Introduction to Radiotherapy Treatment Verification Flashcards
(37 cards)
Define Image-Guided RadioTherapy (IGRT)
The process of frequent imaging throughout a course of radiotherapy to direct the treatment, position the patient, and compare patient anatomy to their planning scan.
What are systematic errors?
Uncertainties with consistent magnitude and direction.
What are random errors?
Uncertainties that vary in direction and magnitude, meaning that they average out over a large enough sample size.
Are positioning errors in radiotherapy treatment random or systematic?
They can be both
Which radiotherapy treatments are most impacted by random errors? Why?
Treatments with fewer fractions. This is because random errors should average out over a course of treatment; however, with fewer fractions, this won’t be as effective.
Do random or systematic errors have a bigger impact on treatment?
Systematic as they don’t average out over the course of treatment.
State 5 sources of uncertainty in radiotherapy treatment delivery
1) Organ motion
2) Setup error
3) Intrafraction motion
4) Geometric accuracy of position and treatment systems
5) Delineation
How can uncertainty due to organ motion be minimised?
Immobilisation
What is immobilisation?
The methods of keeping patients in a reproducible position in order to minimise organ and tumour movement throughout planning and fractional scans. The patient should still be comfortable, as this will impact intra-fraction movement.
Give 5 examples of immobilisation devices
- Face masks
- Stirrups
- Wing boards
- Vac bags
- Abdominal compression
Describe 2 ways that could immobilise a patient’s internal anatomy
1) Bladder/rectal emptying/filling protocols for pelvis patients
2) Abdominal compression to reduce diaphragm movement
What does DIBH stand for?
Deep Inspiration Breath Hold
Define Deep Inspiration Breath Hold (DIBH)
An immobilisation method where patients hold their breath at maximum inhalation to minimise chest motion. This also means that the patient is only treated at one point in their respiratory cycle, reducing uncertainty in patient position.
Give an example of when DIBH may be used
For breast patients in order to spare heart tissue
State the 2 types of reference images used for anatomy comparison in image-guided radiotherapy
1) The planning scan (3D and/or 4D imaging)
2) A digitally reconstructed radiograph (2D imaging)
What is a digitally reconstructed radiograph?
A simulation of a 2D X-ray, created from 3D imaging data
State 4 common pre-treatment imaging methods
1) MV imaging
2) kV imaging: planar radiopgraphs
3) kV imaging: Cone beam CT (CBCT)
4) 4D CBCT
How does MV pre-treatment imaging work?
MV imaging uses the linacs MV beam to produce an image, so the image and treatment isocentre align.
What are the pros and cons of MV pre-treatment imaging?
+ Uses the treatment beam, which ensures alignment
- Poor image quality
How does kV pre-treatment imaging work?
kV imaging uses a separate X-ray source with a lower energy to generate images, which optimises image quality and concomitant dose.
What are the pros and cons of kV pre-treatment imaging?
+ Optimised image quality
- Requires a separate source, meaning that alignment of the two beams is necessary
kV imaging systems can be _____ mounted or ____ mounted.
Linac
Room
What is a fiducial marker?
A small object used as a reference point in X-ray images when bones don’t accurately delineate tumour position.
State 4 key requirements if using fiducial markers
1) The patient must be well enough to undergo marker insertion
2) Sufficient markers are needed to see tumour rotation
3) Markers must be well distributed throughout the tumour
4) The markers must not migrate
