Wk 12 - 4D Techniques and Application Flashcards
intrafraction motion
movement within the patient during a fraction
interfraction motion
motion between fractions (eg organ filling, emptying variations)
if motion can be controlled
- tumour margins could be reduces
- OAR/normal tissue sparing can be improved
- potentially dose escalate
- improve clinical outcomes
motion management solutions include
- enhanced immobolisation
- bodyfix
- respiratory management
- ABC device
4DCT
- captures the location and movement of your tumor and the movement of your body’s organs over time
- 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
- DICOM exported to TPS
- full tumour trajectory tracking
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
creating ITV
- drawing on individual imaging sets
- combining all phase based GTV with automargin
- using specialty image sets
- maximum intensity projection
speciality image sets
is a feature that combines 2 or more image sets to make a specialty image
- max intensity projection (MIP)
- minimum intensity projection (MinIP)
- average intensity projection (AvgIP)
MIP
- 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
MIP considerations
- lower lobe lung lesion on free-breathing CT shows defined separation between the lesion and liver
- the lesion on the 4DCT MIP gives the impression that the tumour is attached to the diaphragm making the inferior extent of the tumour difficult to define
MinIP
- displays the minimum CT number of all the pixels at the same spatial location over the respiratory cycle
- shows where some part of the tumour is always present throughout respiration
- useful for liver tumours that present as low density areas
AvgIP
- displays the average CT number of all the pixels at the same spatial location over the respiratory cycle
- shows where tumour is usually
- considered to be appropriate for use as the treatment planning CT
- reducing the need for an additional planning scan
- lowering radiation exposure to patient
DCAT
dynamic conformal arc therapy
- similar to VMAT as the treatment is delivered continuously as the gantry rotates around the patient, but without segments dissecting the target
- user specifies target and avoidance structures with margins
- fast planning and delivery
- target remains inside the open field for entire treatment
- limited dose shaping but can provide a steep dose fall off
Discuss the interplay effect
- Describes the effect on a given MLC position and instance of radiation delivery with the position of the tumour in the respiratory-induced motion cycle at the same instance.
- Could cause an underdosing of the PTV
- The interplay effect is seen to average out over 30 or more treatments but in SBRT setting it is not clear how it impacts dose distribution
Can dose calculation be performed on the MIP/minIP, why or why not?
- 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
