IGRT Technologies Flashcards
What is image guidance?
- IGRT involves any use of imagnig to aid decision making
What is the main IGRT decision in RT?
- whether to treat and how and when to treat
- confidence in accuracey of treatment delivery is crucial in the decision making process
What is the goal of RT?
- goal to maximise the therapeutic ratio
- deliver tumourcidal dose to target
- minimise dose to OAR
Why do we use IGRT?
- reduce PTV margins
- reduce setup errors
- account for organ motion
- increase accuracey of beam placement
- increase precision of dose delivery
- lead to reduced toxicity
- permit dose escalation
What is the clinical benefit of IGRT for prostate?
- significant reduction in late urinary toxicitiy
- no difference in PSA relapse-free surivial
- high risk patients, significant improvment was obseved at 3 years
What are the IGRT technologies?
- planar
- volumetric
- non-ionising
- new technologies
What is planar MV imaging?
- portal imaging
- projection using treament beam
- used for setup pre-treatment target verification
- low contrast between soft tissue and bone
- useful for soft-tissue/air interface
What are the types of MV imaging?
- liquid ion chamber EPID
- a silicone EPID
What sites if MV still used?
- whole brain
- breast
- MLC shape verification
What are the cons of kV imaging?
- more expensive then MV
- requires additional linac hardware
What are the pros of kV imaging?
- better resolution and constrast then MV
- lower imaging dose
- similar workflow to MV
What are fiducial and why are they used for prostate?
- small gold seeds
- surrogate for prostate and allows physicians to overcome bladder and rectal filling and patient motion
What are pros of fiducials?
- surrogate of prostate motion
- fast method of localisation
- staff have great confidence when aligning (low intra and inter-observer variability)
What are the cons of fidicuals?
- expense
- invasive procedure and risk of infection
- some patients ineligible e.g. warfarin dependency
- rely of three discrite points to localise prostate
- evaluation of nearby organs and deformation of target is difficult
What is the imaging for CK?
- two ceiling mounted KC sources
- two floor-mounted aSi detectors
What is the exactrac imaging?
- two floor mounted kV source
- two ceiling-mounted aSi detectors
- available add on to linac
- allows frameless radiosurgery
What are the differences between CK and exactrac imaging?
- CK kV source on ceiling where as ET kV source in floor
- CK imaging every 15 seconds where as ET imaging when floor is rotated
What is the imaging for VERO SBRT?
- ring gantry similar to tomo but gantry and couch can move unlike tomo
- MV source (unrestricted imaging angles) and kV source
How many sources can be used with VERO imaging?
- can do MV or MVCT with one source but two gives better quality
- best for movable tumours but can be used for all parts of body
What are the in room CT technologies
- fan beam:
- CT on rails
- tomotherapy - cone beam
- CBCT (kV)
- MVCT (MV)
What is CT on rails?
- diagnostic CT directly opposite
- single couch for both gantries as couch rotates between the linac and CT
- CT slides over patient
- assumes fixed relationship between the isocentre of CT and linac
Why is CT-on-rails no used in AUS?
- expensive and significant decrease in departmental efficiency
What is the workflow of CT on rails?
- patient setup on CT couch
- CT scan done
- table rotated 180 degrees
- patient positioned at treatment iso
- CT reviewed
- table adjusted
What is the comparison between CT on rails and CBCT?
- couch sag occurs at CT on rails gantry which can’t be corrected
- image quality better for CT on rails
- time lag between image aquisition and treatment increases organ motion (prostate)
What is the helical tomotherpay imaging?
- MV source placed on ring gantry
- MV fan beam
- provides less tissue contrast then kV CT but less artefacts for high atomic material
- dose higher then CBCT
What are the two models for CBCT projections?
- full fan
- hald fun
When is a full-fan used?
- head region because of its narrow FOV
What does CBCT allows?
- daily iso localisation
- monitor patient throughout treatment
- can replan/adapt treatment depending on weight loss, disease progressio or response
What is an imaging using electromagnetic transponders?
- Calypso
- commonly for prostate but can be used for any site
What are the pros of Calyso?
- continous, real time monitoring of prostate
- non-ionizing = no additional radiation dose
What are the cons of Calypso?
- expense
- training
- not all patients suitable (abdomen measurment)
What are the components of Calypso?
- electromagnetic transponders
- 4D electromagnetic array
- 4D console
- infrared cameas
- 4D tracking system
How do the electromagnetic transponders work?
- permanently implanted in prostate priot to EBRT
- transmits RF wave to calypso system
- inactive until energised by calypso system
How does the 4D electromagnetic array work?
- electromagnetic energy source excties the Beacon transponders
- recievers detect the transponder coordinates
What happens when not using Calypso for VMAT?
- rapid, high dose rate treamtne that may miss target
- motion doesnt stop once the arc begins
- target position at setup doesn’t always translate to target position during treatment
Why is ultrasound not popular?
- inaccurate treatment localisation
- ultrasound probe pressure
- inter-user variability
What are the pros of ultrasound guidance?
- 3D US is inexpensiv
- non ionising
- real time guidance method
What is a 3D ultrasound guidance example?
- Clarity
- US probe calibrated to same isocentre as CT system
How does Clarity work?
- serie of relfective markers used to track its position in space
- enables the volumtric US image to be automatically fused with planning CT
- provides additional info for contouring
- no extra appointment time needed
What is BAT?
- 3D ultrasound used for patient alginment in treatment room
- required 90 seconds for scanning and repositioning
- treatment couch position tracked using same system as US probe
What sites are clairty and BAT used for?
- breast (post-op cavity definition)
- prostate
What are some cons of 3D ultrasound?
- hard to incoroporate due to increased training time
What is a form of optical tracking?
- varian RPM
What is the varian RPM?
- non-invasive, video based system
- uses an infrared tracking camera and a reflective marker array
- relfectors act as surrogate for patients respiratory cycle
What does the RPM measure?
- the patient’s respiratory pattern and range of motion
When is RPM used?
- breath hold procedures
- lungs, liver, pancreas and breast
What is optical surface imaging?
- utilises two or three ceiling mounted 3D camera units, designed to image the patient at simulation or treatment
How does alignRT work?
- reference surface model is produced by importing controus from CT data or acquired 3D surface at sim
- each treatment fraction, system verifies patient posiiton
- couch shifts calculated to correct for inconsistencies between actual and planned positions
What is a con of alignRT?
- no information on internal motion
What is Halcyon?
- multi-arc VMAT
- 800MU/min dose rate
- 6MV
- 4 revolutions per min
- flattening filter free
- 100cm wide bore
What is the IGRT for halycon?
- 15 sec for kV-CBCT
- 7 sec for 2D MV image
What is MRIdian system?
- real time MRI guidance
- uses 3 cobalt sources for treament
What is the advantage of MRI IGRT?
- ability for treatment adaptation based on functional imaging
- superior soft tissue contrast
What is a challenge with MRI linac?
- bore size if 70cm
- increased complexity of imaging will impact clinical decision making
- safety and QA implication
- education for RO and RT
What is the future of real time tracking?
- MLC shape able to follow taret through IGRT
What are the three frequency of IGRT?
- periodic (weekly)
- daily pre-treatment
- real-time tracking
What are the types of ionising IGRT?
- Linac
- MV
- kV/CBCT
- fiducial based - CK and exactrac
- kV - Tomo
- MVCT - VERO
- MV
- kV
- CBCT - CT on rails
- Halcyon
- CBCT
- MV
What are the types of non-ionising IGRT?
- Calypso
- ultrasoud (Clarity and BAT)
- optical surface imaging (Align RT)
- MRI linac
