CT Simulation & Motion Management Flashcards
what is QA
is the overall process which is supported by quality control activities.
QA encompasses all the planned and systematic actions needed to ensure that a product or service will meet the given requirements for quality. In radiation therapy, this involves a range of procedures, activities, actions, and staff groups to maintain the consistency and safety of the medical prescription
what is QC
describes the actual mechanism and procedures by which one can assure quality
QC is a subset of QA and involves activities that impose specific quality on a process. It entails evaluating the actual operating performance characteristics of a device or system, comparing it to desired goals, and taking action on the difference. For instance, QC in radiation therapy might involve regular checks of linac output constancy, mechanical tests like the front pointer, safety tests with survey meters, and checks of meteorological equipment
CT daily QA
- alignments of lasers to gantry plane
- CT number - water
- image noise
- spatial accuracy
Most likely RT would be responsible for the daily tests.
CT monthly test
- orientation of lasers (parallel and orthogonal to imaging plane)
- spacing of wall lasers for patient localisation lasers to imaging scan plane
- orientation of CT table relative to imaging scan plane
- accuracy and reproducibility of digital indicators for vertical and longitudinal motion
- CT number accuracy for different materials
- 3D spatial accuracy
Typically performed by local physicist
what happens if Spatial Accuracy are out of tolerance?
tolerance is +/- 1mm
difference = CT scanned image of object dimension - actual object dimension
how to measure
- this should be verified by CT scanning, using a phantom of known dimensions
- this should be verified different scan protocols
why is it important
- RT planning relies on accurate reproducible representation pf the patient dimensions and shape including
- geometric miss of target - either under or over treating
- image distortion can lead to misleading in causing inappropriate dose dumping to the wrong area
what happens if lasers are out of tolerance
tolerance : +/- 2mm
- Treatment room lasers are well defined and precise localisation of the treatment isocentre, the CT must possess the same relationship to the CT image centre.
- can affect the ability to identify skin marks and required for reproducible patient position for setup
- accuracy needs to be comparable to treatment machines lasers
- Contact Physics/relevant staff if the lasers are out of tolerance so they can be recalibrated ( this might require additional tools and checks such as alignment, tilt/skew/positioning and reference wall marks to be updated if required)
what happens if noise is out of tolerance? + how to perform/check
- Should be performed for head and body phantoms
- Check your phantom is placed in the centre of the imaging bore (you can use table height to find the centre quickly)
- Manufacturer typically provide phantoms and software to
automatically measure noise - image quality directly affects the ability to identify and delineate target volume and surrounding critical structures
- this can impact treatment planning as suboptimal images may cause omission of target volume or inadvertent delineation of normal structures
- noise is very sensitive parameter in overall imaging performance of scanner
what happens if HU for water is out of tolerance
- CT scanner is calibrated to give 0 HU for water, and the relationship between the relative electron density to CT numbers are mapped in the TPS (CT to ED curve or CT to density curve)
- deviation may indicate equipment fault in beam hardening, or image reconstruction software issues
- incorrect density corrected dose calculation are used in treatment planning, so incorrect CT numbers to density relationship can cause dose calculation errors
- incorrect water HU will lead to incorrect relative HU
- external definition also rely on accurate HU delineating the skin and air threshold
CT annual QA
- table positioning
- gantry tilt accuracy
- scan localisation from scout
- radiation profile width-slice collimation
- CTDI - patient dose delivered
- spatial resolution - high contrast resolution
- contrast resolution - low contrast resolution
- field uniformity
Typically performed by physicist at predetermined frequency or after service/repairs
two fundamental sources of positioning error in dose delivery
- determination of the tumour position as a function of time
2.calibration of the spatial relation between the tracking coordinate system and the beam delivery coordinate system
MIP + advantages and disadvantages
maximum intensity projection =
advantages
- one 3DCT gives information that encompass the entire range of tumour motion and TIV delineation can be performed in a timely manner
limitations
- ITVs created using MIPs are smaller and may result in geometric miss compared to ITV delineation using the whole 10 phase 4DCBCT
- difficult to see if near motion - eg. mediastinum, chestwall or diaphragm
- cant be used if tumour is near a high density edge - structures will get blurred out
- any irregularity in breathing during 4DCT acquisition will result in post processing artifacts
- limited to contouring only - higher CT HU units than a regular CT
limitations of 4DCT
- largest uncertainties for small tumours with large amplitude of motion with short period of oscillation
- longer scans can lead to increased motion artefacts
- larger the velocity, increases the volumetric deviations - typically 4DCT overestimate the volume of tumour
- contouring can also be performed on the MIP or average image or 10 phase 4DCT for the ITV definition of tumour excursion in all directions absolute volume and positioning deviations
- gating systems rely on an the interpretation that the external signal and its periodicity are reflective on the motion of the underlying tumour
- can be used in conjunction with other types of tracking technology to give real time tumour motion
SGRT advantages + disadvantages
advantages
- non ionising and non invasive
- high accuracy and resolution of 3D surfaces
- clinical trials postural visualisation
- patient feedback
- independent from linac gantry motion
- real time motion management
- gating and breath hold capability
- faceless mask option
- may not require SGRT on CT
- Faceless mask option (watch out for blink motions creating false gate
Disadvantages
- Equipment/Financial resources
- Additional training and increase of time
five (5) motion management strategies
- internal Target Volume (ITV) - any treatment of a target volume which encompasses the entire range of
motion of the lesion.
2.Free-breathing Gating - any treatment where delivery of the beam is limited to a portion of the respiratory cycle as the patient breathes normally.
3.Breath-hold Gating - any treatment where the delivery of the beam is limited to a portion of the respiratory
cycle which is extended by the patient holding their breath. Note that this strictly refers to breath-hold gating as a motion management strategy, rather than a means of optimising tumour geometry (as is the case in most deep inspiration breath-hold breast treatments utilising tangential beams).
4.Mid Ventilation - any treatment where the target volume is defined using the time- weighted average
position of the tumour.
5.Tumour Tracking - any treatment in which the treatment beam is modified/repositioned to account for the motion of the target.
Phase based binning pros and cons
Pros
- The breathing cycle is diving into equal time points
- Closely depicts the actual movement over time
- Allows calculation of the mid ventilation phase
Cons
- If the patient has any inconsistent breathing patterns it leads to 4D CT artifacts
