Inverse Planning Techniques
for IMRT
Ping Xia, Ph.D.
University of California-San Francisco
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 targets & sensitive structures,
as well as their dose constraints.
AAPM 2004, course TH-A-BRA CE
Inverse Planning Is Less Forgiving
Adding Artificial Structure
• Only treat contoured tumor targets.
• Only spare contoured sensitive structures.
70 Gy, 59.4 Gy, 54.0 Gy, 45 Gy
1
Volume Delineations
Volume Delineation
Sensitive Structure
Delineation
• 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) …
• How to define target volumes?
• How to contour sensitive structures?
• How many sensitive structures should
be contoured?
What are Serial and Parallel Organs ?
• A Serial organ is damaged if one of its subvolumes is damaged.
• A parallel organ loses its functionality only if all
sub-volumes of the organ are damaged.
2
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
Tumor Margin vs Beam Margin
3D Tumor Margin or 2D Tumor Margin
• Tumor margin: position uncertainties
localization uncertainties
• Beam margin: Beam penumbra
1.5 cm block margin = 0.8 cm tumor margin
+ 0.7 cm beam margin
3 mm superior
3
Dose Constraints
• Inverse planning requires us to specify dose constraints
to all structures.
Dose Constraints
• Inverse IMRT planning becomes a trial-error process in
searching for a proper dose constraint specification.
• Improperly specified dose constraints will result in
inferior plans
Tell Me Your Dream
Treatment Goals
Rx doses:
• Full dose to the tumor target
• Zero dose to sensitive structures
Impossible !!!!
95 % GTV > 70 Gy at 2.12 Gy
95 % PTV > 59.4 Gy at 1.8 Gy
Tolerance doses:
Spinal Cord: Max < 45 Gy,
Brain Stem: Max < 55 Gy,
Parotid glands: mean dose < 26 Gy,
Optic structures: Max < 54 Gy,
4
Realty and Physics Limitations
60, 50, 42, 34 Gy
70%
• 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.
40%
15 Gantry angles
Everything Equally Important
Systematic Trial-and-Error
5
See What We Get
Everything Equally Important
• Rx 70% to 66 Gy
7% of GTV underdose,
10% of CTV underdose,
• Max-dose
RT-eye = 64 Gy, LT-eye =61
RT-OPN = 56 Gy, LT-OPN = 57 Gy
Brain Stem = 46 Gy
Chiasm = 54 Gy
Tumor Important
Tumor 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
6
What We Can Get
See What We Get
Critical Structures Important
Critical Structures Important
• 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
7
Compromise Solution
Final Solution
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
8
Beam Selections
Beam Angle Selection
• Are more beams better than fewer
beams?
• Equal spaced beam angles?
• Non-coplanar beam angles?
Not necessarily
Non-coplanar Beam Angles
• Rx 80% to 66 Gy
5% of GTV underdose,
8% CTV underdose,
• Max-dose to critical structures
RT-eye = 62 Gy, LT-eye =63 Gy
RT-OPN = 49 Gy, LT-OPN = 51 Gy
Brain Stem = 39 Gy
Chiasm = 53 Gy
15 Beam Angles
• Rx 82% to 66 Gy
4 % of GTV underdose,
4% CTV underdose,
• Max-dose to critical structures
RT-eye = 63 Gy, LT-eye =62 Gy
RT-OPN = 53 Gy, LT-OPN = 56 Gy
Brain Stem = 33 Gy
Chiasm = 56 Gy
9
9 beam angles
15 beam angles
Plan Refinement
Non-coplanar beam angles
70 Gy, 60 Gy, 54 Gy, 45 Gy
Plan 1
Plan 1
Plan 2
10
Plan 2
Plan 1
Plan 2
70 Gy, 59.4 Gy, 52 Gy
Evaluation of IMRT Plans
Plan Evaluation
• Define endpoints
• Dose volume histogram (DVH)
• Dose distributions on every CT slice
(Rx, hot spot, cold spot)
11
Plan Acceptance Criteria
Head and Neck Tumor:
> 80% isodose line to the GTV
70 Gy > 95% of GTV (2.12 Gy/day)
59.4 Gy > 95% of CTV (1.8 Gy/day)
54 GY > 95% of CTV2 (1.64 Gy/day)
Plan Acceptance Criteria
• Sensitive Structures:
Serial Structures: Maximum dose
Cord < 45 Gy, 1cc < 40 Gy
Stem < 54 Gy, 1 cc < 54 Gy
Optic structures < 54 Gy
Mandible < 70 Gy
Temporal lobe < 70 Gy
Plan Acceptance Criteria
• Parallel Structures: Mean dose
Parotid < 26 Gy~ 30 Gy
Inner ear < 50 Gy
Isodose Distributions
Other Structures: as low as possible
Oral cavity
sub-mandibular gland
Larynx
12
70.0 Gy,
59.4 Gy,
54 Gy
70 Gy,
59.4 Gy,
45 Gy
Cold spot
Hot-spot
Three Dimensional Examination
Class Solutions
70 Gy
60 Gy
70 Gy
60 Gy
6 mm superior
13
T1-2 Nasopharyngeal Cancer
Review Old Plans
• Review previous clinically accepted plans
9 plans for T1-2 Nasopharyngeal patients.
16 plans for T3-4 Nasopharyngeal patients.
(Xia, P et. al, IJROBP, in press)
T3-4 Nasopharyngeal Cancer
Structures
Max Dose (Gy)
42.7
36.4
34.2
Spinal Cord
42.2
33.0
26.7
Brain Stem
55.3
43.1
40.0
Optic Nerve
41.6
34.4
31.6
Eye
32.8
21.9
19.6
Mean Dose (Gy)
Parotid Gland
T-M joint
Dose to 5% Vol.
(Gy)
21.5
30.6
40.4
22.2
13.5
Dose to 50% Vol.
(Gy)
Dose to 10% Vol.
(Gy)
19.7
25.8
37.6
18.8
9.8
Dose to 80% Vol.
(Gy)
26.8
33.8
25.1
30.5
17.9
26.7
41.4
38.3
31.3
PTV70
Dose to 10% Vol.
(Gy)
Chiasm
Dose to 50% Vol.
(Gy)
Max. Dose
(Gy)
27.5
38.3
50.9
23.7
25
Mean Dose
(Gy)
Parotid
Gland
T-M joint
Mid./Inner
Ear
24 plans for oropharyngeal patients.
Dose to 5% Vol.
(Gy)
Structures
Chiasm
Spinal Cord
Brain Stem
Optic Nerve
Eye
Dose to 80% Vol.
(Gy)
27.8
24.6
18.7
38
36.7
31.5
49.6
49.8
42.2
PTV70
PTV70
PTV60
PTV60
PTV60
Cord
Lt Parotid
Rt Parotid
Brainstem
AntiPTV
Parotids
Middle/Inner Ear
Q block
14
Neck skin
airway
Artificial structure
79.2, 70.0, 59.4, 54.0, 45.0 Gy
79.2, 70.0, 59.4, 54.0, 45.0 Gy
Nasopharynx
Tumor Target –
Oropharyngeal Cancer
GTV
Vol.
(cc)
D99%
(Gy)
D95%
(Gy)
D1cc
(Gy)
V93%
(cc)
PTV-70
76.7
± 47.3
69.3
± 1.4
71.2
± 1.5
80.2
± 2.6
0.1
± 0.1
PTV-60
690.4
± 274.1
54.3
±4.7
60.6
± 2.7
80.8
± 2.0
9.0
± 4.8
Parotids
PTV
cord
PTV2
stem
Rx IDL 85.8% ± 2.0%
15
Sensitive Structures – Oropharyngeal
Cancer
Spinal
Cord
Brain
Stem
Mandible
D1cc
(Gy)
D1%
(Gy)
42.6
± 3.5
43.5
± 9.8
71.6
± 2.9
40.2
± 3.8
40.1
± 10.1
67.7
± 3.0
Parotid
Ear
TMJ
Mean
(Gy)
Median
(Gy)
26.1
± 3.2
24.2
± 8.6
26.9
± 7.6
23.5
± 3.5
23.3
± 8.9
26.2
± 7.5
Oropharynx
78.2 Gy
70.0 Gy
59.4 Gy
54.0 Gy
45.0 Gy
Oropharynx
Oropharynx
GTV
Brain stem
Parotids
PTV
cord
78.2 Gy, 70.0 Gy, 59.4 Gy, 54.0 Gy, 45 Gy
16
Class Solutions
• Class solutions can be applied to patients with
same or similar types of cancer
• Streamline treatment planning can significantly
improve planning efficiency.
• Planning turn around time has been reduced from
one week to two days.
• Actual planning time for a typical head and neck
case is about 4-8 hours, including contouring ,
printing, waiting, coffee break…
Simplify IMRT Plans
Tumor Target –
Oropharyngeal Cancer
Seeking Simple IMRT Plans
• Five oropharyngeal cases were planned using five
different beam angle arrangements.
• The criteria for plan acceptance are based on
RTOG protocols (RTOG-0022)
• Five patients were not limited to early stage as in
RTOG protocol.
Submitted to Int. J. Radiat. Oncol. Biol. Phys
Vol.
(cc)
D99%
(Gy)
D95%
(Gy)
D1cc
(Gy)
V93%
(cc)
PTV-70
76.7
± 47.3
69.3
± 1.4
71.2
± 1.5
80.2
± 2.6
0.1
± 0.1
PTV-60
690.4
± 274.1
54.3
±4.7
60.6
± 2.7
80.8
± 2.0
9.0
± 4.8
Rx IDL 85.8% ± 2.0%
17
Sensitive Structures – Oropharyngeal
Cancer
0O
320O
Spinal
Cord
Brain
Stem
Mandible
D1cc
(Gy)
D1%
(Gy)
42.6
± 3.5
43.5
± 9.8
71.6
± 2.9
40.2
± 3.8
40.1
± 10.1
67.7
± 3.0
Parotid
Ear
TMJ
Mean
(Gy)
Median
(Gy)
26.1
± 3.2
24.2
± 8.6
26.9
± 7.6
23.5
± 3.5
23.3
± 8.9
26.2
± 7.5
30O
290
280O
0O
340O
40O
O
80O
90O
260O
240O
120O
200
160
O
230O
130O
O
9 Equally Spaced
8 selected angles
0O
300O
0O
60O
270O
90O 270O
90O
240O
210
O
150
O
65O
120O
210
150
O
180O
7 selected angles
Forward plan
295O
7 angles from
MSKCC
O
230O
130O
Five beam angles
18
PTV70
Isodose line covering 95% of GTV
PTV70
PTV70
0.89
isodose line (%)
PTV60
PTV60
PTV60
Cord
Lt Parotid
Rt Parotid
Brainstem
0.88
0.87
0.86
0.85
0.84
9 angles
AntiPTV
8 angles
7 angles
Parotids
7 angles
(MSKCC)
5 angles
FPMS
Q block
Endpoint doses to sensitive structures
Target Volume Coverages (V70/V59.4)
GTV/CTV
4500.00
4000.00
98.00
Dose (cGy)
Volume (%)
100.00
96.00
94.00
92.00
Mean dose to parotid
Max dose to 1 cc of cord
3500.00
3000.00
2500.00
90.00
9 angles
8 angles
7 angles
7 angles
(MSKCC)
5 angles
FPMS
2000.00
9 angles
8 angles
7 angles
7 angles
(MSKCC)
5 angles
FPMS
19
Treatment delivery time
25.00
Time (min)
20.00
15.00
10.00
5.00
0.00
9 angles
8 angles
7 angles
7 angles
(MSKCC)
5 angles
7000 cGy
6000 cGy
4500 cGy
2500 cGy
Seeking Simple IMRT Plans
7000 cGy
6000 cGy
4500 cGy
2500 cGy
• For simple H&N cases (oropharyngeal),
5-6 beam angles with 60-80 segments
~ 15-20 minutes.
• For complex H&N cases (naso, sinus),
7-8 beam angles with 100-130 segments
~ 20 – 30 minutes.
Submitted to Int. J. Radiat. Oncol. Biol. Phys
20
Special Clinical Problems-Skin
Dose Problem
Patient with marked skin reaction
Skin Dose Investigation
Patient Skin Dose Problem
• Multiple tangential beams decrease skin sparing.
Opp. Lateral
IMRT w/ skin
included
IMRT w /skin
excluded
IMRT w/skin
excluded + skin
spare
w/
mask
w/o
mask
w/
mask
w/o
mask
w
/mask
w/o
mask
w/
mask
w/o
mask
Ave. Daily
dose (Gy)
1.53
+ 0.39
1.31
+ 0.31
1.82
+0.13
1.66
+ 0.15
1.53
+0.16
1.25
+0.17
1.44
+0.12
1.17
+0.10
Ave. Total
dose (Gy)
50.63
43.12
60.10
54.64
50.34
41.10
47.54
38.70
• Bolus effect, due to the use of the headshoulder mask, increases skin dose about 15%.
• In order to cover superficial nodes, the inverse
planning system increases beam intensity on
the neck skin.
• Neck skin may be contoured as a sensitive structure
to avoid high dose on the neck skin.
Lee, N, et. al. IJROBP, 2002.
21
100
Volume (%)
80
60
40
Non-Skin Sparing Plan
Skin Sparing Plan
20
0
0
10
20
30
40
50
60
70
80
Dose (Gy)
Take Home Messages
• Inverse planning is not intuitive but easy to establish
class solution for a specific cancer.
• Know the realistic goals, find the upper limit and lower
limits for both dose conformity and uniformity.
• Systematically research for compromise solution
– Find a proper dose constraints while
Patient neck skin sparing
Take Home Messages
• Once you know the upper and lower limits,
simplify IMRT plan as much as possible to reduce
treatment time, unnecessary radiation…
• Develop your own class solutions
starting with 9 –11 beam angles
– Find a optimal beam angles while
keeping the same dose constraints
22