fundamental issues optimization for imrt (i) -fundamental ... · check anatomy • clean up tumor...

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Optimization for IMRT (I)- Fundamental Issues

Ping Xia, Ph.D.

University of California-San Francisco

AAPM 2005, course MO-B-T-6E

Fundamental Issues

• 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.

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

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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

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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.

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.

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?

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) …

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How many normal structures for prostate case?

• 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?

Rind Structure Used In Prostate Planning

R. Price, et. al. IJROBP Vol. 57, 843–852, 2003

R T L T

2 9

3 0

3 1

3 2

3 3

3 4

3 5

3 6

3 7

3 8

Do

se (

Gy)

A B C D E

Differences in Mean Dose to Parotid Glands

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Overlapped Structures

• 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

Tumor Margin vs Beam Margin

• What does it mean 1.5 cm block margin

• Beam margin: Beam penumbra ~ 0.7 cm

• Tumor margin: position uncertaintieslocalization uncertainties ~ 0.8 cm

3D Tumor Margin or 2D Tumor Margin

3 mm superior

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.

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General rules of beam angle selection

• 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.

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

Poster,SU-FF-T-72

PTV Target

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Right Optic nerve

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Left Optic nerve

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ClinicBeam selection9 beams12 beams15 beams

Right Eye

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Left Eye

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Right Lens

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Brain Stem

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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

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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.

Serial and Parallel Structures in a Typical H&N Case

….Mandible, TMJ

Neck skinTemporal Lobes

Tongue/ oral cavityOptic structures

Inner/middle earsBrain stem

Parotid glandSpinal cord

Parallel StructuresSerial Structures

Dose Tolerance to Some Organs

D 25% <65GyBladder

Max < 70 GyMandibleD20%<65Gy Rectum

Max< 65 GyTemporal Lobe

Mean <30GyLiver

Max <50 GyRetinaMean < 15GyKidney

V45Gy<5%

Or as less as possible

Small BowelV20Gy< 35%

(both lungs)

Lung

DoseOrgansDoseOrgans

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, 1cc < 45GyBrain Stem: Max < 55 Gy, 1% <54 GyParotid glands: mean dose < 26 Gy,Optic structures: Max < 54 Gy,

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Dose Tolerance of Normal Tissue

• A classical reference for dose tolerance of normal tissue is:

Emami B, et al. “ Tolerance of normal tissue to therapeutic irradiation”, IJROBP, 1991; 21 (109-122).

• Discuss with your radiation oncologists for their updated dose tolerance to various organs, and special requirement for each specific case.

Systematic Trial-and-Error

• Start with 7-9 beams to find a well balanced dose constraint.– 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.

70 Gy, 60 Gy, 54 Gy, 45 Gy

Tumor Important

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• 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

Tumor Important Critical Structures Important

Critical Structures Important

• Rx 75% to 66 Gy

6 % of GTV underdose,

7% CTV underdose,




Brain Stem = 42 Gy

Chiasm = 51 Gy

Final Solution

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Final Solution

• Rx 80% to 66 Gy

6% of GTV underdose,

8% CTV underdose,




Brain Stem = 46 Gy

Chiasm = 54 Gy

70 Gy, 60 Gy, 54 Gy, 45 Gy

Equal important

Critical structure

Tumor important

Compromised

How to modify plans that are not acceptable

Check Anatomy

• 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.

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Plan 1

Plan 2

Plan 1 Limiting factor

Looking for Unreasonable Dose Limits

Plan 2

Balance Between Dose Conformity and Uniformity

Plan Evaluation – How to sell your plan

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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.

Uniformity Vs Conformity

• Uniformity and conformity are often trade-off with each other.

• A great dose gradient often scarifies dose uniformity

Evaluation of IMRT Plans

• Define endpoints• Dose volume histogram (DVH)

• Dose distributions on every CT slice (Rx, hot spot, cold spot)

PTV70 Scoring

No Variation Minor Variation Major Variation

95% of PTV70 is at, or above 70 Gy, or/and

99% of PTV70 is at, or above 65.1 Gy and

95% of PTV70 is at, or above 65.1 Gy and

No more than 20% of PTV70 is at, or above 77.0 Gy

No more than 5% of PTV70 is at, or above 80.5 Gy

Failure to achieve either No Variation or Minor Variation

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PTV59.4 Scoring

No Variation Minor Variation Major 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


Failure to achieve either No Variation or Minor Variation

PTV50.4 Scoring

No Variation Minor Variation Major Variation95% of PTV50.4 is at, or above 50.4 Gy and /or99% 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


Failure to achieve either No Variationor Minor Variation

Parotid Scoring

No Variation Minor Variation Major VariationMean dose to either parotid is at or less than 26.0 Gy or

40% of either parotid receives less than, or equal to 30.0 Gy

50% of either parotid receives less than 30.0 Gy or

20 cc of the combined parotid glands receive less than 20.0 Gy

Failure to achieve either No Variationor Minor Variation

Other Organ at Risk

Dose Limit and Criteria

Temporal lobes 60 Gy or 1% of Vol. < 65 Gy

Brainstem*, optic nerves, chiasm*

54 Gy or 1% < 60 Gy

Spinal Cord+ 45Gy or 1 cc of Vol. < 50.0 Gy

Mandible ,T-M Joint 70 Gy or 1cc < 75.0 Gy

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Plan Acceptance Priority

3Mean dose to ears

3Mean dose to Parotid

295% CTV > Rx dose

295% GTV >Rx dose

1Brain Stem < 54 Gy –60 Gy

1Spinal Cord < 45 Gy- 50 Gy

Isodose Distributions

Cold spot

70 Gy,59.4 Gy,45 Gy

Hot-spot

70.0 Gy,59.4 Gy,54 Gy

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6 mm superior

Three Dimensional Examination

70 Gy60 Gy

70 Gy60 Gy

Simplify IMRT Plans

Poster, SU-FF-T96

25 segments 50 segments 100 segments

75, 70, 59.4, 45, 35 Gy


75, 70, 59.4, 45, 35 Gy

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75, 70, 59.4, 45, 35 Gy

Dose Volume Histograms of Tumor

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Average Conformality Index

00.10.20.30.40.50.60.70.80.9

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GTV CTV1

Structures

C.I.

98 segments

64 segments

50 segments

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Average Maximum Dose to Icc of Serial Structures

01000200030004000500060007000

brain stem spinalcord

mandible chiasm templelobe

Structures

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64 segments50 segments25 segments

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Average Mean Dose for Parallel Structures

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Average number of segments

Ave

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Total MUs Vs. Number of segments

Seeking Simple IMRT Plans

• 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.

Posters,SU-FF-T-96, 98

Absolute Volume receiving > 2Gy

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Pelvis H&N Brain

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Absolute Volume (cc) Receiving > 5 Gy

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Inverse Planning Tips

• 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.

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

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