MRI Guided Radiation Therapy: Brachytherapy

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MRI Guided Radiation Therapy Brachytherapy

• Robert Cormack
• DFCI/BWH Cancer Center

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IGRTBrachytherapy

• Image guided radiation therapy
• XRT
• BRT
• Permanent prostate brachytherapy
• Temporary cervical brachytherapy
• Summary

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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)
• Applicator placement
• Imaging planning
• Irradiate
• Repetition interval 6h to weeks

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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

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XRT Localize Target

• Daily pretreatment imaging
• Localize VO
• Adjust
• Patient to plan
• Plan to patient
• Ignores motion after localization

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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

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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

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BRT Process

• Brachytherapy
• High dose gradients (1/r2)
• Multiple independent radiation sources
• Permanent
• Plan
• Deliver
• Confirm
• Temporary (Multiple)
• Applicator placement
• Imaging planning
• Irradiate
• (Repeat)

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BRT Permanent (Prostate)

• Introducing foreign objects (N20, S100)
• Artifacts
• Anatomy distortion
• Suboptimal guidance modality/geometry
• CT poor soft tissue
• TRUS no seeds
• MR Low field/slow
• Lithotomy position
• Time frame
• Implant 1 hour time pressure
• Treatment days anatomy changes

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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

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Permanent BRT Adaptive Planning

• Intraoperative Planning
• Adaptive Dosimetry
• 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

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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

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BRT Temporary (Cervix TO)

• 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
• 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)
• Significant target changes from fraction to
  fraction

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BRT Temporary (Cervix Int)

• MR Image guidance
• Low field
• Lithotomy position
• Multiple sequences required
• Only visual feedback
• Planning
• LDR adjustments to source loading
• HDR dwell times
• 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
• Blind insertion
• Iterative CT
• Poor anatomy

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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
• 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

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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