position in each phase of the respiratory cycle to arrive at an average position of the tumor for each treatment”

Where are we going?

New machines slated for install likely around mid March

All will have - 2D MV imaging- 2D kV imaging- 3D kV-CBCT abilities

Some (or maybe just 1) will have- RPM imaging capabilities- 6 Degrees of Freedom couch motion- ?

The factors for consideration in image registration and set-up evaluation and their relevance.

What assessment tools are available?• View several planes, and availability of visual methods to

compare image to reference (spyglass, colourwash, checker board, toggle function)

How many degrees of freedom are available?• Availability of inclusion/exclusion of rotation• Will affect the flexibility of the system, the ability to use

different correction protocols, and the precision of the match results

What functions are available for registration?• entire image or a ROI, choice of intensity levels, and soft

tissue or bony anatomy selection is available

The factors for consideration in image registration and set-up evaluation and their relevance.

Is manual or automatic match available?• Affects time to match, training, decision making, and match

results, ability to check registration

Is rigid- or non-rigid-matching used?

Can the couch correction be applied remotely?• Affects efficiency of on-line verification, correction & patient

safety• If the couch can be moved remotely, is visual monitoring

available and/or collision detection systems? Is the movement mechanically limited?

What is the resolution of couch movement?• Affects accuracy but is also dependent on couch calibration

Where are we going?

IGRT Working Group

Charged with helping lead the integration of IGRT at our clinic– Determine starting point/site for implementing IGRT program

– Identify & Define Training Needs and Establish Training Program

– Establish & implementing site-specific protocols & procedures

– Evaluate, track efficiency and efficacy of IGRT practices

Where are we going with IGRT here?

Likely endpoints• An efficient and clearly defined set of IGRT processes / protocols

– Better patient treatments– More confidence in treatment– Customized treatment– Ability to image daily if needed– Less subjectivity– Better image quality– A program that would allow us participate in trials requiring IGRT– Clearly defined protocols– Automated couch moves

• Programs for– Well defined quality control and assurance for

• Patient specific procedures, protocols, and patient QA: an auditing system• Machine-related QA

– Education and training• Clear indication of the resource implications

Thanks…

and stay tuned..

Image guided radiation therapy (2011)

Health & Medicine

Transcript of Image guided radiation therapy (2011)