Fundamental Issues
Optimization for IMRT (I)
- Fundamental Issues
Ping Xia, Ph.D.
University of California-San Francisco
•
•
•
•
How many structures should be contoured?
How to deal with overlapped structures?
What are proper dose constraints?
Beam angle selection - how many beams are
enough?
• How good is good enough for an IMRT plan?
• How to evaluate IMRT plans? Or how to sell
IMRT plans.
AAPM 2005, course MO-B-T-6E
Forward vs. Inverse Planning
• Conventional forward planning mostly depends
on geometric relationship between the tumor and
nearby sensitive structures.
• Inverse planning is less dependent on the
geometric parameters but more on specification of
volumes of tumor & sensitive structures, as well
as their dose constraints.
“Inverse Planning Is Less Forgiving”
• Only treat specified tumor targets.
• Only spare specified sensitive structures.
Dr. James Purdy
1
To know what you want
• Inverse planning requires us to know precisely
what to treat and what to spare.
• How to compromise if treatment requirements are
conflicting.
Tell Me What you want
• Full dose to the tumor target
• Zero dose to sensitive structures
Impossible !!!!
Talk with Radiation Oncologists
• Know patient specific planning requirements.
• Know physician’s wishful thinking.
• What to compromise if you can not achieve
planning requirements.
• Is uniform dose important?
• Most importantly, What is Rx dose and daily
fractional dose.
Target and sensitive structure
delineations
2
Delineation of target volumes and
sensitive structures becomes a very
important yet time consuming task
in inverse planning because this is a
way that we provide spatial
information to the optimizer.
Volume Delineations
• How to define target volumes? –totally
leave to radiation oncologists.
• How to contour sensitive structures? – also
leave to radiation oncologists if you can wait
and they have time to do so.
• How many sensitive structures should be
contoured?
Learn Anatomy 101
• It becomes essential for physicists and
dosimetrists to know cross sectional
anatomy.
• Understand image differences in various
imaging modalities - CT, MRI, PET.
How many normal
structures for H&N Cases?
• About 24 sensitive structures need to be
contoured
• Lt & Rt parotid, optic nerves, eyes, lens, inner
ears, TMJ ( 12).
• Spinal cord, brain stem, chiasm, brain, temporal
lobes, larynx, mandible, tongue, airway, apex
lung, neck skin, thyroid (12) …
3
How many normal structures for
prostate case?
Rind Structure Used In Prostate Planning
• Rectum, bladder, colon, penile bulb, small bowel,
femur heads, pelvic bones.
• Artificial structures – e.g. planning rinds
• Determine superior and inferior borders of the
rectum
• Determine small bowel contours – only circular
structures, or the entire pelvic cavity.
• Should large bowel separate from small bowel?
R. Price, et. al. IJROBP Vol. 57, 843–852, 2003
Differences in Mean Dose to Parotid Glands
A
B
C
D
E
38
37
36
Dose (Gy)
35
34
33
32
31
30
29
RT
LT
4
Overlapped Structures
Tumor Margin vs Beam Margin
• From optimizer eye view:
– Overlapped structures cause ambiguity during
optimization
• From physician eye view:
– PTV will overlap with other sensitive structures.
– CTV can be some normal tissue – e.g. CTV can be a
part of the lung.
• From physicist eye view:
– optimize plan with no overlap structures if possible.
– Evaluate plan with overlapped structures
3D Tumor Margin or 2D Tumor Margin
• What does it mean 1.5 cm block margin
• Beam margin: Beam penumbra ~ 0.7 cm
• Tumor margin: position uncertainties
localization uncertainties ~ 0.8 cm
How many beams and
what beam angles?
• For head and neck cancer, in average, 8 beams are
sufficient.
• For prostate cancer, five to seven beams are
enough
• For breast cancer, two tangential beams are still
the best.
3 mm superior
5
General rules of beam angle selection
Brain Tumor
Tolerance doses:
Optic nerves: Max dose < 54Gy
Lens: Max dose < 6 Gy
Chiasm: Max dose < 54 Gy
Brainstem: Max dose < 54 Gy
Eyes: Max dose < 45 Gy
•
•
•
•
Avoid critical structures
Choose shortest pathway to irradiate the tumor.
Keep large beam separation if possible.
Beam angle selection becomes important if the
tumor is not centrally located.
• Depending on the optimizer of each planning
system, theses rules may not be applicable.
Poster,SU-FF-T-72
Right Optic nerve
PTV Target
Clinic
100
100
Volume (%)
Volume (%)
Clinic
9 beams
60
12 beams
15 BEAMS
40
Beam selection
80
Beam selection
80
20
9 beams
60
12 beams
40
15 BEAMS
20
0
0
10
20
30
40
Dose (Gy)
50
60
70
80
0
0
5
10
15
20
25
Dose (Gy)
6
Right Eye
Left Optic nerve
100
100
Volume (%)
Volume (%)
60
Clinic
Beam selection
9 beams
12 beams
15 beams
80
Clinic
Beam selection
9 beams
12 beams
15 beams
80
40
60
40
20
20
0
0
0
5
10
15
0
20
5
10
15
20
25
Dose (Gy)
Dose (Gy)
Left Lens
Left Eye
100
Volume (%)
60
40
100
Clinic
Beam selection
9 beams
12 beams
15 beams
80
Volume (%)
Clinic
Beam selection
9 beams
12 beams
15 beams
80
60
40
20
20
0
0
5
10
15
Dose (Gy)
20
25
0
0
5
Dose (Gy)
10
15
7
Right Lens
Brain Stem
100
100
Volume
60
Volume (%)
Clinic
Beam selection
9 beams
12 beams
15 beams
80
40
Clinic
Beam selection
9 beams
12 beams
15 beams
80
60
40
20
20
0
0
5
10
15
Dose (Gy)
20
0
5
10
15
20
25
30
35
40
Dose (Gy)
Dose Constraints
Dose Constraints
• Inverse planning requires us to specify dose
constraints to all structures.
• Inverse IMRT planning becomes a trialerror process in searching for a proper dose
constraint specification.
• Improperly specified dose constraints will
result in inferior plans
8
What are Serial and Parallel
Organs ?
• A Serial organ will be damaged if one of its
sub-volumes is damaged.
• A parallel organ will lose its functionality
only if all sub-volumes of the organ are
damaged.
Dose Tolerance to Some Organs
Organs
Dose
Organs
Dose
Lung
V20Gy< 35%
(both lungs)
Small Bowel
Kidney
Mean < 15Gy
Retina
V45Gy<5%
Or as less as
possible
Max <50 Gy
Liver
Mean <30Gy
Rectum
D20%<65Gy
Temporal
Lobe
Mandible
Bladder
D 25% <65Gy
Max< 65 Gy
Max < 70 Gy
Serial and Parallel Structures in a
Typical H&N Case
Serial Structures
Parallel Structures
Spinal cord
Parotid gland
Brain stem
Inner/middle ears
Optic structures
Tongue/ oral cavity
Temporal Lobes
Neck skin
Mandible, TMJ
….
Treatment Goals
Rx doses:
95 % GTV > 70 Gy at 2.12 Gy
95 % PTV > 59.4 Gy at 1.8 Gy
Tolerance doses:
Spinal Cord: Max < 45 Gy, 1cc < 45Gy
Brain Stem: Max < 55 Gy, 1% <54 Gy
Parotid glands: mean dose < 26 Gy,
Optic structures: Max < 54 Gy,
9
Dose Tolerance of Normal Tissue
Systematic Trial-and-Error
• A classical reference for dose tolerance of normal
tissue is:
• Start with 7-9 beams to find a well balanced dose
constraint.
Emami B, et al. “ Tolerance of normal tissue to
therapeutic irradiation”, IJROBP, 1991; 21 (109122).
• Discuss with your radiation oncologists for their
updated dose tolerance to various organs, and
special requirement for each specific case.
– Find the upper limits
– Find the lower limits
– Find a compromise solutions
• With a well balanced dose constraint, find a set of
beam angles that should be clinically deliverable
and practical.
Tumor Important
70 Gy, 60 Gy, 54 Gy, 45 Gy
10
Tumor Important
Critical Structures Important
• Rx 84% to 66 Gy
4 % of GTV underdose,
5% CTV underdose,
• Max-dose to critical structures
RT-eye = 71 Gy, LT-eye =64 Gy
RT-OPN = 66 Gy, LT-OPN = 69 Gy
Brain Stem = 48 Gy
Chiasm = 59 Gy
Critical Structures Important
Final Solution
• Rx 75% to 66 Gy
6 % of GTV underdose,
7% CTV underdose,
• Max-dose to critical structures
RT-eye = 63 Gy, LT-eye =64 Gy
RT-OPN = 51 Gy, LT-OPN = 51 Gy
Brain Stem = 42 Gy
Chiasm = 51 Gy
11
Final Solution
• Rx 80% to 66 Gy
6% of GTV underdose,
8% CTV underdose,
• Max-dose to critical structures
RT-eye = 60 Gy, LT-eye =62 Gy
RT-OPN = 55 Gy, LT-OPN = 56 Gy
Brain Stem = 46 Gy
Chiasm = 54 Gy
Equal important
Tumor important
Critical structure
Compromised
70 Gy, 60 Gy, 54 Gy, 45 Gy
Check Anatomy
How to modify plans that
are not acceptable
• Clean up tumor volume.
• Understand geometry limiting factor – are
there overlapped structures?
• Is PTV extended to the outside of skin
surface – limited by most PTS.
• Is tumor volume close to the skin surface –
leave 3-5 mm away from the skin surface.
12
Plan 1
Looking for Unreasonable Dose Limits
Plan 2
Plan 1
Limiting factor
Balance Between Dose Conformity and Uniformity
Plan Evaluation – How to
sell your plan
Plan 2
13
Realty and Physics Limitations
• Single beam penumbra ~ 7-8 mm, from 90%
- 20% iso-dose lines ~ 10%/mm
• IMRT iso-dose lines are also limited by this
radiation physics.
• Scatter dose from multiple beams makes the
beam penumbra shallower.
Uniformity Vs Conformity
• Uniformity and conformity are often tradeoff with each other.
• A great dose gradient often scarifies dose
uniformity
PTV70 Scoring
Evaluation of IMRT Plans
• Define endpoints
• Dose volume histogram (DVH)
• Dose distributions on every CT slice
(Rx, hot spot, cold spot)
No Variation
95% of PTV70 is at,
or above 70 Gy,
or/and
Minor Variation
Major Variation
95% of PTV70 is at,
or above 65.1 Gy and Failure to achieve either
No Variation or Minor
Variation
No more than 20% of No more than 5% of
PTV70 is at, or above PTV70 is at, or above
77.0 Gy
80.5 Gy
99% of PTV70 is at,
or above 65.1 Gy and
14
PTV59.4 Scoring
No Variation
95% of PTV 59.4 is at,
or above 59.4Gy
and/or
99% of PTV59.4 is at,
or above 55.2 Gy and
No more than 20% of
PTV59.4* is at, or
above 77.0 Gy
Minor Variation
PTV50.4 Scoring
Major Variation
Failure to achieve
95% of PTV59.4 is at, either No Variation or
or above 55.2 Gy and Minor Variation
No more than 5% of
PTV59.4* is at, or
above 80.5 Gy
Parotid Scoring
No Variation
Minor Variation
Major Variation
Mean dose to either 40% of either parotid
parotid is at or less receives less than, or
equal to 30.0 Gy
than 26.0 Gy or
50% of either parotid
Failure to achieve
receives less than
either No Variation
30.0 Gy or
or Minor Variation
20 cc of the
combined parotid
glands receive less
than 20.0 Gy
No Variation
95% of PTV50.4 is
at, or above 50.4 Gy
and /or
99% of PTV50.4 is
at, or above 46.9 Gy
and
No more than 20%
of PTV50.4* is at,
or above 77.0 Gy
Minor Variation
Major Variation
95% of PTV50.4 is Failure to achieve
at, or above 46.9 Gy either No Variation
or Minor Variation
and
No more than 5% of
PTV50.4* is at, or
above 80.5 Gy
Other Organ at Risk
Dose Limit and Criteria
Temporal lobes
Brainstem*, optic nerves,
chiasm*
Spinal Cord+
Mandible ,T-M Joint
60 Gy or 1% of Vol. < 65 Gy
54 Gy or 1% < 60 Gy
45Gy or 1 cc of Vol. < 50.0 Gy
70 Gy or 1cc < 75.0 Gy
15
Plan Acceptance Priority
Spinal Cord < 45 Gy- 50 Gy
1
Brain Stem < 54 Gy –60 Gy
1
95% GTV >Rx dose
2
95% CTV > Rx dose
2
Mean dose to Parotid
3
Mean dose to ears
3
Isodose Distributions
70.0 Gy,
59.4 Gy,
54 Gy
70 Gy,
59.4 Gy,
45 Gy
Cold spot
Hot-spot
16
Three Dimensional Examination
Simplify IMRT Plans
70 Gy
60 Gy
70 Gy
60 Gy
6 mm superior
25 segments
50 segments
75, 70, 59.4, 45, 35 Gy
100 segments
Poster, SU-FF-T96
25 segments
50 segments
100 segments
75, 70, 59.4, 45, 35 Gy
17
Dose Volume Histograms of Tumor
25 segments
100 segments
Volume (%)
50 segments
120
GTV-100seg
100
CTV-100seg
80
GTV-50seg
60
CTV-50seg
40
GTV-25seg
20
CTV-25seg
0
0
75, 70, 59.4, 45, 35 Gy
10
20
30
40
50
60
70
80
90
Dose (Gy)
Average Maximum Dose to Icc of Serial
Structures
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
98 segments
64 segments
50 segments
25 segments
GTV
CTV1
Structures
Dose (cGy)
C.I.
Average Conformality Index
7000
6000
5000
4000
3000
2000
1000
0
98 segments
64 segments
50 segments
25 segments
brain stem
spinal
cord
mandible
chiasm
temple
lobe
Structures
18
Total MUs Vs. Number of segments
Average Mean Dose for Parallel Structures
1000
5000
Average Total MUs
900
98 segments
64 segments
50 segments
25 segments
3000
2000
1000
800
700
600
500
400
300
200
100
0
lt ear
rt ear
lt parotid
0
rt parotid
25 seg
Structures
50 seg
65 seg
100 seg
Average number of segments
Absolute Volume receiving > 2Gy
Seeking Simple IMRT Plans
Beam selection
12000
• Simple IMRT plans can reduce treatment
time.
• Reduce total body dose to patients.
• Reduce potential dosimetric errors with
using fewer small fields and small MUs.
clinic
9 beams
10000
12 beams
Volume (cc)
Dose (cGy)
4000
8000
15 beams
6000
4000
2000
0
Pelvis
H&N
Brain
Posters,SU-FF-T-96, 98
19
Absolute Volume (cc) Receiving > 5 Gy
16000
14000
12000
Volume (cc)
Inverse Planning Tips
Beam selection
Clinic
9 beams
12 beams
15 beams
10000
8000
6000
4000
• Inverse planning is not intuitive but easy to
establish class solution for a specific cancer.
• Learn cross sectional anatomy.
• Know the realistic goals, find the upper limit and
lower limits for both dose conformity and
uniformity.
2000
0
Pelvis
H&N
Brain
Inverse Planning Tips
• Systematically research for compromise solution
– Find a proper dose constraints while
starting with 7-9 beam angles
– Find a optimal beam angles while
keeping the same dose constraints
• Once you know the upper and lower limits, simplify
IMRT plan as much as possible to reduce treatment time,
unnecessary radiation…
Acknowledgement
Thai Binh Nguyen
Erica Ludlum, M.S.
Clayton Akazawa, CMD
Jeff Bellerose, B.S.
Lynn Verhey, Ph.D.
All Radiation Oncologists in the Department
20