CT and motion management Flashcards
Role of ARPANSA and WHO
Australian Radiation Protection and Nuclear Safety Agency
World Health Organisation
- Reports on radiation incidents
- Assesses international radiation risk profiles
- Reports determined that most radiation mistakes either occur from dosimetry or stem from it
Why do incidents happen
Lack of Alertness
* causing accidental exposures
Lack of Procedures/Checks
* Not comprehensive, documented or followed
Training and Understanding
* Lack of qualified or well trained staff with unnecessary educational background or specialised training
Responsibilities
* gaps and ambiguities in the functions of staff along lines of authority
Lack of supervision to follow QA process
Chain of QA process
- assessment of patient
- diagnosis and staging
- decision to treat
- prescribing
- immobilisation and positioning
- simulation, imaging, volume delineation
- margins of volumes
- 3D imaging
- planning
- Radiation delivery
- intra fraction motion and management
Swiss Cheese Model
An accident or incident occurs only where there is an alignment of vulnerabilities
* Demonstrates the value of the ‘defence in depth’ approach to radiation safety
‘Defence in depth’ –> several independent controls contribute to overall safety
Daily CT QA tests
Alignment of lasers to gantry plane
* probable issue
* reasonable importance
CT Number
* Use of water phantom
* very important
* check through air calibration
Image noise
Spatial accuracy
- Most likely RT’s would be responsible for daily tests
- Important to review the test, understand the tolerance and advise if results are out of tolerance
Spatial Accuracy: Definition and Tolerance
Definition:
Difference between object dimensions in a CT image and the actual object dimensions
Tolerance:
+/- 1 mm
How to Measure Spatial Accuracy
Should be verified by CT scanning, using a phantom of known dimensions
Should be verified across scan protocols
What is the importance of Spatial Accuracy?
RT requires accurate and reproducible representation of the patient dimension and shape (e.g., skin contour, internal organs)
Image distortion can lead to inappropriate dose dumping to the wrong area
Inaccuracies can lead to over- or under dosing
How to Perform Laser Check
Use Wilke Phantom or Block to assess laser geometry and accuracy
Physicists can also pre-mark out the laser marks on the opposing walls
Important to note:
Daily tests only check intersection of lasers, indicating centre
* It may not necessarily check the skew and tilt present
Lasers: Definition and Tolerance
Definition:
* Alignment of the Gantry Lasers with the centre of the imaging plane
Tolerance:
+/-2mm
Tolerance depends on accuracy of treatment procedures
* Individual departments may have specific tolerances
Why is the Laser Check Important?
Treatment room lasers are well defined and allow for precise localisation of the treatment isocentre
* CT must possess the same relationship between patient position and setup
Can affect ability to accurately identify skin marks
Required for reproducible patient patient position on setup
Accuracy needs to be comparable to treatment machine lasers
Need to contact physics if found to be out of tolerance so they can be recalibrated –> this might require additional tools and checks such as alignment and reference wall marks to be updated)
How to perform Noise QA check?
Should be performed for head and body phantoms
Check phantom is placed in centre of the imaging bore (you can use table height to find the centre quickly)
Phantoms are provided by the vendor
Noise: Definition and Common Causes
Definition:
Refers to random fluctuations in pixel intensity that does not correspond to actual variations in tissue density
Arises from statistical variation in the detection of x-ray photons
Common Causes:
* X-ray dose
* Detector efficiency
* electronic interference
Noise: Tolerance
Set by vendors and manufacturers
Importance of Noise QA
Image Quality directly affects the ability to identify and delineate target volume and surrounding OARs
Presence of Noise can impact treatment planning
* suboptimal images may cause omission of target volume or inadvertent delineation of normal structures
Noise is a very sensitive parameter in overall imaging performance of scanner
QA process should allow for quick detection of image degradation
HU: Definition
Amount of radiation attenuated, with a relative comparison between that in water and in air
How to Perform HU QA test:
Need access to water-filled phantom (provided by manufacturer) (Body 32cm or Head 16cm diameter cylinder)
Manufacturer Software Auto-Generated Report to be reviewed following water phantom scan
Importance of HU QA check
CT is initially calibrated to give 0 HU value for water
* Relationship between the relative electron density to CT numbers are mapped in the TPS (CT to ED curve)
Deviation may indicate equipment fault in beam hardening or image reconstruction software issues
External definition also relies on accurate HU delineating the skin and air threshold
Can lead to:
* Incorrect water HU may lead to incorrect relative HU
- Incorrect CT number to density relationship can cause dose calculation errors in dosimetry (e.g., variations +/- 20 HU in soft tissue range can lead to dose changes approx. 1%)
CT Legislative Requirements
Radiation Shielding of Premises:
* Ensure adequate shielding based on workload
- Ensure no changes to shielding integrity (e.g., installation of power point)
- May involve radiation survey in CT room (e.g., only done in commissioning)
Equipment Compliance (Annually): * Ensuring the safe function of the CT
* Ensure proper dose levels have been set
* Independent audit of equipment
Assessment Certification:
* Outlines the tests performed
* Records are kept
- Yellow Sticker (equipment compliance) (accordance of H003:2010 Radiation Safety Standard)
- Green Sticker (premises compliance) (performed every 5 years) (QLD legislation requirement)
** TG66 can be followed for the above tests **
What is outlined in the H003:2010 Radiation Safety Standard
Legislative requirement for radiation equipment describing the standard for radiation apparatus used to carry out computed tomography
Motion management sources of positioning error
Determination of tumour position as a function of time
Calibration of spatial relation between the tracking coordinate system and beam delivery coordinate system
5 motion management strategies:
ITV, free breathing gating, breath hold gating, mid ventilation, tumour tracking
ITV: Definition
Any treatment of a target volume which encompasses the entire range of motion of the lesion
Free Breathing Gating: Definition
Any treatment where delivery of the beam is limited to a portion of the respiratory cycle as the patient breathes normally
Breath-Hold Gating: Definition
Any treatment where the delivery of the beam is limited to a portion of the respiratory cycle which is extended by a patient holding their breath
Mid Ventilation: Definition
Any treatment where the target volume is defined using the time - weighted average position of the tumour (average position of tumour during normal breathing)
Tumour Tracking: Definition
Any treatment in which the treatment beam is modified / repositioned to account for the motion of the target
What is a 4DCT?
Low pitch helical CT is acquired while the patient breathes normally
Breathing Patterns is required to be established
* If patient motion is irregular, it is difficult to ascertain the respiratory trace from the breathing cycle
Geometric average position
Geometric average position means finding the position of the tumor that it occupies most often or is the “center” of its motion range over time
When do we use active motion management
involves actively tracking and compensating for the tumor’s movement during radiation delivery to ensure the radiation beam stays focused on the tumor at all times.
- when tumour motion is large eg. near diaphragm
- highly targeted techniques
Phase Binning: Definition
Based on percentage of the breathing cycle
Phase Binning Cons
If the patient has any inconsistent breathing patterns it leads to 4DCT artifacts
Phase Binning Pros
The breathing cycle is divided into equal time points
Closely depicts the actual movement over time
Allows calculation of the mid ventilation phase
Provides average based on time
Amplitude Binning: Definition
User defines the min and max limits between which the CT acquisition is dependent on the absolute position of the marker regardless of the phase of breathing
Amplitude Binning: Cons
Not suitable for mid ventilation phase
Not possible to read the actual movement of the tumour over time
Amplitude Binning: Pros
Breaks down the breathing phase into equally spaced sections according to the signal amplitude
Overall artifacts are fewer when patient has irregular breathing
Max exhale phase can easily be reconstructed
MIP: Definition
Maximum Intensity Projection
- Post-processing technique
- Highest intensity values along each projection line in a 3D dataset
- Helps highlight regions of high intensity across all phases of the motion cycle
MIP: Advantage
- One 3DCT gives information that encompasses the entire range of tumour motion and ITV delineation can be performed in a timely manner
MIP: Disadvantages
- ITV’s created using MIP’s are smaller and may result in geometric miss compared to ITV delineation using the whole 10 phase 4D CBCT
- Can’t be used if tumour is near a high-density edge (structures will get blurred out)
- Any irregularity in breathing during 4DCT acquisition will result in post processing artifacts
- Limited to contouring only (higher CT HU units than a regular CT)
MIP: Potential Issues Arising
Missing data/slice interpretation
Amplitude variation can lead to truncation of ITV
Different number of slices in 4D dataset
Can lead in incomplete MIP
Commissioning: CT for 4DCT
Camera Calibration:
* Assess accuracy of the camera determination of the amplitude of AP motion
* Assess limitation of optimal distance from camera
* Accuracy of the timescale
4DCT Phase Binning (Scanning Quasar Phantom)
* Cylindrical inserts inside a body shaped phantom and motion can be programmed in the SUP/INF direction to follow any arbitrary motion pattern with variable speed and range of motion
- Lung is simulated using a cedar wood insert (0.4 g/cc) with a plastic sphere (30 mm or 15 mm diameter)
- Periodic motion of the tumour was simulated by driving the inserted cosine pattern with variable breathing period expected in actual breathing motion
- Platform is synchronised to move AP while the lung insert moves SUP/INF
Evaluation:
* Correct binning in the breathing phases using RPM signal
* Reconstruction motion artefacts - volume accuracy
* Limitation in fast breathing pattern (e.g., may require a lower pitch and slower couch motion)
* Reconstructed phases of breathing cycle
* Additional reconstructed images (average, MIP)
RPM QA
Ensure reconstructed images are free of artfacts
Accuracy of camera determination of the amplitude of AP motion for treatment and CT
Accuracy of determination of the excursion of tumour motion
Accuracy of the reconstructed MIP image for determination of excursion of tumour motion
4DCT limitations
Largest uncertainties for small tumours with large amplitude of motion with short period of oscillation
Optimisation for:
* dose patient
* duration of the scan
* limit of couch drive control
* speed of gantry rotation
* Speed to noise ratio for each of reconstructed phases in 4D dataset
* ‘Normal’ breathing rate (12 to 20 BPM)
Limitations of RPM with 4DCT
- longer scans can lead to increased motion artefacts
- larger the velocity, increases the volumetric deviations-typically 4DCT overestimate the volume of tumour
- gating systems rely on an interpretation that the external signal and its periodicity are reflective on the motion of the underlying tumour
why do we do 4DCT qa
20% of 4DCTs have artefacts that impact ITV generation
If it is incorrect, can lead to geometric miss
Review your 4DCT before proceeding
Assess patients before hand
How to use Varian RPM
Gating using an external respiration signal using an infrared reflective plastic box serving as an external fiducial marker placed on the anterior abdominal surface
Location is chosen to maximise AP motion
Benefits of using Varian RPM for Motion Management
If using gating → less straining for patients than breath hold
Can be used for breath hold
Minimise toxicity
Enables dose escalation (SBRT treatment) highly conformal dosimetry
Doesn’t use consumables (ABC)
Limitations of using Varian RPM for Motion Management
Increase treatment time
Only a surrogate for tumour movement
How to commission the RPM for the Linac
Camera Calibration:
* Accuracy of the camera determination of the amplitude of the AP motion
* Limitation of optimal distance from camera
* Accuracy of the timescale
Camera use
* Connectivity and functionality with integrating the reference breathing trace
* Beam interrupt / beam hold
CT HU tolerance
+/-5HU
Motion management in SIM
4DCT - ITV or MIP
Notional assessment and action levels
Fluoroscopy
Motion management on treatment
CBCT
4DCBCT
Clips
SGRT
Breath hold
Compression belt
MR Planning drawbacks
Geometric distortion - caused by hardware induced effects such as magnetic field in homogeneity and gradient nonlineraity
High cost and shielding
Still requires a CT
Patients that are non-MR compatible
MR Planning: option 1
Fusion of Diagnostic MR to planning CT
- challenges in accurately registering diagnostic Mr images to the RT flat table tops
- limitation of setup
- dosimetric error from mis registration
- multiple imaging appointments
MR planning: option 2
Use MR as primary imaging modality for planning
- require high geometric accuracy
- synthetic CT image generation
- DRR or 3D reference image for pre treatment image verification for linac equipped with CBCT imaging
- MR to MR. Image core gist ration for mRI linac
SGRT advantages
Non ionising and non-invasive – no implants necessary and tattoo-less option
•High accuracy and resolution of 3D surfaces
•Clinical Trials Postural visualisation (e.g. breast, extremities, chin)
•Patient Feedback
•Independent from linac gantry motion
•Real time motion management
•Gating and Breath hold capability – beam hold and 4DCT with GateCT
•May not require SGRT on CT
•Faceless mask option (watch out for blink motions creating false gate
SGRT disadvantages
Equipment/Financial resources
•Additional training and increase of time
Align RT
Multi camera system- insure that at least 2 cameras can see the patient at any time during gantry rotation
•Structured Light or Speckle pattern projected on the patient and the reflection is captured
•The deformation of the reflect detected on the camera system is reconstructed into 3D surface is measured and compared to reference surface (Day 0 or Day 1 or CT) or TPS contours
•External Imaging System
•Useful for superficial, tumours’s motion that move with respiration, tumours that don’t move, or help initial patient setup, gamify patient setup with patient.
•Option to possibly reduce the use of GA in paediatrics
•Coincidence with kV/CBCT, MV isocentre and SGRT isocentre
•Beneficial for non-coplanar tracking
•Evaluates multiple points on the patient for whole postural setup accuracy and no cheating with arching backs
•Faceless mask option (watch out for blink motions creating false gate)
•Tracking breathing
•4DCT
•DIBH
•EEBH
Commissioning tests - SGRT
- connectivity to R&V system
- FOV: occlusion with gantry rotation
- Spatial drift and reproducibility
- reproductivity of spatial accuracy
- Setup the iso of the SGRT using CBCT, kV and MV
- End to end testing
- standard operating procedures and guidelines
Limitations and other considerations for SGRT
Findings from Commissioning and Recommendations
•Warm up drift effects- may need to be left on at all times or wait until fully warm up
•Field of view limitations- e.g. couch height, geometry of large patients, camera obstruction
•Room lighting conditions – follow the departmental ambient light requirements
•Blocking of cameras- can lead to geometric accuracy issues
•HD mode can be slow (use according to recommendations e.g. SRT)
•Region of Interest optimisation
•Skin Tone
•Frame Rate
•Accessories
SGRT Daily QA
Safety - Check interlocks are functional
•Static Localisation
•Positioning accuracy - the isocentre of the SGRT using CBCT and kV MV imaging
•Accuracy of applied shifts to treatment couch
•Needs to be robust, quick and easy to setup
•Static Localisation Accuracy – Performed by RT
•Relative Camera Position – performed by RT
•Reviewed by physicist
SGRT Daily QA failure
•Misalignment between treatment isocentre and CBCT isocentre
physics need to perform Iso-cal check
•Couch rotation offsets from the treatment isocentre
verify with Winston-Lutz
•Camera has been moved
•Coordinate system
•Thermal Drift – short and long term
•Reproducibility
Rationale for Daily CT QA
- The goals of a CT-simulation QA program are to assure safe and accurate operation of the CT-simulation process as a whole.
- The tests are designed to detect potential errors that can affect accuracy of target and normal structure delineation and treatment simulation.
