Image Evaluation Flashcards
What is verification?
the process that enables us to be certain we are treating the tumour volume as planned. In ensuring that the right radiation dose has been given to the right place, two measures are needed- geometric and dosimetric verification (i.e diodes).
Verification is a double check with the gold standard. It facilitates assessment of the accuracy of treatment set up. Use DRRs (for KV/bony anatomy) and planning CT (for CBCT) as the gold standard. Check field sizes/MLC shapes, check what’s been planned is what is being delivered.
What are port films?
MV portal radiographs shows bony anatomy and air spaces. Physical films were positioned parallel to the treatment site during exposure. They were easy to use, produced a permanent hardcopy and required minimal training. However, they gave poor contrast resolution, poor image quality, inconvenient, and don’t demonstrate organ motion.
What are EPI’s?
Electronic portal images: assess daily variations in patient set up and eliminate geometric treatment delivery errors (gross error, random, systematic). They can be used to establish tolerance levels for anatomic site and evaluate immobilisation devices. IMRT and new techniques require accurate reproducibility and safe reduction of margins is only possible if the magnitude of geometric errors is known, so EPI are required.
What is a double exposure?
Jaws are opened to take an image including more anatomy than is in the field. The single exposure is overlaid making it easier to match to.
What is a movie loop?
This films the MLCs moving in and out of the field; it is a recording of imaging, not static. Particularly useful for breast.
What is the imaging process?
Input the reference image, outline the match structures, draw the field edge, acquire the portal image, match the portal image to the reference image, measure deviations
What is involved in image evaluation?
Measuring of field displacement errors Reference image comparison Bony landmarks Fiducial markers/clips Relate the above to target volume
What is IGRT?
Image-guided radiotherapy.
Ensures accuracy of irradiating the PTV to improve tumour control, reduce toxicity, and give better quality of life.
What types of IGRT are there?
2D MV, 2D KV, 3D CBCT
What are fiducials used for?
3 fiducials are inserted into the prostate in a triangular shape making it easier to match to. However they can migrate.
What is CBCT?
With a single 360 degree rotation of the gantry CBCT scans can be reconstructed. Bony and soft tissue matching can be done. It is multi-planar
What is gross error?
An unacceptably large error that could under-dose part of the CTV, or over-dose an OAR. It can be caused by incorrect patient, anatomical size or patient orientation, incorrect field size, shape or orientation, incorrect isocentre position of unacceptable magnitude
What is systematic error?
the systematic component of any error is a deviation that occurs in the same direction and is of similar magnitude for each fraction throughout the treatment course. These errors are introduced at localisation, planning or treatment delivery. They may refer to an individual or population.
What is random error?
a deviation which can vary in any direction and magnitude for each delivered fraction. They occur at the treatment delivery stage in patient set up, target position and shape, and can be intra-fraction errors.
What reference anatomy is used for the pelvis?
Ant - Symphysis pubis (both sides)
Obturator foramen
Pelvic side walls
Top of the acetabulae
Lat –
Anterior symphysis pubis
Sacrum and sacral promotorium
Top of the acetabulae
