DCAT Flashcards
How to solve motion problems
Treat patients faster
Motion management
What kind of motion can be controlled by 4DCT
Intrafractional motion
Why do we reduce motion
Tumour margins could be reduced
- OAR / Normal tissue sparing can be improved
- Potentially dose escalate
- Improve clinical outcomes
Motion management solutions
Enhanced immobilisation
Respiratory management
4D imaging
4DCT
- Respiratory cycle correlated CT Scan
- Acquisition of multiple images at slice positions, each image tagged with breathing signal
- Images are sorted based on breathing
signal - full tumour trajectory tracking
- DICOM exported to TPS
ITV
The range where tumour can be due to motion
Overall target is still large but it is not irradiating as much normal tissue as normal PTV
Cine view
The Cine View gives a simulation of the tumour
and the tissues that move as the patient breathes
* You can select the image sets that are in the cine and change the orientation of the image
* This view is available for transverse, sagittal and coronal slices
How to create ITV
- Drawing on individual image sets: takes long
- Combining all phase based GTV with Automargin - Using Specialty Imagesets
- Maximum Intensity Projection (MIP)
Lowest diaphragm
Inhalation
Specialty image pixel values
MIP Maximum Intensity Projection
- MinIP Minimum Intensity Projection
- AvgIP Average Intensity Projection
MIP
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: anatomically misrepresent
- brighter as it is taking higher intensity
MIP challenges
Sometimes difficult to define edge
For e.g.- tumours may seem like it is attached to the diaphragm when in reality it is not
MiniIp
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
- shows where the tumour is (100% of the time it is there)
AvgIP
Average Intensity Projection:
- Displays the average CT number of all the pixels at the same spatial location over the respiratory cycle.
- Considered to be appropriate for use as the
treatment planning CT
* Reducing the need for an additional planning scan
* Lowering radiation exposure to patient
Used for treatment planning
Reduces radiation exposure as an additional image is not required
What happens if planned on MIP or MINIP
More MU as more modulation would be required for OAR constraints
Deeper depth and more deposition of energy
Maximum dose for MIP is much lower
Most variation of dose on skin
What is DCAT
DCAT is similar to VMAT as the treatment is delivered continuously as the
gantry rotates around the patient, but without segments dissecting the target
User defines: gantry start and stop, couch and Colli, relative weighting for arcs, constant dose rate
Why DCAT
DCAT is similar to VMAT but without segments dissecting the target, mitigating the mLC interplay effect
Is better for lung as it is a moving target which sometimes be missed due to MLC interplay of VMAT
DCAT advantages
Differing control points - oar sparing
Conformality
Variable dose rate
Variable target margin
Accounts for motion
Faster
Maximise dose rate
Most robust
DCAT disadvantages
Only suitable for simple targets
More central and spherical in shape
Worse dose distribution
How is interfractional motion controlled for lung treatments
Adaptive RT
Which bin indicates max exhale
10th bin
Treatment considerations
Motion:
- physiological movements - respiration and cardiac motion (systematic)
- Uncontrollable movements - swallowing, coughing (random)
Imaging artefacts:
- lengthen/shorten apparent targets
- target displacement: gaps in image
Dose delivery artefacts
Increasing target volume sizes
- increased treatment portal sizes
-increased OAR and normal tissue irradiation
