MR Linac QA Flashcards
Process of introducing MRI linac into a centre
Machine installation (vendor)-> machine acceptance (vendor) -> beam data collection -> beam modelling (vendor)-> beam model validation -> going live
Bunker modifications
Floor Structure:
• Unlike conventional LINACs, MRI-LINAC requires a trench in the floor.
• Large components are used for positioning.
Faraday Cage:
• Installed at the end of the magnet system and board.
• Includes aluminium sheeting and ceiling panels forming a closed cage.
• The Faraday cage isolates the magnet from ambient RF signals.
Floor Construction:
• The floor section is constructed using copper sheets to support magnetic field stability.
Power and magnetic field setup (PPS)
• A small trench is required in front of the magnet assembly.
• Magnetic Field Ramp Up: Establishes a permanent 1.5T magnetic field.
• Shimming: Adjustments made to ensure field uniformity.
Acceptance Testing and Bunker Surveys
Acceptance Tests:
• Device acceptance tests are performed by electrical engineers without sign-off by physicists.
• Radiation survey of the bunker is conducted.
Depth Measurements:
• Performed during Device Acceptance Testing (DAT) for both the radiation MV and MRI systems.
Nearby LINAC Checks:
• Investigations to confirm that the MRI has no impact on nearby LINAC systems.
x-ray beam, leakage from head of gantry, bunker
Mechanical tests are done for gantry, collimator
Medical physicsts need to sign off on all acceptance testing complete
Commissioning: Beam data collection
• Performed using vendor-supplied water tank and chambers.
• Electrophysicists collect:
• PDD (Percent Depth Dose) data.
• Beam profiles and output factor data.
On site medical physics team then performs comissioning measurements, this data serves as a baseline for future QA
Beam data is collected for modelling of TPS, testing of beam bodel and end to end testing
Commissioning: Beam modelling
• Collected data is sent off-site for TPS (Treatment Planning System) modelling.
• Additional validation of PDD and beam profile data can be requested by physicists.
• Data collection takes approximately 2 weeks.
Commissioning: Training of staff and emergency preparedness
• Focus on mock scenarios to prepare for potential emergencies in the room.
Model verification
• IMRT RT plans are created using the vendor array.
• Plans are compared to the TPS using gamma criteria of 3% in 3mm.
• In-House Patient QA Systems:
• Utilizes array-based measurements and film for patient QA.
• Gamma Analysis:
• A measure of similarity between two data sets (distance and percentage difference).
Pre-live QA
• Pre-Live Audits:
• Australian Clinical Dosimetry Services perform an audit before the system goes live.
• MR Expert Audits:
• An MR expert is contracted to ensure compliance with AUSTRALIAN RANZCAR guidelines.
Extra qa measurements
• Fringe Magnetic Field:
• Measurements of fringe magnetic fields around the bore.
• Acoustic Noise:
• Acoustic noise levels within the MRI bore are measured for safety.
Commissioning and QA challenges on MRIS
Challenges:
* Lack of laser system
* Bore dimensions can restrict size of equipment
* Requires MR safe equipments
* Radiaiton behaviour in a magnetic field
* Unique/lack of QA phantoms
* Limited training
* Machine design: unique beam energy, ring gantry, no light field, limited bore, limited couch motion, full integration with imaging system)
Unity system description
• Uses 7MV Flattening Filter-Free (FFF) beam.
• Operates at 6 revolutions per minute (rpm).
• Collimator and Beam Steering:
• No collimator rotation.
• No beam steering capabilities.
• Beam Central Axis:
• Always orthogonal to the MVI (Mega Voltage Imager).
• The MVI is not used for clinical imaging but is crucial for QA applications.
• Field of View (FOV):
• Limited to 22 cm by 9 cm.
• Beam Stopper:
• A 12 cm thick aluminium beam stopper is located below the MVI.
Patient positioning system on unity
• The couch (Patient Positioning System) has limited motion:
• Can only move longitudinally (along the sup/inferior direction).
• Remains static during treatment.
• Daily Adaptive Planning:
• The treatment plan is adapted daily to accommodate patient anatomy and positioning.
• QA Check:
• QA only needs to check motion in the superior direction.
Equipment positioning
• No Light Field:
• No light field is available to help orient equipment in the bore.
• MV Imaging for Positioning:
• AP (Anterior-Posterior) and LR (Lateral-Right) orthogonal MV images are used to position equipment.
• Sagittal Laser:
• Provides a nominal positioning aid but is not accurate for precise equipment positioning due to its attachment to the cage.
• The laser may shake or move when opening/closing the cage door.
QA machines and equipment for Unity
• MR-Compatible QA Tools:
• MR-compatible arrays, ion chambers, and detectors are used for QA tasks.
• Solid Water: • Used as a substitute for regular water during testing. • Water Tanks: • A range of water tanks are used, with vendor-provided water tanks operated by the vendor physicist. • Vendor-Provided Phantoms: • Phantoms dedicated to specific QA tasks are provided by the vendor. • These are used to ensure MV iso-centesis termination (coincidence of MV and MR isocentres). • Important for adaptive plans to ensure alignment between MR and MV systems.
Pre-treatment reference plan QA
Prior to first fraction we perform independent secondary MU check, we use MU Tuna
Given adaptive nature, indepentn MU check is the only QA that can be done on the plan before it is delivered to the paatient
MR imaging is used after secondary MU check to verify patient position follow RO approval beam data is sent to MOSAIQ. At this point, we confirm that data was correctly transferred. There are multiple options for this
After correct data transfer, treatment can commence
Data match and workflow management
• Background Tasks: Some sensors perform database queries in the background to confirm data match between Mosaic (the oncology information system) and the Truth and Planning System.
• Groups:
• Certain groups ensure the total monitor units (MU) and number of segments match before treatment.
• Motion Management C sequences assess the target position, using MR imaging while the MV (megavoltage) beam is running.
Impact of MRI and MV beam on quality
- MR Imaging & MV Beam Impact:
* Investigate potential impact of MRI imaging on O symmetry (output symmetry).
* Check how the MV beam affects MR image quality.
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Patient hearing protection
• Audiology Study: Combination of ear defenders and ear plugs effectively protects patients from hearing loss due to loud MR scans.
Adapt to position
• A reference plan is created based on simulation setup images.
• The reference data set is rigidly registered to the daily MR image.
• The isocenter position is updated.
• The plan is recalculated to maintain or improve target coverage.
• Electron density information is gathered from the reference data (CT or MR images).
Adapt to shape
• Adapts the plan to the patient’s new anatomy for that day.
• Contours are updated based on the new patient anatomy.
• The plan is recalculated on the daily MR image with bulk density assignment based on CT data.
QA MRI guidelines and testing
• International and national guidelines exist for performing QA on MRI-LINAC.
• In Australia, the ACPSEM is creating a QA document for MRI-LINAC.
• Tests are typically divided into:
• Mechanical tests: for hardware.
• Dosimetric tests: for radiation dose measurements.
• Imaging tests: for ensuring image quality.
• Test Frequencies: Tests are categorized by how often they are performed (e.g., daily, monthly) and their tolerance levels.
Designing QA programs
• QA programs consider:
• Workflow requirements.
• Guidelines and recommendations.
• Local conditions and available equipment.
• Focus is on minimizing machine downtime while ensuring safety.
Commissioning key steps
Beam data collection
Beam modelling
Staff training
Model verification
End to end testing
