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
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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