mri guided radiation therapy: brachytherapy robert cormack dfci/bwh cancer center

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MRI Guided Radiation Therapy: Brachytherapy Robert Cormack DFCI/BWH Cancer Center

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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)• 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