IGRT Technologies Flashcards
Why do we use IGRT?
The goal is to maximise the therapeutic ratio.
Deliver high dose to the target whilst minimising dose to organs at risk.
Increasing precision and accuracy can allow for dose escalation.
Account for organ motion.
Can also reduce setup error.
What are the types of planar IGRT technologies?
Planar MV imaging
Planar KV imaging
Fiducial markers- prostate
Stereoscopic kV imaging
What is planar MV imaging?
Traditional method of ‘portal imaging’.
Relatively low contrast between soft-tissue and bone.
Useful for soft-tissue/ air interfaces.
Still useful but becoming less common.
What is planar KV imaging?
Has largely repanced MV portal imaging for setup.
Pros: Better resolution and contrast than MV.
Lower imaging dose
Similar workflow to traditional MV methods.
Widely availiable and utilised.
Cons: Requires additional LINAC hardware.
What are fiducial markers (IGRT)?
Routinely used for prostate irradiation
Small gold seeds
Surrogate for prostate to match to fiducuals to overcome factors such as:
Bladder filling, Rectal filling and patient motion.
What are the strengths and weaknesses of fiducial markers?
Strengths: FMs surrogate of prostate motion
Very fast method of localisation
Greater staff confidence due to lower intra and inter obervability
Weakness:
Associated expense
Invasive procedure = risk of infection at implantation
Some patients ineligible, e.g. Warfarin dependency
Rely on few (three) discrete points to localise the prostate
Evaluation of nearby organs and deformation of the target is difficult
What are examples of volumetric guidance modalities?
In-room CT technologies Fan beam such as: CT-on-rails and.. ..Helical TomoTherapy (HT) imaging Kilovoltage such as ... ... MV cone beam CT and kV cone beam CT
What is CT on rails and describe the workflow?
A diagnostic CT is directly opposite or orthogonal to the Linac.
There is a single couch for both gantries.
The couch rotates between the LINAC and the CT.
Assumes fixed relationship between the isocentres of the two systems
WORKFLOW:
• Patient setup on CT couch (BBs placed on reference marks)
• CT scan performed
• Table rotated 180°
• Patient positioned at treatment isocentre (BBs)
• Diagnostic quality CT reviewed;
• Table position adjusted.
Is CT on rails effective?
Studies have shown that the couch sag that occurs at the CT on rails gantry cannot be totally corrected for.
Other factors that can affect accuracy are:
- the action of rotating the couch
associated time lag between image acquisitions (prostate motion).
EPI image qulaity and outlining uncertainties
What is Helical TomoTherapy (HT) imaging?
MV beam source placed on a ring gantry.
Xenon ion chamber array allows CT reconstruction from treatment beam
The MV fan beam provies lesstissue contarst than kV CT, but less artefacts from high atomic number material (such as metal hips)
What is an MV conebeam CT (MVCT)
Uses the treatment beam and EPID to acquire cone beam CTs.
Uses standard hardware - no additional arms.
Similar to HT: Has low contrast (high energy) and is better at imaging higher density material.
Same isocentre of imaging and treatment.
Dose several times greater than kV.
What is a kV cone beam CT (CBCT)?
Extra arms added to the gantry (kV source and flat panel detector).
The OBI-CBCT (Varian) has two bow-tie filters.
The full bowtie filter is used for the FULL-FAN mode (half-scan) which are usually head and neck patients (because of the narrow field of view).
The half bowtie filter is used for the HALF-FAN mode (full scan).
What is are kv Cone beam CT’s (CBCT) useful for?
Much more than daily isocentre localisation - positioning.
Able to monitor the patient throughout treatment.
Volumetric rather than few discrete points.
Useful for adaptive planning (can see Disease progression, Disease response and Weightloss).
What are non-ionising radiation image guidance modalities?
Non-ionising = no additional radiation dose.
Electromagnetic transponders such as The Calypso system and 4D electromagnetic array.
Ultrasound guidance
Optical tracking - Varian RPM
Optical surface imaging.
What is the Calypso electromagnetic transponder system?
transponder Approx: 8.5mm in length
is permanently implanted in the prostate prior to EBRT.
Transmits RF (radio frequency) waves to the Calypso system (which then tracks movement)
Inactive until energised by the Calypso System.
Studies have shown that this method increases accuracy and decreases setup and treatment delivery times. Also allows for the reduction of treatment margins which increases patient QOL.
