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

2. IGRT:Brachytherapy Image guided radiation therapy
XRT
BRT
Permanent prostate brachytherapy
Temporary cervical brachytherapy
Summary

3. IGRT XRT Process BRT Process LINAC Simulation & planning
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

4. 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

5. XRT: Localize Target Daily pretreatment imaging Localize VO Adjust
Patient to plan
Plan to patient
Ignores motion after localization

6. 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

7. 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

8. BRT: Process Brachytherapy Permanent Temporary (Multiple)
High dose gradients (1/r2)
Multiple independent radiation sources
Permanent
Plan
Deliver
Confirm
Temporary (Multiple)
Applicator placement
Imaging & planning
Irradiate
(Repeat)

9. 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

10. 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

11. Permanent BRT: Adaptive Planning
Place
Needle
Image
Seeds
Anatomic
Geometric
Dosimetric
Adjust Plan
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

12. Permanent BRT: Implant ConfirmationCT
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

13. 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

14. 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

15. 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

16. 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