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
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
Medical Physics
Geometric optimization
0 50 100 150 200 250 300 3500
500
1000
1500
2000
2500
Are
a (
mm
2)
Gantry Angle (Deg)
Area of VOI vs. Gantry Angle
0 50 100 150 200 250 300 3500
500
1000
1500
2000
2500
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
Medical Physics
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
Medical Physics
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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
Medical Physics
Conventional Class Solution
COLLISION
COLLISION
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Standard cranial template
NAVIGATION OF THE MAP
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
rgen
t sp
arin
g
α
URGENT SPARING
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Dose fall-off toward specific OARs % improvement of mean dose, 16 acoustic neuroma patients
URGENT SPARING
Medical Physics
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