interfraction motion
motion between fractions
- soft tissue shifts relative to bones are well known to occur in many sites (head and neck, thorax, abdomen and pelvis)
- tend to occur day to day regardless of immobolisation and localisation methods used
- considerations include patient preparation strategies (bladder and bowel filling)
intrafraction motion
motion during delivery of a fraction
- primarily a function of the extent of the target motion during treatment
- voluntary patient motion (wriggling, scratching)
- involuntary motion (respiratory, peristalsis, cardiac motion, coughing, sneezing, swallowing)
patient preparation to reduce motion
- immobolisation devices
- bowel and bladder preparation
- compression techniques
- education and compliance
goal of IGRT
to reduce uncertainties and maximise the reproducibility of treatment delivery by
- improving set-up accuracy and
- accounting for organ motion
IGRT aims to minimise the PTV margins, thereby reducing the overall irradiated volume and sparing uninvolved tissue
fiducial markers - prostate
- routinely used for localising prostate irradiation
- small gold seeds implanted under trans-rectal US guidance
- aims to overcome prostate motion due to factor such as
- rectal filling
- bladder filling
- patient motion
- has become standard of care over last 10 years in AUS
strengths 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
weakness of FMs
- associated expense
- invasive procedure = risk of infection at implantation
- some patients ineligible (Warfarin dependency)
- rely on few (three) discrete points to localise the prostate
- particularly when used with planar imaging
- evaluation of nearby organs and deformation of the target is difficult (if not possible)
- not gas on images opposite may indicate seminal vesicle motion
‘CT on Rails’
- diagnostic CT directly opposite (or orthogonal to)
- single couch for both gantries
- couch rotates between the linac and CT
- CT gantry “slides” over patient
- assumes fixed relationship between the isocentres of the two systems (CT and linac)
describe 4DCT
Images sampled at every position of interest along patients longitudinal axis. Each image tagged with breathing cycles, Images sorted based on breathing cycle, 3DCT dataset produced corresponding to breathing phase
what happens when the tumour motion > CT scanning speed
smeared tumour position
what happens when the tumour motion < CT scanning speed
tumour position and shape heavily distorted
margins for free breathing with conventional CT
deformed tumour shape
will need large PTV margin to account for numerous uncertainties
margins for free breathing, 4DCT using ITV approach
medium sized PTV
margins for free breathing or breath-hold, 4DCT using gating (at exhale or breath-hold)
small PTV margin
free breathing, 4DCT using mid-ventilation approach
small PTV margin
