TRAJECTORY-

BASED TREATMENT

PLANNING AND

DELIVERY FOR

CRANIAL RADIOSURGERY

JAMES ROBAR, PHD, FCCPM

LEE MACDONALD, MSC

CHRISTOPHER THOMAS, PHD, MCCPM

DALHOUSIE UNIVERSITY, CANADA

Medical Physics CLASS SOLUTIONS IN SRS/SRT

• Identical arc arrangement at cardinal angles for all cases

• No patient-specific customization to the arc arrangement

• Cranial cases are highly variable with regard to PTV and OAR geometry

Medical Physics PATIENT-SPECIFIC ARC TRAJECTORY

• Establish patient-tailored dynamic arc trajectories

• May involve coordinated gantry and couch motion

• Designed to minimize dose to OARs without compromising PTV coverage

TRAJECTORY

BASED PLANNING

ALGORITHM CONSIDERATIONS

Medical Physics

Algorithm Objectives

Overlap between PTV and OAR Relative Depth

PTV

OAR

QUANTEC Dose Weighting Promote Rapid Dose Fall-off

PTV

OAR

FOUR CONSIDERATIONS

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Geometric optimization

0 50 100 150 200 250 300 3500

500

1000

1500

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2500

Are

a (

mm

2)

Gantry Angle (Deg)

Area of VOI vs. Gantry Angle

0 50 100 150 200 250 300 3500

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1000

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3000

Are

a (

mm

2)

Gantry Angle (Deg)

Area of Overlap vs. Gantry Angle

PTV

Brainstem

PTV

Brainstem

Overlap

OAR OVERLAP in FOUR-PI

Medical Physics

EXAMPLE: brainstem map • Creates a suitability

ranking for every couch-gantry position

• Unique map for every patient

• Condenses three dimensional relationships between structures

• Higher penalty assigned with OAR in front of PTV

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OAR OVERLAP in FOUR-PI

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A composite OAR map

All OARs

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Brainstem

Left Optic Nerve

Right Optic Nerve

Eyes

Lenses

Chiasm

OAR OVERLAP in FOUR-PI

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20

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SAMPLE OVERLAP MAPS

TRAJECTORY

BASED PLANNING

NAVIGATING THE MAP

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• Overlay treatment arcs to visualize their exposure to OARs.

• Measure the amount of cost present in an arc, and remodel the treatment.

• Establish safe collision zones.

COLLISION

COLLISION

NAVIGATION OF THE MAP

Medical Physics

Patient Example

• Right Acoustic Neuroma. – Brainstem

– Eyes

– Lenses

– Optic Chiasm

– Optic Nerves

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NAVIGATION OF THE MAP

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Conventional Class Solution

COLLISION

COLLISION

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Standard cranial template

NAVIGATION OF THE MAP

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Fixed-Couch Optimization

COLLISION

COLLISION

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NAVIGATION OF THE MAP

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COLLISION

COLLISION

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DYNAMIC TRAJECTORY

Medical Physics DYNAMIC TRAJECTORY

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COLLISION

COLLISION

TRAJECTORY

BASED PLANNING

EXAMPLES AND RESULTS

Medical Physics DOSE REDUCTION ~ 20% (N=30)

RL Macdonald, C Thomas, Dynamic trajectory-based couch motion for improvement of radiation therapy trajectories in cranial SRT, Medical Physics 42, 2317 (2015)

Medical Physics MINIMIZING MODULATION

RL Macdonald, C Thomas, Dynamic trajectory-based couch motion for improvement of radiation therapy trajectories in cranial SRT, Medical Physics 42, 2317 (2015)

Medical Physics

Rewarding dose fall-off toward specific OAR

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U

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URGENT SPARING

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Dose fall-off toward specific OARs % improvement of mean dose, 16 acoustic neuroma patients

URGENT SPARING

TRAJECTORY

BASED PLANNING

DELIVERY

Medical Physics DELIVERY (SPEED 8X)

Medical Physics

NEXT STEPS

For FourPi

Medical Physics

Collimation Example

BAD GOOD

Medical Physics Dynamic Collimator Arc

Arc 1

Arc 2

Arc 3

Arc 4

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4Pi with Dynamic Collimation

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BRAINLAB

IMPLEMENTATION

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USE

R T

EMP

LATE

FourPi in BRAINLAB CRANIAL SRS ELEMENT

• PTV/OAR overlap • OAR radiological depth • OAR on source side • Gantry stop/start

deviations • Table deviations

ASSIGN PENALTIES

…

FIND ALL POSSIBLE ARCS

FIND BEST POSSIBLE SET

Medical Physics SUMMARY

Approach minimizes overlap of PTV and OARs in dynamic trajectories

Multiple OARs accounted for in determining trajectory, each can be individually weighted

Can reward regions of approach which allow most rapid fall-off toward a specific OAR

Can be applied to optimization of fixed-couch arcs or simultaneous gantry / couch trajectories

Medical Physics

THANK YOU!

Medical Physics