Notes Flashcards
3 means that comprise TPS
-input data
-calculate
-output data
what does does calculation accuracy depend on?
-algorithm
-modeling of actual clinical radiation beams
quaterly TPS tests (or after hardware or software upgrade)
-CPU/server
-digitizer (check accuracy of known contour)
-electronic plan transfer- check that is transfer to treatment console
-plotter/printer- test by comparing against known contour
-backup recovery
-CT geometry/density
annual TPS tests
Check constancy of dose calculations using a standard set of at least four clinical plans covering a range of geometries, energies and modalities including extreme scenarios likely to be encountered clinically. Check DVH constancy.
Also check simple cases corresponding to beam data used for commissioning (PDDs, profiles)
End-to-end test performed as realistically as possible (anthropomorphic phantom; use immobilization devices)
how do you do the measuremnet for virtual source?
must be done in air with appropriate build up cap since only the primary beam is expected to follow the ISL (according to the position of the virtual source)
is dose verification sufficient QA of TPS?
-verifications do not check that CT sim images are correct with respect to the patient. Need e.g., QUASAR phantom with various inserts of known HU to check this aspect of the TPS.
what is in TG-53?
also includes information for administrators on required staffing levels, time commitment required to commission a TPS, a description of the roles of different staff members, and a description of the treatment planning process
different MLC aperture options
to middle of leaf end, to inner corner, to outer corner
-check these
some imaging artifacts
-finite voxel size
-partial volume effects
-streaking from heterogeneities
-MR distortion
what is involved in acceptance testing of TPS?
-CT input
-anatomical description- make sure you can view contours
-beam description- make sure all beam functions work
-photon and electron beam dose calculations- test out all MLCs, SSDs, inhomogeneities etc
-dose display and DVH
-no measurements are carried out for acceptance test- just make sure all features are working
overview of commissioning steps for TPS
-imaging inout (does imaging work, is orientation correct, is integrity maintained)
-anatomical structure considerations (display etc)
-dosimetry- is TPS calculating properly
-establish QA procedures
-train staff
-IT considerations
image input considerations at commissioning
-check for correct # of pixels, pixel size, slice thickness
-check DICOM formats are compatible
-check that multiplanar reconstruction and DRR work
-check that image orientation is correct
-check that text is correct
-check that window and level work
-check that image correction tools are working and that original and modified images are correctly identified
-check that image registration works
-chck that conversion of of CT number to electron density was done properly
TPS commissioning steps related to CT scanner
check that CT scanner delivers expected dose, has adequate image quality, and has correct RED curve
what kind of curve is RED curve?
bilinear
-a line is fit to data below HU=0; a different line is fit to data above HU=0
-slope of RED vs HU is smaller for higher RED/HU
-energy dependent, but studies showed same HU curve can be used for CT scanners between 120-140 keV; error > 1 %
what to do with RED curve for HU> 6000?
set plateau- 3.92
-makes sure you don’t end up with ridiculous values if the curve were extrapolated
-check this also as part of commissioning
mass densities of common materials
-air: 0.001 g/cc
-lung: 0.3 g/cc
-fat/muscle: 0.9-1.1 g/cc
-bone: 1.1 - 1.8 g/cc
-metal implant: 3.8 g/cc
HU values of common materials
air= -1000
fat = -20-100
muscle/blood = 40-60
lung = -300
2 HU related curves in Eclipse
-curve of RED vs HU and curve of mass density vs HU
-RED are used for scaling kernels and distances in AAA
-mass densities are used in eMC algorithm
in RED phantom, why do e measure the HU with inserts at various positions?
-make sure calibration curve is correct across the FOV
water insert in middle of RED phantom
used to renormalize the rest of the measurements, to correct for scanner fluctuations
for what scan parameters do you do the RED curve?
-should check for all possible kV for all protocols in use
-verify that variation with typical CT scanner energies is negligible
-if a protocol is noisy, acquire and average multiple scans
features relating to anatomical structure to check in TPS
-structure types work as intended (ie target could be different than OAR)
-display works
-contouring tools work
-auto-segmentation or auto-contouring works
-missing contours are handled as expected
-verify margin expansion/contraction
-verify that end of structures are based on contours
-check that HU overrides have intended effect
-test contouring under different window/level settings
-verify body contour is working- no dose should be displayed outside the body contour
beam limits to be aware of
-MU limit per arc (1000 for conventional, 6000 for stereo)
-max allowable SSD for extended SSD tx
-max gantry speed
-max MLC leaf travel speed
-min dose grid size should be pixel size of image used for planning
machine configuration vs beam configuration
-energy independent features fall under machine configuration
-energy dependent falls under beam configuration
MLC parameters to check when TPS commissioning
-lef width
-number of leaves
-over-travel beyond midline
-leaf transmission
-min gap between opposing leaves
also check jaw parameters: max over-travel beyond midline, jaw positions specified in beam config model)
default values to be aware of
dose calculation uncertainty
grid size
penumbra slope
5-10%/mm for single field
what is entered into TPS during commissioning?
machine-related beam parameters and radiation data from measurements in a scanning water tank.
- For photons, need to measure CAX PDDs (for various FS), beam profiles at various depths and for various FS, diagonals for the largest FS at various depths, output factors for different FS, attenuation factors for wedges, compensators and trays (more details on this later).
* Special techniques such as beam junctions, SRS/SRT/SBRT, etc. require additional commissioning tasks (e.g., QA of half blocked fields, more stringent jaw positioning requirements for beam junctions; small field output factors for stereo, possibly including MLC defined fields)
For electrons, want to also take measurements that characterize the bremsstrahlung tail.
what do you compare TPS computed results with?
(1) measurements you have done yourself (even in the simple cases, it is useful to ensure that the commissioning measurement data was input correctly), or (2) published measured results obtained using a similar (but not identical) machine (e.g., TG-23, output factors) [make sure measurement conditions e.g., SSD are the same]. Can also do (3) MU check using hand calculation in simple situations or using independent software such as RadCalc (assuming RadCalc is already commissioned).
-could also use independent physicist
-could use IROC (Imaging and radiation oncology core)
how to assess inhomogeneity correction of TPS?
-try slabs of material and compare results with hand calc (not very clinically relevant)
compare different hand calc inhomogeneity corrections
- RTAR method does not depend on the location of the inhomogeneity relative to the point of interest.
- Batho power law method does take into account the location of the inhomogeneity and can be used for estimating doses to points within an inhomogeneity as well as below it.
- Both of these methods assume that the inhomogeneities have infinite lateral extent.
- RTAR method only corrects the primary component of dose, but does not address the change in scattered dose.
- Use of TMR or TPR is recommended above TAR because TAR includes inherent backscatter (it doesn’t cancel in ratio because denominator is in air), which varies with field size.
when comparing 2D dose measurement vs TPS, should you align based on dose distribution?
No, because you would miss errors in localization
how to check that couch kicks are working properly?
use a phantom with embedded detectors
how to test TPS ability to calculate scattered dose
measure under blocks/jaws
what to check for normalization?
check the diffrent methods- that they work
difference between algorithm and calculation verification
algorithm- checks that it is working from a math perspective
calculation- compares calculated and measured doses over a range of representative clinical situations
what could discrepancies in comparison with measurements be related to?
-could be due to errors in input data used for commissioning, limitations of algorithm, could be related to the software..
how to check things like isodose contours, colour wash, line profiles, dose at a point
-can export as dicom, determine line profile and compare with TPS display
-can also compare beam profiles on dosimetrically equivalent machines using gamma analysis
- Isodose surface can be checked by creating an artificial dose distribution in e.g., python, importing this into TPS and making sure the isodose surfaces generated by the TPS are as expected. 3D objects in the TPS are typically represented using a mesh (a series of 3D coordinates). This mesh can be analyzed further using e.g., API (simply comparing isodose volumes would be a simple way to do the comparison, but this does not test spatial coincidence).
how to check DVH display in TPS?
-use isodose distribution and known volume of a structure to check DVH at a few points on the curve
how to check plan evaluation tools: plan sums/differences?
Can check this by exporting the two dose distributions, redoing the calculation in e.g., python, and comparing against the TPS calculation. Could also do a point by point spot check.
what should you check on the treatment plan report?
: check that all information on treatment plan parameters (e.g., accessories) and patient information are correct, graphical display of dose distributions on different planes, DVHs
3 steps for establishing ongoing QA procedures for TPS
1) measurement procedure and how often it is carried out
2)comparison with baseline, tolerance and action levels
3)actions necessary if discrepancy is outside of tolerance
examples of TPS QA
-assess CT data transfer by scanning phantom with know densities
-reference conditions test (expect to get 1 cGy/MU)
-independent check of MU
-E2E test
-in vivo dosimetry
o Recalculate to check the constancy of dose calculations (including comparison of DVHs) using a standard set of clinical plans covering a range of geometries, energies and modalities (consider most extreme scenarios likely to be encountered clinically). This is useful to do after software update.
IT considerations
-check linac can communicate with TPS
-when can parameters be modified- ie can someone deleted a wedge after plan is approved with wedge
-investigate ability to delete linked objects
-multi-user environment- someone should not be able to change something in plan while someone else has it open
Evaluate software rules for calculation validity: if a change is made that will affect the dose distribution (e.g., turning on/off inhomogeneity corrections, changing beam aperture, etc.), then this should force a new dose calculation.
relational database
-data from ARIA is stored as this
-organize data into tables (relations) of columns and rows (tuples), with key identifying each row
-row representes instance of that entity and column represents values attributed to that instance
SQL
structured query language
for querying and maintaining the database
TPS positional errors on 3D TPS
-body contours
-collimator setting/display
-aperture definition/display
-beam location
-1 mm
-3 mm expected for PTV margin expansion
-contouring on another data set and transferring contaours to CT dataset adds 2-5 mm uncertainty associated with registration
TPS positional errors for traditional (2D planar images) TPS
-5 mm - order of many cm for positional errors
-5-10 mm for PTV margin expansion since this is done manually
-1-2 cm for transferring contours since this is done manually
TPS error for gantry, couch, and collimator angles
-< 1 degree
-for traditional TPS, 1 degree gantry angle and lack of couch/colli capability
TPS error for dose on central 80% of beam width
<1 %
-traditional TPS: > 10%
TPS error for dose in penumbra
1-5 mm (corresponding to 5-10% dose unertainty- this is motivation for 5%/5 mm IMRT gamma critera)
traditional TPS: 2-5 mm
TPS dose to norm point error (in blocked field)
2%
traditional TPS: expect 10% since scatter under blocks is not modelled
TPS error for dose in block penumbra
1 mm
> 1 cm for traditional TPS
published data in TG23
- TG-23 provides data from measured test cases on two example clinical treatment units. These data represent a variety of different scenarios (e.g., basic PDDs for different FS & SSD, half beam blocks, slab phantoms, oblique incidence, wedged fields) and represent benchmarks for comparison against computed values. These data are good for assessing trends but will not necessarily agree at 2-3% level since these data are measured on different treatment machines located elsewhere in the world. Using published data such as TG-23 is a quicker alternative to doing these measurements yourself (also represents a practical alternative if you don’t have access to a particular phantom. The data in TG-23 is not to be used for patient treatment, or clinical use whatsoever
QUASAR
quality assurance system for advanced radiotherapy
-assesses CT image acquisition and transfer to TPS
-CT image reconstruction
-DRR
-contouring and anatomical volume manipulation
-DVHs
-RED
-new version of QUASAR has apertures that can check MLC display
TPS purchase steps
-assess need
-request tech specs and prices from vendor
-vendor presentations/demonstrations/site visits
-tender process
-selectrion criteria (i.e. essential, important, useful, not needed)
-acquire list of known customer reported issues and make assessment based on this
-purchase
measurements required for typical TPS
-crossline and inline profiles at 5 depths for 14 field sizes (Varian only requires one)
-diagonal profiles at 5 depths for 40x40 FS
-PDDs for 14 field sizes (1-40 cm)
-output factors for a matrix of x and y jaw sizes
-reference conditions and dose/MU at reference condition
-o Can also specify absolute point doses (optional) for different jaws positions and measurement positions within water tank. These may improve estimation of the mean radial energy curve and intensity profiles.
why should you make sure your scans are symmetric before entering them in TPS?
-software assumes symmetric- only uses one side
-If the measured full profile is not perfectly symmetric, then the software presumably takes an average of both sides to carry out beam configuration
