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 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

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 Permanent Temporary (Multiple) 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 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

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