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

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Acknowledgement

• Slides from

• Morten Høyer

• Richard Pötter

• Larry Marks

• Stewart Gaede

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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