Notes Flashcards
1
Q
3 means that comprise TPS
A
-input data
-calculate
-output data
2
Q
what does does calculation accuracy depend on?
A
-algorithm
-modeling of actual clinical radiation beams
3
Q
quaterly TPS tests (or after hardware or software upgrade)
A
-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
4
Q
annual TPS tests
A
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)
5
Q
how do you do the measuremnet for virtual source?
A
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)
6
Q
is dose verification sufficient QA of TPS?
A
-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.
7
Q
what is in TG-53?
A
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
8
Q
different MLC aperture options
A
to middle of leaf end, to inner corner, to outer corner
-check these
9
Q
some imaging artifacts
A
-finite voxel size
-partial volume effects
-streaking from heterogeneities
-MR distortion
10
Q
what is involved in acceptance testing of TPS?
A
-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
11
Q
overview of commissioning steps for TPS
A
-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
12
Q
image input considerations at commissioning
A
-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
13
Q
TPS commissioning steps related to CT scanner
A
check that CT scanner delivers expected dose, has adequate image quality, and has correct RED curve
14
Q
what kind of curve is RED curve?
A
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 %
15
Q
what to do with RED curve for HU> 6000?
A
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
16
Q
mass densities of common materials
A
-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
17
Q
HU values of common materials
A
air= -1000
fat = -20-100
muscle/blood = 40-60
lung = -300
18
Q
2 HU related curves in Eclipse
A
-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
19
Q
in RED phantom, why do e measure the HU with inserts at various positions?
A
-make sure calibration curve is correct across the FOV
20
Q
water insert in middle of RED phantom
A
used to renormalize the rest of the measurements, to correct for scanner fluctuations
21
Q
for what scan parameters do you do the RED curve?
A
-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
22
Q
features relating to anatomical structure to check in TPS
A
-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
23
Q
beam limits to be aware of
A
-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
24
Q
machine configuration vs beam configuration
A
-energy independent features fall under machine configuration
-energy dependent falls under beam configuration
25
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)
26
default values to be aware of
dose calculation uncertainty
grid size
27
penumbra slope
5-10%/mm for single field
28
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.
29
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)
30
how to assess inhomogeneity correction of TPS?
-try slabs of material and compare results with hand calc (not very clinically relevant)
31
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.
32
when comparing 2D dose measurement vs TPS, should you align based on dose distribution?
No, because you would miss errors in localization
33
how to check that couch kicks are working properly?
use a phantom with embedded detectors
34
how to test TPS ability to calculate scattered dose
measure under blocks/jaws
35
what to check for normalization?
check the diffrent methods- that they work
36
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
37
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..
38
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).
39
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
40
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.
41
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
42
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
43
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.
44
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.
45
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
46
SQL
structured query language
for querying and maintaining the database
47
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
48
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
49
TPS error for gantry, couch, and collimator angles
-< 1 degree
-for traditional TPS, 1 degree gantry angle and lack of couch/colli capability
50
TPS error for dose on central 80% of beam width
<1 %
-traditional TPS: > 10%
51
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
52
TPS dose to norm point error (in blocked field)
2%
traditional TPS: expect 10% since scatter under blocks is not modelled
53
TPS error for dose in block penumbra
1 mm
> 1 cm for traditional TPS
54
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
55
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
56
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
57
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.
58
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
59
how to do diagonal scans?
-rotate colli but this won't take into account FF effects... may be necessary if diagonal scan resolution is inadequate
-rotate tank and stitch together scans from opposite quadrants
60
how to help ensure stability of detector and consistency of output
o Carry out measurements over the shortest time possible. Periodically repeat base measurements
o Use a reference chamber to account for output fluctuations when making scanning measurements with large fields.
61
do you use continuous scanning or step-by-step scanning?
o For large fields, generally use continuous scan mode to save time (using a reference detector to correct for variations in linac output over time). For small stereotactic fields below 5 cm, the step by step scanning method should be used without a reference chamber (since this will affect the field chamber measurement) – integration time should be long enough to compensate for accelerator pulses and small variations in linac output over time.
62
how to find true CAX in tank?
-measure profiles at various depths
-also scan chamber around to check that tank axes are around
63
why do you avoid measuring near the wall of the tank?
missing scatter
64
how to attach 2 half scans in OmniPro?
join function
65
consderations for orientation of detector
-don't want changing amount of stem in field (consistent)
=shielded dector- hysteresis
-more volume averaging if using larger detector dimension in direction of scan
66
measure profiles vs calculated profiles
-expect measured profiles to be slightly more rounded than the calculated profiles
-some degree of volume averaging in penumbra, even if using diode
67
do you need uniform spacing of data pts for profiles?
yes
68
daisy chaining of beam profiles
Use the ion chamber measurements in the central part of the field, and use the photon diode measurements in the penumbra region. Expect photon diode to over respond to low energy photons, so shift photon diode measurement to make it match up with the ion chamber measurement (renormalize it).
Alternatively, can use a diamond detector to measure the entire profile.
69
how to ensure chamber is on CAX
Make sure the tank scanning axes are level with respect to the water surface by scanning the chamber around and making sure it stays level.
Make sure the vertical scanning axis is aligned properly by scanning the detector up and down and looking at the shadow with respect to the cross hairs.
70
CAX measurement in OMNIpro
built-in software function that measures profiles at various depths and adjusts zero position as needed
71
flatness
-over central 80%\
-max 3 % for 10 cm depth and 100 cm SSD for largest FS
-F = 100 X (Dmax-Dmin)/(Dmax+Dmin)
72
symmetry
-2% (usually < 0.5%)
-S = 100 X (arealeft-arearight)/(arealeft+arearight)
73
optimization process in TPS
-2 term optimization function
-primary term is gamma error for calculated pt vs measured curve, 3mm/1% criteria
-secondary term is penalty term-gives penalities for noise etc, also not physically possible parameters
-parameters related to electron contamination are optimized separately
74
pre-processing of measured data by TPS
-truncate profiles and PDDs that saturated at a constant value
-extrapolate PDD if profiles are measured at depths deeper than max PDD (avoid doing this by measuring both to same depth)
75
quick TPS verifications for photons
o Electron contamination curve as a function of depth should peak ~0.5 mm depth and should drop to zero by 50 mm depth (this will depend on energy)
o Collimator back scatter factors should be >1 for FS < FSref (small FS more scatter from jaws) and <1 for FS > FSref (large FS)
o Wedge transmission curve is normalized to 1 so all values should be [0,1].
o Check that the highest energy in the spectrum corresponds to the nominal energy (since this is the energy of electrons striking the target).
o The mean radial energy determines the average energy of photons after flattening filter. The mean energy decreases smoothly when moving from the CAX to the field edge (the beam is hardest on the CAX when FF is present).
o Check for unphysical situations: increasing mean radial energy curve, increasing intensity outside the field edge, secondary source parameters should make sense (see above).
o Do expect some disagreement in penumbra due to volume averaging depending on chamber that is used.
76
4 sub-sources modelled in TPS
-main diverging beam (contains photons and electrons)
-edge electrons: along opening of applicator/insert
-transmission photons: produced by main photons and electrons in the insert material. Main photons that pass through the insert material without interacting are also included (they have same direction as the main photons but different energy distribution)
-second diverging beam (contains electrons and photons)
77
differences between main and second diverging beam
main: focus is 10 cm below nominal source. Particles are sampled on a plane 95 cm below nominal source, inside shape defined by applicator/insert
second: focus is 50 cm below nominal source (particles still sampled on a plane 95 cm below the nominal source, inside the shape defined by the applicator/insert)
78
required measurements in open fields for TPS
PDD for SPD = 100 cm
absolute dose in water at calibration point
profiles in air at 95 cm (only for open fields)
79
how to measure leaf transmission factor
o Measure the ratio of the measured dose in in an open field and the measured dose when using the same field size with all MLC leaves closed behind the jaws (so that the DLG doesn’t come into play)
o Use average of measurements obtained with the two leaf banks.
o This measurement will depend slightly on depth and field size, so choose geometry that is similar to clinically relevant geometries. Varian provides suggested values
o Varian also suggests having the first leaf pair closed behind the opposite jaw to force the carriage box behind the jaw.
o Orient the ion chamber perpendicular to the leaves so that leaf leakage contribution is not systematic (i.e., ion chamber could end up being directly under a leaf or directly in between two leaves – these situations would lead to different measurements and different DLG values). Can mitigate this issue by performing measurements in several positions; the average value should be used.
o Can use solid water for this.
80
how is rounded leaf end modelled?
* The algorithms model the shape of leaf edges as sharp and take the rounded leaf end into account by shifting the leaf tip positions back by half the value of the DLG in the actual fluence calculation.
-specifying DLG is iterative- Dose verifications are repeated and DLG values are adjusted until the gamma pass rate is as high as possible
81
how to measure DLG
plot readings vs different gap values
-subtract leaf transmission from the readings
-DLG is absolute value of x-intercept of linear plot
82
how is tongue and groove design modelled?
-extend leaf projection in direction perpendicular to leaf motion with extension parameter slightly smaller than real tongue width
83
differents in commissioning/QA for 3D-CRT vs IMRT
-IMRT QA must focus on cumulative delivered dose rather than QA of individual segments
-because of complex IMRT dose, dose should be checked at multiple locations
-with IMRT, cannot use simple portal image to validate critical structure avoidance
-o The temporal nature of IMRT/VMAT/DCAT dose delivery means that integrating dosimetric techniques are needed to evaluate the total dose distribution (components e.g., MLCs are moving while the beam is on). Can’t investigate dose for each individual field separately
84
why use gamma test for dose verifications?
-distance to agreement is overly sensitive in regions of low dose gradient
-dose difference is overly sensitive in regions of high dose gradient
85
what is the evaluated distribution in the gamma test?
the calculated distribution
86
what is image matching based on?
minimizing of the average variance of the ratio of grey values in corresponding points of the two images
A more complex image matching algorithm using mutual information may also be used, which is especially necessary if you want to co-register images obtained with different imaging modalities
87
image matching with mutual information
-iteratively maximize mutual information- maximize ability to provide grey value in one image, given that they grey value in the other image is known
88
how to calculate mutual information
-plot image 1 signal vs image 2 signal
-convert this scatterplot to a 2D histogram by overlaying a grid over the scatterplot, and counting how many points land in each square.
-The mutual information is high when signal in the 2D histogram is concentrated into a few bins, and low when signal is spread across many bins.
89
typical MTF shape
-starts at unity for low f (ideal), then drops to 0 as frequency increases (ROI gets more blurred out as lines get closer together and can no longer be resolved by the imaging system)
90
PROS of thin client system compared to a thick client (e.g., Citrix-based system accessed via the internet):
* The person managing the internet-based system can troubleshoot remotely.
* Software upgrades/updates are easier to manage and can be carried out for everyone at the same time.
* Thin clients are easy to set up (you don’t need to install software directly on the thin client workstation), resulting in lower IT costs.
* The workstation doesn’t need to have much computational power, and don’t require much cooling since computational load is handled remotely.
* No data is lost if a workstation is destroyed or stolen.
91
CONS of thin client system compared to a thick client:
* You must have redundant hardware in place in the event of a server crash, to avoid having a single point of failure.
* You must invest in high-quality servers with a high level of computational power that can maintain high performance of the system across the entire thin client load (large initial cost).
92
what does machine configuration involve?
energy-independent parameters of the linac
nominal energy, available dose rates, MLC ID (leaf width, leaf length, number of leaves, max over-travel), max jaw over-travel, etc.
Note that intra-leaf transmission, inter-leaf leakage and DLG are energy dependent and so these go under beam configuration and are not part of machine configuration.
-couch correction limits
-treatment room equipment safety zone
o Equipment motion permissions (e.g., do you have to be in the treatment room to move the couch)
o MU limits per conventional and stereotactic arc (1000 and 6000 MU, respectively).
o Max gantry rotation speed, max MLC leaf travel speed.
o Max SSD
o Imaging
93
imaging parameters in machine configuration
Imaging workflows (which image acquisition techniques are activated, unused imager position e.g., mid)
Automatic image workflow management (e.g., after CBCT is acquired, automatically activate 3D-3D match workspace).
-customize imaging protocols
94
how to know if auto mAs adjustement is working for CT?
can scan phantoms of different sizes (and having the same composition – could use the two CTDI phantoms for this) using the same scan protocol (i.e., same kVp) and make sure the dose (CTDI/CBDI) is approximately the same in both cases. Note that the amount of mA to use is determined during the scout scan
thought it would be image quality not dose??
95
what are beam types referred to as in beam configuration?
add-ons
example: open field, each wedge, electron applicator, each compensator, blocks, MLCs
96
assigning beam data from one treatment unit to another versus copying beam data from one unit to another
assigning- any changes to beam model with affect all treatment units sharing the beam data
copying: creates independent calculation model
97
deleting beam data vs clearing beam data
clearing: dissociates beam data from a treatment unit while retaining the data
deleting: completely removes it from congifuration system and affects all beam models that use that data
98
checksums
used to protect the data from alteration, to make sure that it hasn’t been changed e.g., in transferring data from one software/computer to another.
o Checksum algorithm should output significantly different value even for small change in input.
99
distributed calculation framework
increases dose calculation speed because calculation jobs can be performed on multiple processors simultaneously. The DCF allocates calculations to available calculation resources.
o You can define global settings for all users. You can also define local settings for a particular user profile or for a shared profile that applies to multiple users. Local settings override global ones.
100
collimator backscatter factors
CBSF values do not need to be taken into account in hand calculations based on tabulated experimental relative dose factors since these experimental values implicitly include the effect of backscatter into the jaws resulting from delivering a particular number of MU.
-<1 for large field sizes and > 1 for small field sizes
-backscatter into ion chamber is a smaller percentage of total dose for big field sizes
101
what is energy spectrum calculation dependent on?
-horns in profile and how horns change with depth
102
source model for AAA
-multiple source
-primary photon source
-extra focal secondary source:models photons that result from interactions in the accelerator head, e.g., in the flattening filter, primary collimators and secondary jaws. This is disabled in FFF beams
-electron contamination source: modelled as depth dependent curve. Also includes photon contamination due to photons created in electron interactions. Electron contamination mainly comes from flattening filter, collimating jaws and air.
-could also be photon wedge scatter source
103
what does phase space give?
o The phase space gives the energy, position, and direction of all particles (photons and electrons) exiting the linac.
104
what does Acuros discretize?
discretizes three space variables (giving the position), two angular variables (giving the direction) and one energy variable. Acuros solves this discretized system of equations to determine the angular and energy dependent photon fluence at every spatial degree of freedom in the computational domain.
105
errors in MC vs Acuros
o In both Acuros BV and MC, there is a trade-off between computational speed and accuracy. In MC, errors are generally stochastic. In Acuros BV, errors are deterministic and primarily result from the discretization resolution in space, angle and energy.
Angular discretization typically results in ray effects.
Energy discretization errors: solution biases present over a large region.
Spatial discretization errors: local solution over/under shooting; generally most pronounced in high dose gradients regions (e.g., in penumbra regions behind shields)
106
what is used for primary photon tracing in acuros?
ray tracing
107
what does ray tracing involve?
modelling energy transfer to the medium and attenuation of incident fluence according to the attenuation coefficient. Ray tracing may also account for ISL and beam hardening (change in spectrum with depth) in addition to attenuation
o Photon transport using ray tracing is in a straight line unlike Monte Carlo photon transport…
108
briefly describe monte carlo algorithm
photon transport, photon interactions (and resulting changes in direction, creation of secondary particles) are explicitly modelled.
o Sample incident photon energy, direction, position and transport to the first boundary.
o Sample distance to the first interaction according to the attenuation coefficient, and transport photon to this interaction location.
o Choose type of interaction and sample energy, direction of new particles and original particle if it still exists.
o Transport photon until it leaves the geometry or its energy is below the transport threshold.
o Also transport secondary electrons, keeping track of delta rays and bremsstrahlung as well; score resulting energy deposition.
o Repeat for all histories
109
of rows of detectors in our CT sim
16
-each row is 0.0625 cm thick
110
CT pitch vs scan time
pitch>1 means undersampling and scan time is shortened; pitch<1 means oversampling and scan time is lengthened)
111
4 generations of CT scanners
o First generation: single detector; x-ray pencil beam; 30 minute scan time. Detector and source translates to acquire entire FOV, then rotates and repeat (“translate-rotate” technique).
o Second generation: multiple (up to ~30) detectors; x-ray fan beam ~ 10°; < 90 s scan time; still using “translate-rotate” technique.
o Third generation: multiple (100s) detectors; x-ray fan beam ~ 50°; ~5 s scan time; no longer using “translate-rotate” technique (source and curved array of detectors move together; “rotate-rotate”)
o Fourth generation: multiple (>2000) detectors in fixed outer ring; x-ray fan beam; scan time of a few seconds; “rotate-fixed” technique
o Other technologies have been added to third and fourth generation scanners:
Helical/spiral acquisition (now used in all modern scanners). Made possible by slip-ring technology (no need to reverse gantry rotation to untwist wires). Allows for continuous acquisition.
Multi-slice CT further sped up image acquisition.
112
brachy TPS commissioning
-similar as EBRT
-test series of plans with film or 2D array
-test various applicators
-make sure source decay is being calculated properly and is taken into account at TCS
-check that source parameters in library are correct (ex. half life)
* Catheter reconstruction features should be tested (e.g., what happens if you delete a point)
* Pay attention to units used and time zones used throughout.
* Transfer of information to TCS (how are dwell times/positions getting truncated if precision is too high)
113
how is electron dose calculated in Eclipse?
eMC
-Varian's implementation of the local to golbal MC method
114
2 steps in eMC (electron monte carlo)
1.-MC simulations of electron transport are done for a well-defined local geometry (EGSnrc is used to simulate transport of incident electrons of various energies through macroscopic spheres of sizes and materials likely to be needed in an actual clinical situation)
-end up with bunch of probability distribution functions of particles emergied from local geometry
-these PDFs are calculated once for a variety of clinically relevant materials, energies, and sphere sizes
2.golbal geometry
-particles are transported through the CT volume im macro steps based on the PDFs generated in the local calculation
-CT numbers are converted to electron density using RED
-A sphere index corresponding to the maximum sphere radius that can be used from the current voxel centre without the sphere reaching into another material is determined for each voxel. The average density within a sphere determines which material is assigned to that sphere.
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differentiate validation vs commissioning
commissioning data collection is dictated by what the vendor requires
-validation data collection is to validate the model (ex. surface contamination, Bresmtrah tail, oblique incidence etc)
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data specifically required for electron MC commissioning
-in air output factors (with build-up cap)
-mass density
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is virtual wedge considered a different energy beam?
no, but physical wedge is
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can you use AAA measured data to input into Acuros?
yes but don't then use ACUROS to input into a 3rd system- use the one that has the measured data
(also still spot-check data)
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focal spot size
-only parameter explicitly recomended to be different between Acuros and AAA
-impacts shape of penumbra and correction factors at small fields
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unique MLC measurement required by Pinnacle
effective radius at end
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TPS in linac MR?
Monte Carlo
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how to measure average MLC transmission factor
-farmer ion chamber
-leaves closed vs leave open
-average values at center and off-center since MLC leaves are different sizes
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DLG input into TPS
-measure DLG for sanity check but iteratively change DLG to optimize TPS performance
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DLG of millenum MLC
6 FFF - 1.5 mm
6 MV - 2.1 mm
15 MV- 2.5 mm
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DLG of HD-MLC
0.3-0.5 mm
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how to measure DLG
-severla gap widths
-measure signal (subract off transmission signal)
-plot dose vs width and extrapolate to 0
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how to measure transmission through MLCs, average
close MLCs under the jaw
measure for both banks and average, average mid and off-mid values if outer MLCs thicker than inner ones
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minimum static leaf gap to ensure MLC leaves don
t collide
0.5 mm
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wedges in beam configutrstion
each beam angle upper and lower tray is a new add-on
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can Eclipse beam configuration handle data entry for measurements at 100 cm SSD but reference at 100 cm SAD?
yes
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typical focal spot size for beam
1 mm
no way to measure it
"fudging" parameter used in beam modelling
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will 1x1 and 2x2 cm2 data be used in beam model?
Eclipse- they will appear but not be used in fitting of the model
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issues with measurements at surface and penumbra
partial volume effects
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once beam is configured, what does "calculate beam data" in Eclipse do?
-yields electron contamination, off-axis energy, intensity profile, energy spectrum
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what is intensity profile?
primary profile
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explain EDW in Varian
weighted open field and jaw closed field
y-jaw sweeps across
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scanning water tank set-up for EDW
-some have 1D array of detectors
-detectors are 1 cm apart
-can shift detector and retake measurements to improve resolution
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separate beam model for stereo?
at some centers
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how to tweak TPS model paameters
-use wide selection of patient plans
-tweak DLG, transmission, focal size
-iterate
-test on wide selection of NEW patient plans
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issue with getting diagonal profile
-with 40x40 cm2 field size already only measure a few cm of umbra. If put tank diagonally and start from origin, won't fit profile in... have to shift tank
some scanning systems have radial scanning to get diagonal profile in
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responsibility of RO
-prescription
-localization of target
-OAR constraints
-approve final plan
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responsibility of physicist
-design and implementation of QA program for treatment planning
-generates treatment machine data necessary
-directs and review dosimetry planning
-determines local QA tests for treatment planning
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responsibility of RT
-positioning, immobilization, verification
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responsibility of dosimetrist
The medical dosimetrist is responsible for patient data acquisition, radiation treatment design, and manual and computer-assisted calculations of ra-diation dose distributions. In consultation with the radiation oncology physicist and radiation oncologist, the dosimetrist generates and documents the chosen treatment plan for each patient. The final plan is reviewed by the radiation oncology physicist and approved by the radiation oncologist
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what may anatomy definition entail?
image registration with MRI, PET
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how to add CT scanner in aria
CT simulator
-RT-admin- here you enter machine limits, types of MLC etc
-clinical data- here you enter eletron and mass density ranges for all materials
-in beam configuration- beam data- CT calibration- here you put RED curve
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why does AAA use electron density curve whereas acuros uses mass density?
mass density- acurose chooses an actual material based on mass density
-AAA- RED is relevant for compton. Not as great for high Z, high E (i.e. pair production)
