4DCT and DCAT Flashcards
Define intra and inter-fractional motion
- Movement with the patient during a single fraction
- inter: movement between fractions. Can be caused by organ filling and emptying variations. Fix using adaptive Therapy
What dose tumour motion control allow us to do?
- Can possibly reduce tumour margins.
- OAR and normal tissue sparing could be improved.
- Potentially dose escalate.
- Improve clinical outcomes.
4DCT?
It is a respiratory correlated CT scan.
Helps us to understand INTRA-fractional motion.
Acquisition of multiple images at slice positions, each image tagged with breathing signal - the slower the acquisition the more signal we get and the more accurate the representation of the breathing cycle is.
Images are sorted based on breathing signal.
DICOM is then imported to TPS.
What is the difference between 4DCT and 3DCT
3D is one set of images (1 dataset).
4D - 10 sets of datasets. A lot of additional data and work.
Define ITV
Internal Target Volume (CTV + IM [motion])
IM - internal margin that corrects for uncertainties in internal motion
What is Cine View?
Gives a simulation of the tumour movement as the patients breathes - available for transverse, sagittal and coronal slices.
In the respiratory cycle what is more stable: inhalation or exhalation?
Exhalation
Why is motion management still important if you take a 4DCT?
Motion uncertainty can be negated by having very stable and reproducible setup - limits patient movement (especially important when doing specialised techniques).
4D imaging does not fix bad stabilisation or prevent motion it just maps it.
What is a Specialty image set
Combination 2 or more image sets to make a specialty image. 3 Types:
MIP- Maximum intensity projection
MinIP - Minimum Intensity Projection
AvgIP - Average Intensity Projection
What is a MIP image?
(Maximum intensity projection)
Displays the maximum CT number of all the pixels at the same spatial location over the respiratory cycle Basically shows entire extent of tumour motion/trajectory
Use with caution for tumours close to the diaphragm or chest wall
What is an MinIP image
Minimum intensity projection.
Displays the minimum CT number of all the pixels at the same spatial location over the respiratory cycle
Basically shows where some part of the tumour is always present throughout respiration
Useful for liver tumours that present as low density areas
What is AvgIP?
Average Intensity Projection
Displays the average CT number of all the pixels at the same spatial location over the respiratory cycle.
This is what we use for the treatment planning CT.
Reduces the need for an additional planning scan - ALARA.
What is VMAT and DCAT?
VMAT is a delivery technique where the Gantry, MLC and Dose rate are all modulated together to create extremely conformal dose distributions. VMAT segments are created to mimic an ideal dose distribution.
DCAT - Dynamic Conformal Arc Therapy. Different to VMAT as it uses an open field that completely encompasses the tumour volume (No moving MLC’s or segments that cut across the PTV).
Why would you use DCAT over VMAT?
Faster planning and delivery.
Target remains inside the open field for the entire treatment.
Has limited dose shaping but still has a steep dose fall off.
Good for simple larger target volumes.
Reduces MLC interplay issues (MLCs interacting with the beam can cause dose uncertainty)
Uses less MU (due to decreased modulation)
More likely to pass QA than VMAT (more reliable and robust).
PTV is always encompassed in the field.
