06 van dyk - definition of target volume & organs at risk

IAEA International Atomic Energy Agency

Definitions of Target Volumes and

Organs at Risk

22 February 2011

Jacob (Jake) Van Dyk

Consultant, IAEA

Professor, University of Western Ontario, Canada

IAEA

Acknowledgement

• Slides from

• Morten Høyer

• Richard Pötter

• Larry Marks

• Stewart Gaede

IAEA

Definitions of Target Volumes and OARs

• Aim

• To introduce target volume and organ at risk

concepts as defined by ICRU

• Specific Learning Objectives

• Introduce ICRU reports 29/50/62/71/83

• Define volume definitions of ICRU

• GTV, CTV, PTV, OAR, PRV

• Describe margins and how they are determined

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Evolution of Modern Radiation

Therapy Technology

• Decrease normal tissue dose

• Tighter margins

Dose Escalation

R Pötter

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MINIMIZE

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1978

Replaces

ICRU 29, 1978

1993 1999

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ICRU 29

• “Target volume” & uniform

prescription concepts

• “2-D era”

1978

• Single slice (or few)

• External contour

• Coplanar beams

• Simple calculations

• Dose prescription to

“ICRU reference point”

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• Specification of volume(s) & dose(s)

• For prescription, recording, reporting

• Purpose

• Consistent treatment policy

• Compare results of treatment - departmental colleagues

• Enable other radiation oncologists to benefit from

department’s experience

• Enable department’s treatment results to be compared

with those of other centers

• Especially multi-centered clinical trials

ICRU 50 - Purpose Replaces

ICRU 29, 1978

1993

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ICRU Volumes

9 ICRU 50

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Gross Tumor Volume (GTV)

• Gross palpable or visible/demonstrable

(imaging) extent and location of disease

• GTVprimary, GTVnodal

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GTV with CT

• Use right window settings

• Use right contrast and delay

Right! Wrong!

Purdy, Sem Rad Oncol 14: 27, 2004

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Intra- and Inter-observer Variability In contouring on CT

United GTV Max. PTV

Agreement GTV

Min PTV

Leunens et al, Radiother Oncol 29: 169; 1993

Inter

Intra

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Inter-observer variation in delineation

for cervix cancer

Dimopoulos J et al. R&O 2008 EPUB

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Clinical Target Volume (CTV)

• Contains GTV and/or subclinical microscopic

malignant disease, which has to be

eliminated.

• This volume needs to be treated adequately to

achieve cure or palliation

Perez et al 1998

IAEA Courtesy D. Yan & M. van Herk

Uncertainties … Inter-fraction

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Intra-fraction … Respiratory Motion

Courtesy S Gaede

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Planning Target Volume (PTV)

• Geometrical concept - To select appropriate beam

sizes/arrangements, accounting for the net effect

of all the possible geometrical variations and

inaccuracies to ensure that the prescribed dose is

actually absorbed in the CTV

2/25/2011 17 Perez et al 1998

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Prostate

Contours for

GTV, PTV,

bladder,

rectum.

CTV=GTV

Non-uniform

margin

Purdy, Sem Rad

Oncol 14: 27, 2004

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PTV

• PTV depends on the precision of tools • immobilization devices

• lasers

• PTV does NOT include margin for dosimetric

characteristics beam

• these will require additional margin during

treatment planning and shielding design • penumbral areas

• build-up region

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Dose Volumes

• Treated volume - enclosed by an isodose surface

selected by rad. onc. as appropriate to achieve

purpose of treatment

• Irradiated volume - receives a dose that is

significant in relation to normal tissue tolerance

2/25/2011 20

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Treated Volume - NSCLC

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Irradiated Volume - NSCLC

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Organs at Risk (OAR)

• Normal tissues whose radiation sensitivity

may significantly influence treatment

planning and/or prescribed dose

OARs

• Lung

• Spinal cord

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ICRU 50 – Dose Reporting

• ICRU Reference Dose

• Dose at ICRU reference point

• Centre of PTV

• Near central axis of beam(s)

• Dmax – Maximum dose in PTV

• Dmin – Minimum dose in PTV

• Dave – Average dose in PTV

• Dmedian – Median dose in PTV

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ICRU 62 - Purpose 1999

• Irradiation techniques have advanced

• 3-D imaging

• Conformal radiation therapy

• More accurately formulate definitions & concepts

• Issues

• Reference points and coordinate systems

• Introduction of

• Internal margin (IM)

• Setup margin (SM)

• Internal target volume (ITV)

• Planning organ at risk volume (PRV)

• Conformity index (CI)

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Reference Points and Coordinate Systems

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Set-up Margin • Accounts for all uncertainties in patient-beam

positioning

• Technical factors

• Patient immobilization

• Machine stability

Internal Margin

• Takes account of variation in size, shape, and

position of CTV in relation to anatomical

reference points, e.g.,

• Filling of rectum

• Movements due to respiration

• Patient related

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Margins

A.

B.

C.

“Global” safety margin

• Accounts for

• OAR

• Decrease of

subclinical spread

from GTV

ICRU 62

22 SMIMCTVPTV

SMIMCTVPTV

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Internal Target Volume (ITV)

• ITV = CTV + IM

• Geometric

• ITV accounts for

motion of CTV in the

patient

• Does not account for

setup uncertainties

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GTV & CTV: NSCLC stage IIIA

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Internal Target Volume (ITV)

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Planning Target Volume (PTV)

Gross Tumor

Volume (GTV)

Microscopic

Spread

Set-up

Errors + +

Clinical Target Volume (CTV)

Internal Target Volume (ITV)

Internal

Motion +

Planning Target Volume (PTV)

On board

imaging

Respiratory

gating Imaging

CT, PET, MRI Biology

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Organs at Risk (OAR)

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Planning Organ at Risk Volume (PRV)

• … Like CTV to PTV

• Accounts for OAR movements

• Shape, size, setup

• PRV = OAR + margin

• PTV and PRV may overlap

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Changes Over Time: ICRU 50 to 62

• PRV: Includes margin around the OAR to compensate for changes in shape and internal motion and for set-up variation.

GTV

CTV

ITV

PTV

TREATED VOLUME

IRRADIATED VOLUME

OAR

PRV

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Conformity Index (CI) in ICRU 62

PTV

volumeTreatedCI

Ideal

CI=1.00

Note: In ICRU 83 discussion on conformity indices:

“…the applicability … of above indices for

reporting results of IMRT is likely to be limited.”

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ICRU 50 & 62 Summary

• Consistent specification of dose and dose

homogeneity are essential

• Facilitates communication

• Improves knowledge of dose-response data

• ICRU 50 & 62 are widely accepted

2/25/2011 38

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Changes Over Time

Purdy, Sem Rad Oncol 14: 27, 2004

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ICRU 83- Purpose

• Irradiation techniques have advanced

• 3-D CRT to IMRT

• More availability of CT

• Additional imaging – CT + MRI, PET, PET/CT,

functional

• Improved conformality

• Reduced doses to normal tissues

• More detailed dose-volume information on TPS

• Use of dose-volume constraints

• Automated optimization, IMRT 40

2010

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ICRU 83

• Dose reporting adapted to IMRT

• Use of DVH

• No Dmin or Dmax , instead …

• D98% and D2%

• Specify median dose, D50%

• Close to “old” ICRU reference dose at “ICRU

reference point”

2010

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ICRU 83 – Dose Specification

~= dose to ICRU

reference point

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ICRU 83 – Dose Accuracy

• Old: dose accuracy ±5%

• New:

• More statistical

• Two regions

• Low dose gradient (<20%/cm)

• 85% of target volume, dose within 5%

• High dose gradient (≥20%/cm)

• Specify distance to agreement

• 85% of dose samples, within 5 mm

2010

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Remaining Volume at Risk (RVR)

• Optimization

• Carcinogenisis

2010

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3-D vs. IMRT

Implications for target definition

• Concave vs. convex targets

• Target volume quality assurance/peer review

• 3-D beams more readily compared to

“historical beams” 2-D beams

• IMRT beams/plans are less able to be

checked via intuition.

• Dose gradients

• Margins

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3D-CRT: Convex Dose Distribution

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IMRT: Concave Dose Distribution

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Convex vs Concave

• Careful definition of the concavity is more

important for IMRT than for 3-D CRT

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Reducing Margins

• “Technical” margins can be reduced by

improving daily setup.

• Daily setup verification by EPID, CBCT,

ultrasounds…

• Automatic repositioning of patients by external

systems (eg, ExacTrac®).

• Automatic repositioning with internal fiducials

(eg, Calypso®).

• …

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Note ...

We may be able to reduce

technical margins ...

but not biological

margins GTV

CTV

ITV

PTV

TREATED VOLUME

IRRADIATED VOLUME

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Summary

• ICRU reports provide …

• Consistency in target volume definition

• Consistency in dose prescription

• Consistency in reporting

• Tumour doses

• Normal tissue doses

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MINIMIZE

THE WORLD’S WAR AGAINST

NORMAL TISSUE DAMAGE

RADIATION

ONCOLOGISTS MEDICAL

PHYSICISTS