Download Image Guided Radiation Therapy: Brachytherapy - NA

MRI Guided Radiation
Therapy: Brachytherapy
Robert Cormack
DFCI/BWH Cancer Center
IGRT:Brachytherapy
• Image guided radiation therapy
– XRT
– BRT
• Permanent prostate brachytherapy
• Temporary cervical brachytherapy
• Summary
IGRT
• XRT Process
– LINAC
• Well defined
geometry/dosimetry
• Treatment at a distance
• Treatment determined by
alignment of target to planned
position
– Simulation & planning
• Patient immobilization
• Imaging
• Target definition/Beam
optimization
• Patient marking
– Many treatments
• Localize target
• Track target
• Repeat next day
• BRT Process
– Many radiation sources
• Individual dosimetry well
defined
• Treatment determined by final
source configuration
• Treatment is (minimally)
invasive
– Permanent
• Plan
• Deliver
• Confirm
– Temporary (Multiple)
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Applicator placement
Imaging & planning
Irradiate
Repetition interval: 6h to
weeks
XRT: Simulation & Planning
• CT (4D) for anatomy
delineation
• Multimodalilty image
registration
• Beam selection and dose
optimization
• Phase selection (4D)
– Assuming reproducible
cycles
– Assuming correlation
between phase and taret
motion
XRT: Localize Target
• Daily pretreatment
imaging
• Localize VO
• Adjust
– Patient to plan
– Plan to patient
• Ignores motion
after localization
XRT: Track Target
• Daily repeated imaging
• Identify fiducials
– Gold markers
– RF devices
• Gate beam if out of spec
• Fiducials correlate to
target
– Change in configuration
– Evolution over treatment
XRT: Summary
• Repeated positioning of patient in
reproducible position (often near
diagnostic scan position) wrt known
radiation source
• Relevant time frame seconds to minutes
• No contact with patient
• Anatomy in ‘rest’ state
BRT: Process
• Brachytherapy
– High dose gradients (1/r2)
– Multiple independent
radiation sources
• Permanent
– Plan
– Deliver
– Confirm
• Temporary (Multiple)
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Applicator placement
Imaging & planning
Irradiate
(Repeat)
BRT: Permanent (Prostate)
• Introducing foreign objects
(N:~20, S:~100)
– Artifacts
– Anatomy distortion
• Suboptimal guidance
modality/geometry
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CT: poor soft tissue
TRUS: no seeds
MR: Low field/slow
Lithotomy position
• Time frame
– Implant ~1 hour: time pressure
– Treatment ~days: anatomy
changes
Permanent BRT: MR (Image)
guided planning
• Modality of choice for
pelvis (low field)
• Efficient VOI definition
– Auto segmentation
– Registering DX imaging
• Efficient planning tools
– Highlight points of greatest
concern to physician
– Make metrics visual
– Consequences of proposed
adjustments
Permanent BRT: Adaptive Planning
Adjust Plan
Plan
• Intraoperative Planning
• Adaptive Dosimetry
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Multiple feedback loops
Consolidate cold spots
Steer hot spots
Under plan as opposed to
over contouring and
planning
– Spare normal structures
• ~3mm displacement from
ideal an produce ~10%
loss of coverage
Place
Needle
Image
Needle
Anatomic
Geometric
Dosimetric
Place
Seeds
Permanent BRT: Implant
Confirmation
• CT
– Seed identification
– Poor anatomy made worse
by artifacts
• MR
– Artifacts obscure anatomy
– Different scans optimize
seed and anatomy
• Time frame
– Edema effects dose and
registrations
– ~4 week
BRT: Temporary (Cervix T&O)
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Tandem & Ovoid
– Applicator geometry determines
treatment
– Minimal need for image guided
placement
– Significant distortion of anatomy
– 2-5 fractions over the course of a
month
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Normal tissue geometry vary from
fraction to fraction
Not possible to create true
cumulative dose distributions
MRI
– Not widely used
– Purely for planning (1st fraction
only)
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Significant target changes from
fraction to fraction
BRT: Temporary (Cervix Int)
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MR Image guidance
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Low field
Lithotomy position
Multiple sequences required
Only visual feedback
Planning
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LDR: adjustments to source loading
HDR: dwell times
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Ability to adjust plan
Cost: hot spots
Cannot make up for poor implant
CT based for geometry
MR anatomy obscured by needles
Fusion appropriate MR-MR and MR-CT
Change in sagittal images highlights
need to adjust over course implant
Elsewhere
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Blind insertion
Iterative CT
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Poor anatomy
BRT: Summary
• Placing many independent radiation sources within patient
(changing) anatomy
• Relevant time frame minutes to hour
– Time should be minimized
– Longer times than XRT
• Process inherently change/displace anatomy configuration
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Edema
Applicators
Multiple image sets
Temporal changes during procedure
• Procedures are not in or near treatment/diagnostic position
• Common challenges
– Feature extraction
– Registrations
– Temporal changes changes across fractions
Image Guided Brachytherapy
Cahllenges
• Common challenges
– Feature extraction
• Auto segmentation
• Contour evolution
– Registrations
• Target definition at time of planning
• Patient to Radiation Sources
– Accounting for temporal changes (anatomy changes across
fractions)
• Common worries
– Validity of snapshot image
– Account for mid-treatment shifts
– QA: Image interpretation, IGRT Process, Algorithms