Current RT Developments - Motion Management Flashcards
What types of motion management are available?
Passive- accept motion exists, attempt to quantify; create treatment to take account of movement prior to delivery (e.g. free breathing 4DCT)
Active - actively reduce tumour motion; real time monitoring; adapt delivery throughout.
e.g.s Compression; gated delivery or breath-hold; tracking
What does compression motion management entail?
Using immobilisation devices to reduce motion so that treatment is not interrupted (i.e. delivered continuously); reproducibility and patient compliance may be an issue. In terms of patient compliance, if treatment is quick (e.g. VMAT FFF) abdominal compression may be better tolerated. Also, may want to think about full duration of course….i..e. patient tolerates abdo being compressed at #1 but at #20 when feeling ill, may not be able to tolerate such compression.
What does Gated delivery or breath-hold motion management entail?
Patient either holds their breath whilst treatment is delivered or patient breaths normally, their respiratory cycle is monitored and beam on/treatment delivery only at a specific point in the cycle. This extends the overall treatment time; possibility of postural changes and patient compliance are also issues.
What does Tracking motion management entail?
Respiratory motion is monitored in real time or predicted with beam on throughout; need to verify treatment delivery.
When imaging with PET-CT, 4D CT and planning CT with breath hold, what visual characteristics does a tumour of the lung present?
During a single respiratory cycle:
PET - tumour will appear blurred over the range of motion
CT (PET-CT) - tumour will have discrete and different positions on adjacent slices (differences in location will be relatively large)
4D CT - tumour will have discrete and different positions on adjacent slices; as the scan is calibrated to capture a single slice repeatedly over the full respiratory cycle, differences in location will be smaller than those on CT (PET-CT) and will be regularly spaced.
3D planning CT with breath hold - discrete, constant location on adjacent slices; however, as patient breaths in and holds, this is somewhat artificial and results in the tumour location being outside the range observed in the other modalities (i.e. beyond the extremes of the normal respiratory cycle).
Respiratory management systems - there are six systems listed in the notes; name the systems and give a short list of features that allow them to track/anticipate respiratory motion.
C-RAD - uses optical surface scanning (i.e. not markers or fiducials) to monitor respiratory cycle and (if needed) adjust patient posture
Vision RT - similar to C-RAD only exception is “3D surface mapping”
Varian RG - uses small fiducial markers that are infra-red reflective to monitor a single area of patient for respiratory motion (via an infro-red/visible camera)
ANZI - pressure sensor on belt around diaphragm (similar is used at Leeds for 4D CT scans)
Calypso - implanted electromagnetic transponders (i.e. planted into tumour and invasive for patients); electromagnetic array detector (which has low attenuation); not a ‘motion surrogate’
Elekta Clarity - used for prostate treatments; US probe at perineum; intrafraction imaging; 3D monitoring of prostate to within a few mm.
What report gives dose estimates for onboard imaging?
AAPM report 179
What are the disadvantages of 4D CT retrospective imaging?
1) large dose to the patient (approx 3x that of 3D CT) - need to consider this is rescan is considered
2) scan length limited due to heating of x-ray tube
3) automatic dose control may not be available for 4DCT acquisitions
4) Imaging anatomy outside of CT normal FOV (the extended FOV) may not be possible; i.e. larger patients may present a challenge, e.g. ipsilateral breast
What is MIP and AvIP?
Maximum Intensity Projection (MIP) is the sum of the value of each voxel over all of the bins; it gives a ‘solid’ region within which the tumour moves during each respiratory cycle.
Average Intensity Projection (AvIP) is the average of the values in each pixel over all bins used. Therefore, a region of tumour movement during each respiratory cycle is delinated (as with MIP) however AvIP gives a higher amplitude in pixels where the tumour spends more time during the respiratory cycle.
What artifacts could be seen in a 4DCT image?
Cardia motion (as not in phase with respiratory cycle). Irregular respiratory patterns; could lead to repeat anatomy imaging (e.g. diaphragm).
What is the main advantage of gated RT?
Treament volume reduced, therefore spare more healthy tissue.
In order to deliver gated RT effectively, what must the patient do?
Maintain regular breathing pattern or incorrect volumes irradiated. Patient usually instructed when to breath in and out (auto-coaching).
Active motion management - Compression devices - can these be used for localisation of the tumour?
AAPM TG 101 report specifies that body frames and fiducial systems are acceptable for immobilization and coarse localisation but NOT sufficient for sole method of localisation. i.e. IGRT required too.
Note, compression does not always work! Also issues with patient compliance.
List main points involved in Deep Inspiration Breath-Hold (DIBH) and list any advantages.
1) patient breaths in deeper than normal and holds breath for around 20 seconds - patient is audio-coached for this (i.e. has audio prompts).
2) CT acquired during breath hold
3) alongside, acquire associated breath trace
Advantages:
For breast treatments, heart is moved away from field. As heart does not have long term recovery from treatment (see Radiobiolgy section), dose to heart is one of the main issues with breast treatments.
List some image treatment verification options.
1) steroscopic 2D kV imagiing
2) 3D kV CBCT
3) 4D kV CBCT
4) 2D kV Fluro
Note both 4D kV CBCT and 2D kV Fluro can be used to monitor motion.
