Motion Management Flashcards
What is interfraction motion
o Motion between fractions
o Soft tissue shifts relative to bones (head and neck, thorax, abdomen, pelvis)
o Tend to occur day-to-day regardless of immobilisation and localisation
What can cause interfractional variations
o Due to adjacent OAR/target organ filling
o Due to changes in tissue geometry
What is Intra-fraction motion
o Motion during delivery of a fraction
o Primarily a fraction of the extent of the target motion during treatment
o Voluntary (wriggling, scratching)
o Involuntary (coughing, sneezing, swallowing, cardiac motion)
How to manage intra-fraction motion
o Consider patient comfort
o Reduction of patient anxiety
o Minimise length of treatment
Patient Preparation and Setup Techniques to Minimise Motion
Immobilisation Devices
Important to maintain reproducibility and accuracy
Can be a source of anxiety – may exacerbate motion
Bowel and Bladder Prep
Consistency of filling can improve positional reproducibility of pelvic organs
Compression techniques
Aim to reduce abdominal motion due to respiration (abdominal compression)
Potentially increase baseline shifts (setup errors)
Education and Compliance
Understanding of positioning equipment, preparation protocols – shown to increase compliance
What is the optimal arm position when treating a lung?
Arms above head
Greater choice of beam angles
Improves target coverage
Sparing of normal tissue
May be unsuitable for certain patients
What can be used to stabilise the arm when treating a lung?
Use a stable arm support in combination with knee support to increase comfort
Generally vac-bags – provide support through arms, neck and torso
Which aspect of the lung is tumour motion the greatest?
Different aspects of lung have variable motion
60% lung tumour – move < 1 cm
35% lung tumour – move 1 – 2 cm
5% lung tumour – move > 2 cm
Motion increases towards the diaphragm and is largest in the liver
Goal of Image Guided RT
Reduce uncertainties
Maximise reproducibility
Improve set up accuracy
Account for organ motion
Sparing NTT
What are fiducial markers and their purpose?
o Routinely used for localising prostate irradiation
o Small gold seed (0.9-3.0 mm) implanted under transrectal US guidance
Benefits of Fiducial Markers
FMs surrogate of prostate motion
Very fast method of localisation
Staff have great confidence when aligning FMs
- very low intra- and inter-observer variability
Disadvantages of Fiducial Markers
o Associated expense
o Invasive procedure = risk of infection at implantation
o Some patients ineligible, e.g. Warfarin dependency
o Rely on few (three) discrete points to localise the prostate (particularly when used with planar imaging)
In which direction is prostate motion most common
Dominant changes were in anterior and/or superior directions.
What can affect the position of the uterus
o Bladder filling significant factor in motion – filled can push posterior and superior
o Pelvic Tilt
What can affect the position of the prostate
Pelvic Tilt
Bladder Filling
Motion Artifacts in CT simulation
Distortions along the axis of motion can either lengthen or shorten the target
(in a somewhat random effect)
If CT scanning speed < tumor motion speed, → smeared tumor image
If CT scanning speed > tumor motion speed, → tumor position and shape captured at an arbitrary breathing phase
If CT scanning speed ~ tumor motion speed, → tumor position and
shape heavily distorted