forward vs. inverse planning inverse planning techniques ... · 1 inverse planning techniques for...

1

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

2

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.

3

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

4

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,

5

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

6

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

7

What We Can Get Critical Structures Important

See What We GetCritical Structures Important



8

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

9

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



10

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

11

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)

12

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

13

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

14

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

15

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%

16

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

17

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%

18

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

19

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

20

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

21

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

22

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

forward vs. inverse planning inverse planning techniques ... · 1 inverse planning techniques for...

Documents