Motion Management Flashcards
What is interfraction motion
o Motion between fractions
o Soft tissue shifts relative to bones (head and neck, thorax, abdomen, pelvis)
o Tend to occur day-to-day regardless of immobilisation and localisation
What can cause interfractional variations
o Due to adjacent OAR/target organ filling
o Due to changes in tissue geometry
What is Intra-fraction motion
o Motion during delivery of a fraction
o Primarily a fraction of the extent of the target motion during treatment
o Voluntary (wriggling, scratching)
o Involuntary (coughing, sneezing, swallowing, cardiac motion)
How to manage intra-fraction motion
o Consider patient comfort
o Reduction of patient anxiety
o Minimise length of treatment
Patient Preparation and Setup Techniques to Minimise Motion
Immobilisation Devices
Important to maintain reproducibility and accuracy
Can be a source of anxiety – may exacerbate motion
Bowel and Bladder Prep
Consistency of filling can improve positional reproducibility of pelvic organs
Compression techniques
Aim to reduce abdominal motion due to respiration (abdominal compression)
Potentially increase baseline shifts (setup errors)
Education and Compliance
Understanding of positioning equipment, preparation protocols – shown to increase compliance
What is the optimal arm position when treating a lung?
Arms above head
Greater choice of beam angles
Improves target coverage
Sparing of normal tissue
May be unsuitable for certain patients
What can be used to stabilise the arm when treating a lung?
Use a stable arm support in combination with knee support to increase comfort
Generally vac-bags – provide support through arms, neck and torso
Which aspect of the lung is tumour motion the greatest?
Different aspects of lung have variable motion
60% lung tumour – move < 1 cm
35% lung tumour – move 1 – 2 cm
5% lung tumour – move > 2 cm
Motion increases towards the diaphragm and is largest in the liver
Goal of Image Guided RT
Reduce uncertainties
Maximise reproducibility
Improve set up accuracy
Account for organ motion
Sparing NTT
What are fiducial markers and their purpose?
o Routinely used for localising prostate irradiation
o Small gold seed (0.9-3.0 mm) implanted under transrectal US guidance
Benefits of Fiducial Markers
FMs surrogate of prostate motion
Very fast method of localisation
Staff have great confidence when aligning FMs
- very low intra- and inter-observer variability
Disadvantages of Fiducial Markers
o Associated expense
o Invasive procedure = risk of infection at implantation
o Some patients ineligible, e.g. Warfarin dependency
o Rely on few (three) discrete points to localise the prostate (particularly when used with planar imaging)
In which direction is prostate motion most common
Dominant changes were in anterior and/or superior directions.
What can affect the position of the uterus
o Bladder filling significant factor in motion – filled can push posterior and superior
o Pelvic Tilt
What can affect the position of the prostate
Pelvic Tilt
Bladder Filling
Motion Artifacts in CT simulation
Distortions along the axis of motion can either lengthen or shorten the target
(in a somewhat random effect)
If CT scanning speed < tumor motion speed, → smeared tumor image
If CT scanning speed > tumor motion speed, → tumor position and shape captured at an arbitrary breathing phase
If CT scanning speed ~ tumor motion speed, → tumor position and
shape heavily distorted
4DCT
- 10 Bins are recorded per breath cycle
- 1 CT dataset is created with the same bin from each breath cycle
-> e.g., Bin 1 (Cycle 1) + Bin 1 (Cycle 2) + Bin 1 (Cycle 3)
-> results in 10 CT datasets - 10 CT datasets are fused together -> creates a single CT dataset in which ITV can be contoured
- allows more accurate estimate of tumour position
Benefits of IGRT
Tightly conformed dose distributions are at
increased risk of missing the moving target
Resultantly, IGRT can provide more knowledge on the target position –> more precisely irradiate the intended target
Benefit of Planar kV imaging
Relatively low imaging dose
Efficient means of isocentre verification
Forms of Volumetric Imaging
Fan Beam (CT on Rails) (Tomotherapy)
Cone Beam (CBCT) (MCVT)
Benefits of Volumetric Imaging
Provide visualisation of soft tissue
Great potential to improve treatment delivery
What is CT on Rails
- Diagnostic CT directly opposite (or orthogonal)
- Single couch for both gantries
- Couch rotates between the Linac and CT
- CT gantry “slides” over patient
- Assumes fixed relationship between the
isocentres of the two systems (CT and Linac)
Disadvantages of CT on Rails
Couch sag that occurs at the CT on rails gantry could not be totally corrected for
Time lag between image acquisition
What is an MVCT
Uses the treatment beam and EPID to acquire
cone beam CTs
Benefits of MVCT
- images high-density images without artefact
(e.g. prosthetic hips) - Same isocentre of imaging and treatment
- Standard hardware – no additional arms
Disadvantages of MVCT
- Low contrast (high energy)
Dose several times greater than kV
Benefit of CBCT (kVCT)
Able to monitor patient throughout treatment
Volumetric rather than few discrete points
Prostate Interfraction Motion Management
Recommend complementary use of implanted FMs and volumetric imaging with CBCT for daily localisation
“Aligning FMs on kVCT provides the best IGRT solution; offering minimal intra-observer error and prostate and OAR deformation information.”
Bladder ‘Plan-of-the-day’
Adaptive RT for Bladder Cancer
Initial plan has generous CTV-PTV margins (based on single CT, bladder empty)
Initial fractions treated with this conventional plan
* CBCT guided soft-tissue match
* CBCTs from first 5 fractions subsequently used to adapt plan
Develop a ‘small’, ‘average’, ‘large’ plan -> chosen to best suit the patient bladder on the day
Can reduce amount of tissue irradiated
How is surface imaging utilised
Utilises two or three ceiling mounted 3D camera units, designed to image the patient at simulation or treatment.
Reference surface model is produced from either: * importing contours from CT data, or by
* acquiring a 3D surface with AlignRT at simulation.
At each treatment fraction, the system images the current patient position
This image is registered to the reference surface
Couch shifts then calculated to correct inconsistencies between actual and planned positions.
What is the difference between gating and tracking
Gating
- beam ON only when target is at desired position or phase
Tracking
- the treatment beam follows the target during the whole cycle
Clarity 3D Ultrasound Guidance - use in simulation
Can be employed at CT simulation to aide
soft-tissue visualization.
Ultrasound probe is calibrated to the same
isocentre as the CT system
Clarity 3D Ultrasound Guidance - use in treatment
Used for patient alignment in the treatment room
* requiring just 90 seconds for scanning and
repositioning
* The treatment couch position is tracked using the same system as the ultrasound probe, providing highly accurate alignment.
Verify target position without ionizing radiation
Provides continuous real time imaging
Deformation and rotations
Most IGRT corrections only account for linear displacements
-> other sources of geometric uncertainty
1. Deformation of organs (adjacent organ position, filling)
2. Patient Weight loss/gain
3. Rotation of tumours and OARs
Suggested bladder volume
150-300cm3
When would you use 4DCT vs gating
4DCT - 8mm and greater motion
Gating - 13mm motion and above
Interplay effect
- observes as the dose deposition occurs when the target is moving
- results in a sub-optimal dose distribution contributed by a lack of synchronisation between the beam delivery and the desired target position (can cause regions of over-dose or and under-dose)
- much less likely with static treatment
Electromagnetic tracking of lung
Anchored transponder
- contains beacon transponder core and has 5mm anchors and legs to secure positioning in small airways
- Calypso monitoring system provides 3D target position -> predicts lung motion during treatment -> provides adjustment to 3D leaf position if necessary (in accordance with breathing cycle)
- Allows for significant reduction in the volume of ITV with MLC tracking delivery
Benefits of Markerless Tracking in Lung Cases
• Markerless 3D monitoring of the proximal bronchial tree (PBT) could help manage this risk by tracking the position and motion of airways relative to the
tumour, while offering a more reliable tracking option than reflective or implanted markers.
• Accurate real-time position monitoring could alert users to interrupt radiation treatment if the PBT position deviates beyond a pre-determined threshold.
Markerless tracking application and benefit in Rt
Example - Lung - MRL
* Automatic tumour tracking triggers a beam-on when target volume is within the boundary, and triggers a beam-hold when the target volume exceeds the boundary
- This allows patients to hold their breath at their own pace by using an adjustable mirror to see the real-time sagittal
projection on the in-room monitor.
Other Benefits
-> real time tracking
-> reduced dose to NTT
-> real time adaptation to organ motion
4DCT disadvantages
Huge dataset
Irregular breathing pattern
Movement
