Download - Forward vs. Inverse Planning Inverse Planning Techniques ... · 1 Inverse Planning Techniques for IMRT Ping Xia, Ph.D. University of California-San Francisco AAPM 2004, course TH-A-BRA

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Inverse Planning Techniques for IMRT

Ping Xia, Ph.D.

University of California-San Francisco

AAPM 2004, course TH-A-BRA CE

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.

Inverse Planning Is Less Forgiving

• Only treat contoured tumor targets.

• Only spare contoured sensitive structures.

70 Gy, 59.4 Gy, 54.0 Gy, 45 Gy

Adding Artificial Structure

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Volume Delineation

Volume Delineations

• How to define target volumes?• How to contour sensitive structures?• How many sensitive structures should

be contoured?

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) …

What are Serial and Parallel Organs ?

• A Serial organ is damaged if one of its sub-volumes is damaged.

• A parallel organ loses its functionality only if all sub-volumes of the organ are damaged.

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R T L T

2 9

3 0

3 1

3 2

3 3

3 4

3 5

3 6

3 7

3 8

Dose

(Gy)

ABCDE

Differences in Mean Dose to Parotid Glands

Tumor Margin vs Beam Margin

• Tumor margin: position uncertaintieslocalization uncertainties

• Beam margin: Beam penumbra

1.5 cm block margin = 0.8 cm tumor margin + 0.7 cm beam margin

3 mm superior

3D Tumor Margin or 2D Tumor Margin

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Dose Constraints

Dose Constraints• Inverse planning requires us to specify dose constraints

to all structures.

• 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

• Full dose to the tumor target• Zero dose to sensitive structures

Impossible !!!!

Treatment GoalsRx doses:

95 % GTV > 70 Gy at 2.12 Gy95 % 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,

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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.

15 Gantry angles

60, 50, 42, 34 Gy

70%

40%

Systematic Trial-and-Error

Everything Equally Important

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See What We Get

• Rx 70% to 66 Gy7% of GTV underdose, 10% of CTV underdose,

• Max-dose RT-eye = 64 Gy, LT-eye =61RT-OPN = 56 Gy, LT-OPN = 57 GyBrain Stem = 46 GyChiasm = 54 Gy

Everything Equally Important

Tumor Important

• Rx 84% to 66 Gy4 % of GTV underdose, 5% CTV underdose,

• Max-dose to critical structures RT-eye = 71 Gy, LT-eye =64 GyRT-OPN = 66 Gy, LT-OPN = 69 GyBrain Stem = 48 GyChiasm = 59 Gy

Tumor Important

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What We Can Get Critical Structures Important

See What We GetCritical Structures Important



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Compromise Solution Final Solution

Final Solution

• Rx 80% to 66 Gy6% of GTV underdose, 8% CTV underdose,


70 Gy, 60 Gy, 54 Gy, 45 Gy

Equal important

Critical structure

Tumor important

Compromised

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Beam Angle Selection

Beam Selections

• 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 Gy5% of GTV underdose, 8% CTV underdose,


15 Beam Angles



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15 beam angles 9 beam angles

Non-coplanar beam angles

70 Gy, 60 Gy, 54 Gy, 45 Gy

Plan Refinement

Plan 1

Plan 2

Plan 1

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Plan 2

Plan 1 Plan 2

70 Gy, 59.4 Gy, 52 Gy

Plan Evaluation

Evaluation of IMRT Plans

• Define endpoints• Dose volume histogram (DVH)• Dose distributions on every CT slice

(Rx, hot spot, cold spot)

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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)

• Sensitive Structures:Serial Structures: Maximum dose

Cord < 45 Gy, 1cc < 40 GyStem < 54 Gy, 1 cc < 54 GyOptic structures < 54 GyMandible < 70 GyTemporal lobe < 70 Gy


• Parallel Structures: Mean doseParotid < 26 Gy~ 30 Gy

Inner ear < 50 Gy

Other Structures: as low as possibleOral cavitysub-mandibular glandLarynx


Isodose Distributions

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Cold spot

70 Gy,59.4 Gy,45 Gy

Hot-spot

70.0 Gy,59.4 Gy,54 Gy

6 mm superior

Three Dimensional Examination

70 Gy60 Gy

70 Gy60 Gy

Class Solutions

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Review Old Plans• Review previous clinically accepted plans

9 plans for T1-2 Nasopharyngeal patients.16 plans for T3-4 Nasopharyngeal patients.

24 plans for oropharyngeal patients.

(Xia, P et. al, IJROBP, in press)

31.338.3 41.4 Mid./Inner Ear

26.7 30.5 33.8 T-M joint17.9 25.1 26.8

Parotid Gland

Dose to 80% Vol. (Gy)


Mean Dose (Gy)

9.8 13.5 25Eye18.8 22.2 23.7 Optic Nerve37.6 40.4 50.9 Brain Stem25.8 30.6 38.3 Spinal Cord 19.7 21.5 27.5 Chiasm



Max. Dose (Gy)Structures

T1-2 Nasopharyngeal Cancer

42.249.8 49.6Middle/Inner Ear

31.5 36.7 38T-M joint18.7 24.6 27.8Parotid Gland


Dose to 50% Vol. (Gy)Mean Dose (Gy)

19.6 21.9 32.8 Eye31.634.4 41.6 Optic Nerve40.0 43.1 55.3 Brain Stem26.7 33.0 42.2Spinal Cord 34.236.442.7Chiasm


Dose to 5% Vol. (Gy)Max Dose (Gy)Structures

T3-4 Nasopharyngeal Cancer PTV70

PTV70

PTV70

PTV60

PTV60

PTV60

Cord

Lt Parotid

Rt Parotid

Brainstem

AntiPTV

Parotids

Q block

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79.2, 70.0, 59.4, 54.0, 45.0 GyArtificial structure

airway

Neck skin

79.2, 70.0, 59.4, 54.0, 45.0 Gy

cord

stem

Parotids

GTV

PTV

PTV2

Nasopharynx Tumor Target –Oropharyngeal Cancer

9.0 ± 4.8

80.8 ± 2.0

60.6± 2.7

54.3 ±4.7

690.4 ± 274.1

PTV-60

0.1 ± 0.1

80.2 ± 2.6

71.2± 1.5

69.3 ± 1.4

76.7 ± 47.3

PTV-70

V93% (cc)

D1cc (Gy)

D95% (Gy)

D99% (Gy)

Vol. (cc)

Rx IDL 85.8% ±±±± 2.0%

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Sensitive Structures – Oropharyngeal Cancer

26.2± 7.5

26.9 ± 7.6

TMJ67.7± 3.0

71.6± 2.9

Mandible

23.3 ± 8.9

24.2 ± 8.6

Ear40.1± 10.1

43.5± 9.8

Brain Stem

23.5 ± 3.5

26.1 ± 3.2

Parotid40.2 ± 3.8

42.6 ± 3.5

Spinal Cord

Median (Gy)

Mean (Gy)

D1% (Gy)

D1cc (Gy)

78.2 Gy70.0 Gy59.4 Gy54.0 Gy45.0 Gy

Oropharynx

Oropharynx

78.2 Gy, 70.0 Gy, 59.4 Gy, 54.0 Gy, 45 Gy

GTV

PTV

Brain stem

cord

Parotids

Oropharynx

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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

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

Tumor Target –Oropharyngeal Cancer

9.0 ± 4.8

80.8 ± 2.0

60.6± 2.7

54.3 ±4.7

690.4 ± 274.1

PTV-60

0.1 ± 0.1

80.2 ± 2.6

71.2± 1.5

69.3 ± 1.4

76.7 ± 47.3

PTV-70

V93% (cc)

D1cc (Gy)

D95% (Gy)

D99% (Gy)

Vol. (cc)

Rx IDL 85.8% ±±±± 2.0%

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Sensitive Structures – Oropharyngeal Cancer

26.2± 7.5

26.9 ± 7.6

TMJ67.7± 3.0

71.6± 2.9

Mandible

23.3 ± 8.9

24.2 ± 8.6

Ear40.1± 10.1

43.5± 9.8

Brain Stem

23.5 ± 3.5

26.1 ± 3.2

Parotid40.2 ± 3.8

42.6 ± 3.5

Spinal Cord

Median (Gy)

Mean (Gy)

D1% (Gy)

D1cc (Gy)

9 Equally Spaced 8 selected angles

0O

200O 160O

240O

320O

120O

40O

80O

0O

30O340O

290O

230O

90O

130O

280O

260O

7 selected anglesForward plan

7 angles fromMSKCC

210O

270O

150O

90O

180O

120O240O

0O

210O 150O

60O

90O270O

300O

Five beam angles

230O

65O

0O

130O

295O

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PTV70

PTV70

PTV70

PTV60

PTV60

PTV60

Cord

Lt Parotid

Rt Parotid

Brainstem

AntiPTV

Parotids

Q block

Isodose line covering 95% of GTV

0.84

0.85

0.86

0.87

0.88

0.89

9 angles 8 angles 7 angles 7 angles(MSKCC)

5 angles FPMS

isod

ose

line

(%)

Target Volume Coverages (V70/V59.4)

90.00

92.00

94.00

96.00

98.00

100.00


5 angles FPMS

Volu

me

(%)

GTV/CTV

Endpoint doses to sensitive structures

2000.00

2500.00

3000.00

3500.00

4000.00

4500.00


5 angles FPMS

Dose

(cG

y)

Mean dose to parotid Max dose to 1 cc of cord

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Treatment delivery time

0.00

5.00

10.00

15.00

20.00

25.00


5 angles

Tim

e(m

in)

7000 cGy6000 cGy4500 cGy2500 cGy

7000 cGy6000 cGy4500 cGy2500 cGy

Seeking Simple IMRT Plans

• 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

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Special Clinical Problems-Skin Dose Problem

Patient with marked skin reaction

Patient Skin Dose Problem• Multiple tangential beams decrease skin sparing.

• Bolus effect, due to the use of the head-shoulder mask, increases skin dose about 15%.

• In order to cover superficial nodes, the inverse planning system increases beam intensity onthe 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.

38.7047.5441.1050.3454.6460.1043.1250.63Ave. Total dose (Gy)

1.17 +0.10

1.44 +0.12

1.25 +0.17

1.53 +0.16

1.66 + 0.15

1.82 +0.13

1.31 + 0.31

1.53 + 0.39

Ave. Daily dose (Gy)

w/o mask

w/ mask

w/o mask

w/mask

w/o mask

w/mask

w/o mask

w/mask

IMRT w/skin excluded + skin

spare

IMRT w /skin excluded

IMRT w/ skin included

Opp. Lateral

Skin Dose Investigation

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807060504030201000

20

40

60

80

100

Non-Skin Sparing PlanSkin Sparing Plan

Dose (Gy)

Volu

me

(%)

Patient neck skin sparing

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

starting with 9 –11 beam angles– Find a optimal beam angles while

keeping the same dose constraints

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

Download - Forward vs. Inverse Planning Inverse Planning Techniques ... · 1 Inverse Planning Techniques for IMRT Ping Xia, Ph.D. University of California-San Francisco AAPM 2004, course TH-A-BRA

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