MRI Guided Radiation Therapy: Brachytherapy Robert Cormack DFCI/BWH Cancer Center.
Post on 27-Dec-2015
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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)• 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)– Applicator placement– Imaging & planning– Irradiate– (Repeat)
BRT: Permanent (Prostate)
• Introducing foreign objects (N:~20, S:~100)– 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
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
• 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
Plan
Place Needle
ImageNeedle
Place Seeds
Anatomic
Geometric
Dosimetric
Adjust Plan
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)• 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
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
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
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
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