Notes Flashcards
1
Q
steps for selection and purchase process
A
-assess needs of clinic
-compare specs based on request for info to manufacturer
-site visits to another center that has linac of interest
-tender process
-decide what is essential, important, preferred
-compare responses from manufacturers
-purchase contract
2
Q
describe tender process
A
-RFP (request for proposal)- bid request
-• specifications, capabilities, service, training, warranty, costs and delivery times
3
Q
example of RFP details for CT sim
A
-system performance: stapial resolution, noise, contrast, dose
-set of typical clinical images of human anatomy for various example scan parameters
-list of local purchasers to facilitate site visits
-architectural info: size, weight, electrical/thermal requirements
-radiation exposure levels around CT gantry for shielding design
-details of warranties, service contracts, training, projected costs for services after warranty expiration
-can send out bid questionnaire
4
Q
what is included in purchase contract
A
-manufacturer
-make/model
-specification document
-acceptance test procedures, with associated tolerances
5
Q
what is verified during acceptance testing
A
-performance of equipment must meed or exceed contract agreement
-environment free of radiation and electrical hazards
6
Q
who is present during acceptance testing
A
vendor
physicist
7
Q
how are acceptance tests designed?
A
-any deficiencies discovered and rectified have minimal impact on tests previously done- order matters
8
Q
define commissioning
A
-process through which all machine characteristics relevant to clinical use are investigated, measured, and recorded
-develop baselines of system performance
-prepare equipment for clinical service (acquire data to carry out radiation dose calcs)
9
Q
should commissioning data always be independently double checked?
A
yes
-external audits like IROC are recommended when commissioning new equipment that the centre hasn’t dealt with before, and for special techniques
10
Q
is vendor present during commissioning?
A
No
11
Q
time required for commissioning
A
4-6 weeks after acceptance testing
-1.5 weeks for 2 photon energies
-week for data collection
-1-2 weeks electrons
-1-2 weeks analysis and report writing
12
Q
what is required to bring unit into clinical operation
A
-acceptance testing
-commissioning
13
Q
why is commissioning important?
A
RT is directly related to accuracy in delivered dose to patient which is dependent on accuracy of beam data
14
Q
define golden beam data
A
-attempt by vendors to standardize machines to have identical characteristics
-can purchase golden beam data which includes most of data required for TPS commissioning
15
Q
cons of using golden beam data
A
-variations in parameters between beams with same nominal energies
-on site changes (ex beam steering) will not be modeled in golden data and therefore TPS)
-speed of jaws varies, which affects wedge characteristics
- If only subset of the data is checked, there may be acceptable agreement, but some clinical setups may have multiple errors that combine to produce unacceptable results.
-still need to spot check (don’t need to check entire suite of measurements that you would need for beam configuration from scratch)
16
Q
pros of golden beam data
A
-less risk of catastrophic error
-faster commissioning because only a spot check is required
17
Q
2 additional steps once commissioning is done
A
-perform independent audiot of collected data and report
-backup whole dataset (raw and processed)
18
Q
survey equipment required
A
o Geiger Counter – to detect presence of radiation.
o Large volume (= higher sensitivity) ion chamber survey meter – to quantify amount of radiation
o Neutron survey equipment for units operated above 10 MeV (Bonner spheres, long counters, bubble detectors, BF3 counters)
19
Q
dosimetry equipment required
A
-small volume ion chamber or diode to measure in rapidly changing dose gradients, small fields
-PP chamber for buildup region, electrons (required for < 6 MeV)
-diodes- PDI can be taken as PDD
-diodes are useful for small fields and high dose gradient regions due to their small sensitive volume yet high sensitivity due to high density.
20
Q
what equipment require calibration certificate?
A
reference thermometer, barometer, hygrometer
-o Field devices should have cross calibration performed prior to initial use, following malfunction and repair, and every year, except for barometers, which should be cross-calibrated every 6 months. Can compare barometer with local airport system (corrected for altitude difference).
21
Q
list of acceptance testing safety tests
A
-interlocks
-warning lights
-patient monitoring
-training
-radiation survey
-collimator and head leakage
22
Q
goal of safety testing
A
-eliminate possibility of unplanned/inappropriate irradiation of people
23
Q
list of interlocks
A
-door, beam off, key, motion, emergency off, emergency power failure illumination, electron applicator jaw setting interlocks, wedge jaw setting, beam stopper interlocks, dosimetry, MLC (ex. FS too big, attempt to use with electrons)
Interlocks should be directly linked to machine operation
Emergency off interlocks disable power to motors that drive the treatment unit and couch and disable power to some of the radiation producing elements of the treatment unit (to prevent beam)
24
Q
describe the radiation sruvet steps with order of regions surveyed
A
For linacs above 10 MeV, need to survey for neutrons as well as photons
Use the highest energy photon beam at the highest dose rate
First perform preliminary calibration and beam quality check of the highest energy photon beam
First locate hotspots with Geiger counter, then use ion chamber type survey meter to quantify leakage currents
First area to survey = control console
Survey primary barriers with largest FS, with collimator rotated to 45 degrees (corresponding to largest horizontal and vertical extents of FS), no phantom in the beam, beam pointing toward barrier.
Survey secondary barriers with largest FS, with a phantom in the beam, beam pointing toward wherever will result in the highest reading (or according to orientation for particular calculation point which should correspond to worst case scenario).
Always want to survey in “worst case” conditions (highest energy, highest dose rate)
Check that transmission through beam stopper is as specified.
25
list of mechanical tests at acceptance testing
-collimator axis of rotation
-photon collimator jaw motion
-light and radiation field congruence
-gantry axis of rotation
-couch axis of rotation
-radiation isocenter
-ODI
-gantry angle indicators
-collimator field size indicators
-patient treatment table motions
26
collimator axis of rotation acceptance level
radius < 1 mm
27
how do you do the collimator jaw motion test
Ensure jaws open symmetrically about collimator axis of rotation: Use a dial indicator (movement of probe/feeler is indicated on dial readout) to find position of each jaw in pair, rotating the collimator 180 degrees to get reading of other jaw in pair. Difference between jaws in a pair should be < 1 mm. Do this for each pair of jaws.
Check that the two sets of collimator jaws are perpendicular to each other using gantry at 90 or 270 and place level on jaws.
Check that collimator angle readout is correct at cardinal angles using level.
28
how do you verify that light field axis and collimator axis are congruent?
verify that image of the cross hairs is coincident with the collimator axis of rotation (using front pointer). Rotate collimator; deviation should be < 1 mm
29
acceptance limit for symmetry of collimator jaw images
< 1 mm
30
acceptance limit for light-to-rad test
< 2mm
• Check for asymmetric jaws as well. There is more stringent tolerance (<1 mm) for asymmetric jaws since these may be used for field junctions/beam matching in e.g., breast and supraclavicular node treatments. In this case, there are uncertainty contributions from both fields. This is also true for jaw position indicator accuracy tests.
31
gantry axis of rotation acceptance limit
radius < 1 mm
32
describe couch axis of rotation test
Put graph paper on couch. Mark position of cross hairs. Rotate couch and note movement of cross hairs. Radius should be < 1 mm.
33
mechanical isocentre acceptance limit
-collimator, gantry, and couch should intersect in a sphere of radius < 1 mm (0.5 mm for stereo)
34
is radiation isocenter determined for each energy?
yes
35
what is radiation isocenter determined for?
Determined independently for each component of the accelerator that can rotate (couch, gantry, collimator) using star shots or use Winston-Lutz test to test various components simultaneously.
36
describe gantry radiation isocenter test
Can place film in plane of gantry rotation (i.e., sandwiched between two slabs of solid water). Mark location of mechanical iso. Obtain star shot image (use thin rectangular field), avoiding gantry angles 180 apart (to avoid entrance and exit overlap).
• Note that this method does not test isocentricity of the gantry in the direction parallel to the gantry axis of rotation. Performing Winston-Lutz test with gantry and collimator rotation would make such lack of isocentricity apparent but would not allow you to separate out the gantry and collimator contributions.
o To test longitudinal isocentricity, place the film flat on the couch and irradiate a series of thin rectangular fields (with long axis perpendicular to axis of rotation) at different gantry angles. The resulting dose distribution should appear as a stripe, which will be blurred if there is a lack of isocentricity. To determine baseline FWHM that should be expected if there is no blurring, irradiate a film with a single anterior field for comparison.
37
collimator radiation isocenter test
irradiate film in plane perpendicular to CAX or irradiate EPID in star shot pattern. Repeat with field defined by both sets of jaws.
• Note that couch and gantry tests cannot be done using EPID.
38
acceptance test limit for radiation isocenter
2 mm tolerance on radius of region where all axes intersect. ALSO, centre of this region should be within 2 mm of mechanical isocentre. Tolerances are 1 mm for stereotactic machines.
radiusordiameter??
39
increment distances checked for ODI at acceptance testing
5 cm
40
how to check patient treatment table motions at acceptance testing
Vertical motion: put graph paper on couch, mark location of cross hairs, move couch up and down, ensure cross hairs remain at same position.
Similarly, check horizontal motion as well: put graph paper or appropriate jig on couch, align it with cross hairs, move couch and ensure that the cross hairs stay aligned (not drifting away from line on jig/graph paper that they were previously lined up with)
41
table flex?
Check that flex in longitudinal and lateral travel with and without load is within specification
Max load typically 550 lbs
42
• What would you do if there was a patient to be treated who weighed 570 lbs?
-max load typically 550 lbs
-It is the physicist’s job to advise and provide data (e.g., quote the couch limit), but the decision to treat should be made by the head of the RO department in this case. Physicist can call vendor and ask for advice. Could also consider using something to support the couch where it is extended beyond the base. Must consider that if the couch breaks, this could be harmful to the patient being treated, and to other patients whose treatments would be delayed. If the patient is treated, then must carry out couch QA (motion alignment, position and rotation readouts, maximum range, sag under typical load, brakes, travel speed) after treatment.
43
list of dosimetry tests at acceptance testing
-photon and electron beam energy (%dd(10)x and R50)
-photon and electron beam uniformity (symmetry, flatness)
-photon and electron penumbra
-electron beam Brems contamination
-monitor characteristics (linearity, sability, dose rate independence, gantry angle independence, dose rate accuracy, backup counter)
-collimator transmission
-surface dose
-MLC
44
describe beam energy acceptance test
PHOTONS: Typically determine %dd(10)x = value of PDD for 100 cm SSD, 10x10 cm2 FS, 10 cm depth in a water phantom, excluding electron contamination
ELECTRONS: Typically determine R50, the depth past dmax in a water phantom where the dose drops to 50% of its maximum value, at 100 cm SSD, for a 10x10 cm2 field.
NOTE: Purchase contract acceptance test procedure tolerance may specify range of acceptable %dd(10)x values. In practice, can ask the vendor representative to tune the beam to a particular value even though this is not in the purchase contract (and they will likely say yes because they don’t want to lose your business). Adjusting the beam energy in-house is not recommended because the clinic would be liable for any damage to the machine resulting from mistakes made during this process.
45
flatness acceptance test limits
-Dmax and Dmin within central 80% of beam
-< 3% at 10 cm depth and 100 cm SSD for largest field size available
-F= 100 X (Dmax-Dmin)/(Dax+Dmin)
-• Typically specify flatness and symmetry at dmax in addition to at 10 cm.
46
limitation for beam horns
Typical limitation on beam horns at dmax is 5% for a 40x40 cm2 field (at 100 cm SSD).
Looking at horns at shallow depths gives a good indication of beam energy
47
symmetry acceptance test limit
Measure the areas under the left and right sides of the profile (OR consider any two points equidistant from CAX)
S= (area left- area right)/(area left + area right)
< 2 %, usually < 0.5 % is achievable
-• Typically specify flatness and symmetry at dmax in addition to at 10 cm.
48
where do you do flatness/symmetry test for electreons
at some particular depth, probably near Dmax
49
for what planes do you do flatness/symmetry tests
principal planes and diagonals
50
uniformity index
area enclosed by the 90% isodose divided by the area enclosed by the 50% isodose
-can be measured along with flatness, symmetry
51
where are penumbra defined?
-usually get 80-20% penumbra
-10 cm depth for photons, near dmax for electrons
52
describe linearity test
Linearity: use ion chamber in phantom. Readings (collected ionizations vs MU/time requested should produce a straight line).
• If the line does not pass through the origin, then there is an end effect (known as shutter error for some orthovoltage units and for teletherapy units containing e.g., Co-60)
o Positive x-intercept corresponds to less radiation delivered than indicated by the monitor setting.
o In orthovoltage case, it is due to output buildup as generating voltage builds up [in linacs, it is due to non-zero response time of monitor chambers]
53
multiple start-stop method for determining end effect (alternative to plotting ionizations/time)
end effect = ((In-I1)/(nI1-In)*T
T is total MUs
In is ionization after (n-1) interruptions
I1 is ionization after no interruptions
-derive from D1 = Ddot(T+end effect) and Dn= Ddot(T+na*end effect), Ddot is true dose rate, Dn is measured with interruptions and Di without
-negative end effect = less radiation delivered than monitor setting
54
should the linac monitor chamber vary with T and P?
No, this would indicate it is not sealed properly
55
describe arc therapy test
Set number of MU and number of degrees for the desired arc. Deliver it and check that values [according to control console] are within 1 MU and 3 degrees (for arc termination location) of set values. Test for all energies, relevant angle ranges (try variety of start/end points), various total doses.
56
describe collimator transmission test
Must check for both sets of jaws.
Compare measurement with jaws open (at dmax on CAX with the largest field) to measurement with jaws closed (move chamber to shadow of one set of jaws).
-should be as specified per vendor
57
how is surface dose test done?
Extrapolation chamber is the gold standard for this task
• Extrapolation chamber is a parallel plate chamber that has variable air cavity sensitive volume, which allows for varying the depth of measurement
Determine surface dose also when block trays are in place.
-beam contamination must be within limits
58
describe MLC acceptance tests
-actual vs programmed position
-leaf speed
-iner and intra-leaf leakage
-scatter from MLC-contribution to suface dose
-verify performance doesn't change withg gantry angle
-leaves move parrallel and perpendicular to jaws
-penumbra
59
summary of steps for commissioning
-acquire all data
-organize data in database
-enter data into TPS
-verify accuracy of data entry (i.e compare TPS output with measurements)
-develop dosimetry, TP, and treatment prpcedures
-establish QA tests and procedures
-train personnel
-o If beam matching among multiple machines, agreement of beam profiles should be within 1%.
-o Need second independent check of commissioning – independent physicist, compare with published data or golden beam data, external audits
60
can you use scaling of data taken at different SSDs?
-only as sanity check
Various things vary with SSD in a way that cannot be corrected in a simple manner:
Electron contamination, primary dose in small field sizes, scatter dose, head scatter (may obey ISL from source at position of flattening filter), energy spectrum off axis varies with off axis angle due to divergence, penumbra (amount of volume averaging varies).
61
EPOM rules for PDD measurements
To account for Pgr, Remember to shift PDD measured with cylindrical ion chamber 0.6 rcav upstream from centre of chamber (closer to source) for photons and 0.5 rcav upstream for electrons. EPOM for parallel plate chamber is inside surface of entrance window (no shift required). Typically no shift required for diode.
62
photon PDD measurements for commissioning
-various FS, to depths of 40 cm
63
issue with FS <4X4 cm2
(relevant for stereotactic treatments) require special attention due to source occlusion, lack of lateral CPE on CAX, volume averaging across chamber volume.
o Criterion for small field given in IAEA TRS-483: beam half width or radius has to be at least as large as rLCPE plus half the size of the external volume of the detector, where rLCPE is the lateral charged particle equilibrium range, which is defined as the minimum radius of a circular photon field for which collision kerma in water and absorbed dose to water have reached the values determined by broad beam TCPE.
64
describe volume averaging
-when measuring small field with large volume chamber, volume averaging will make dose reading appear too low
65
why not use diodes for large fields?
-PDD fall-off will not be as steep as it should because diode over-responds to low energy photons due to PE enhancement with high Z silicon. There is more low energy phantom scatter at deoth
66
how is dmax affected by FS? (photons)
• Expect dmax decreases with increasing FS above 5x5 cm2 due to increased scatter (from flattening filter and collimator jaws having increasing surface area exposed to radiation). Below 5x5 cm2, dmax increases with increasing A due to establishing lateral equilibrium.
67
how do you measure PDD for soft wedge?
must integrate dose at each depth over the entire jaw sequence.
68
how to measure PDDs for electrons, commissioning
-PP
-scan to depth Rp+ 10 cm (capture Bremss tail)
-o If using diode designed to measure electron beams, it may not respond properly to photons, so ion chamber should be used to measure brems tail.
-• Due to steep fall off of electron beams (especially for low energies), it is extremely critical to establish the correct water surface in order to obtain accurate PDDs.
69
how does dmax vary with FS for electrons?
expect dmax to decrease for fields with dimensions smaller than the range of the electrons due to loss of lateral scatter equilibrium reaching the CAX (scatter from collimation system also changes).
70
where is EPOM for PP chamber
inside surface of entrance window
71
output factors for small fields
For small fields, use output correction factors given in IAEA TRS-483. Can use daisy chaining to renormalize data obtained for small fields using e.g., diode.
72
rule of thumb for small field detectors
: detector dimension parallel to scan direction (or in plane perpendicular to beam incidence for output factor measurements) should be < 1/3 of smallest field dimension.
• Criterion given in TG-106: if profile varies by more than 1% over the detector diameter, then detector is too big.
73
where do you measure output factors for commissioning?
-dmax or 10 cm depth
ie calibration condition
74
how do you approximate output factors for rectangular fields?
square root of product of output factors for the 2 square fields
75
why do we use big water tank for output factor measurements?
water phantom must be large enough to ensure full lateral buildup and full backscatter beyond depth of measurement
76
collimator exchange effect
affects output factors due to two sets of jaws being at different heights
77
Sc is due to ?
air
flattening filter
jaws
monitor chamber
78
how to measure Sc?
• For photons > 4 MV, buildup cap required to achieve full buildup in air becomes impractically large. Solution:
o Use mini-phantom which consists of a solid column of water-equivalent material of cross-section sufficient to achieve lateral scatter equilibrium, and of thickness sufficient to position the detector at a depth where electron contamination becomes negligible and full buildup is achieved [want conditions as close to CPE as possible].
Scatter contribution to the ion chamber is constant for field sizes larger than its cross-section (i.e, Sp remains constant and cancels in ratio, so that Sc can be determined).
79
issues with measuring Sc for small fields?
• For small fields, buildup cap may approach or exceed the FS and phantom scatter becomes an important, unwanted contribution to the measurement. Solutions:
o Use extended SSD approach to ensure FS >> phantom size
In this case, must also collect reference field measurement at extended distance.
o OR use high density buildup cap or mini-phantom
If a high-Z mini-phantom is used, then correction factor to account for resulting change in beam fluence spectra may be required.
o Minimum field size determined by the requirement that there be at least 1 cm flash around mini-phantom
80
Does AAA need Sc?
Nope
81
how to determine Sp?
divide output factor by Sc
82
output factors for electrons
• Typically determined at dmax at the standard SSD (e.g., 100 cm)
• Measure for all available cones (a.k.a. electron applicators). Some manufacturers design cones to reduce the penumbra, while others use the cone to scatter electrons off the side to improve flatness.
83
what about patient specific cutout factors?
have to determine case-by-case cutout factor
-have to scan around for dmax since output factor is defined as ratio at dmax and dmax shifts to shallower depths for smaller FS
84
what should output factor plot resemble?
plot of output factor versus field size should yield a smooth curve with slope that is steep for small fields and relatively flat for large fields
85
pros of MLCs vs custom blocks
• Versatile (can do variety of aperture shapes throughout treatment without having to create and install a new block each time)
• Remote positioning (no heavy lifting for therapists, less danger of injury)
• No need to deal with cerrobend, which exposes therapists to toxic fumes.
• Less time consuming than custom block making. Also quicker setup at the treatment unit.
• No beam contamination resulting from trays that hold blocks.
• Less storage space required
• Dynamic leaf motion provides opportunities for fluence modulation (VMAT)
86
cons of MLCs vs custom blocks
• Discrete leaves means limitations in aperture shape (not a smooth boundary, is jagged, limited resolution)
• Leakage between, through, and at closed leaf ends (DLG) must be quantified and taken into account in TPS. Leakage is especially a concern for IMRT, other high MU per fraction treatments, and with higher energies (definitely want to be using jaw tracking!)
• More complex QA: must QA leaf speed, accuracy, reproducibility. Must also determine intra- and inter-leaf leakage, DLG, output factors for MLC defined fields, penumbra of MLC defined fields (characterization/commissioning)
• With rounded leaf ends and leaf motion perpendicular to CAX (as in Varian truebeam), penumbra due to leaf transmission remains ~constant with leaf position in the field; however, the penumbra is larger than it would be with collimator jaws or custom blocks, especially if divergent block aperture is used.
87
MLC pros compared to circular cones
• Dose distribution inhomogeneity with cones when used to treat irregularly-shaped targets
• Planning with multiple isocentres is time consuming with cones
• Multiple isocentres require patient repositioning shifts for cones
• RTT must manually install the appropriate cone on the gantry treatment head.
88
MLC cons compared to circular cones
• Rounded leaf ends (Varian) produce a more gradual fall-off (i.e., larger penumbral region)
• MLCs are further away from patient surface, resulting in larger geometric penumbra due to finite source size.
• MLCs create a jagged aperture due to discrete leaves
• When used with intensity modulation (IMRT or VMAT), leakage through and in between leaves is a concern, especially with high MU per fraction (use jaw tracking!)
89
do we need MLC data nowadays?
typically it is already modelled in TPS
-Data only required for fields defined by primary jaws
-In this case, MLC acceptance testing is still required, and MLC shaped field measurements are still needed for verification of the models as part of TPS commissioning. Even if MLC if modelled in TPS, still need to specify some MLC parameters: intra-leaf leakage, inter-leaf leakage, DLG and effective target spot size in X and Y directions. These can all be iteratively modified to achieve best agreement with dose verification results (effective target spot size is commonly adjusted to achieve better agreement in penumbra of open fields).
• Find best gamma pass rate across a wide variety of different plans.
• Make sure the EPID or film is properly calibrated before carrying out this procedure (don’t want to mask other errors). E.g., compare 2 dosimetry systems and make sure they agree.
90
what parameters need to be quantified for MLCs
• Light/radiation field coincidence
• Inter- and intra-leaf leakage
• Tongue and groove effects
• Penumbra
• Dosimetric leaf gap for rounded leaf ends abutted.
• Positional accuracy/reproducibility, speed
o This can be investigated with MLC log file evaluation
-many of these can be measured with film, detector array, or EPID
91
values for MLC leaf transmission
-should be < 2 % of isocenter dose
typical values (% of isocenter dose)
-through leaves- 1.5-2.5%
-interleaf- 2- 3,5%
through closed MLC leaf ends - 12-28%
-jaws- < 1 %
92
transmission through cerrobend blocks
3.5-5%
93
how do you determine leakage through and in between MLC leaves?
measure dose distribution with the leaves close (valleys = transmission through leaves; peaks = transmission between leaves)
94
how do you ensure chamber is on CAX?
), take measurements at two collimator angles 180 degrees apart – adjust chamber position until readings are equal within 1%.
95
where is wedge angle measured
between isodose and line perpendicular to CAX
96
for how many FS and depths do you measure wedge angle?
Typically determined for one FS and depth (although may be a function of FS and depth if the wedge affects fluence spectrum – if is a strong function of FS, then measure for a large range of FS).
97
how do you center wedge?
Once chamber is centred, repeat two measurements with wedge (not collimator) rotated through 180 degrees. Reading should differ by <5% for 60 degree wedge and <2 % for a 30 degree wedge. Larger discrepancies may indicate that the side rails are not symmetric about the CAX.
98
wedge factors for soft wedges
Wedge factors for soft wedges are typically 10-30% higher (closer to unity) than the corresponding physical wedges. EDW wedge factors are ~independent of depth because beam quality is ~not changed by these wedges.
99
star profile for commissioning
measure profile at 10 degree intervals
This can be done by tank
• software or by manually rotating the tank. DO NOT rotate collimator since this does not capture flattening filter effects.
100
transverse beam profiles for commissioning
-stars, diagonals, in-line, cross-line
-various depths and FS
-some TPS require measurements in air with build-up cap
101
cross-line and in-line profiles for wedges
In the non-gradient direction, expect “rounding off” due to oblique incidence of the beam resulting in a longer pathlength through the wedge at positions away from the CAX.
102
how to ensure high SNR in small volume chamber
-decrease scan speed or increase integration time
103
varian requires crossline or inline profiles for commissioning?
just one or the other
-assumes symmetric profile so correct for symmetry before acquiring the profile data...
- Can average crossline and inline scans, or you can measure just one or the other, and do a few spot checks to make sure crossline and inline agree (expect this to be the case for modern linacs such as Varian TrueBeam).
For wedges, do need both crossline and inline scans
104
how to measure diagonal profiles
-rotate tank
-"stitch" together scans from opposite quadrants
-mirror a one sides half scan (but have to verify minimal asymmetry)
-average out --/++ and -+/+-
105
recommendations for measuring entrance dose and interface dosimetry for photons
Can use extrapolation chamber. However, availability is limited and surface dose measurement is very time consuming.
Use fixed-separation parallel plate chamber instead. These may over-respond in buildup region/at surface due to their relatively large separation compared with the extrapolation chamber [resulting in volume averaging] and small guard ring [which interferes with ion pair production in sensitive volume? Affects fluenc?]. Hence use one with small plate separation and wide guard ring.
Use thin window parallel plate chamber. Note that use of detector with thin window is challenging in water phantom because these are difficult to waterproof, and hydrostatic pressure may result in deformation of the window.
Typically not needed for model-based TPS such as AAA (although may carry out such a measurement for validation purposes).
Surface dose measurements should always be carried out with both polarities (expect large Ppol) – average the absolute values of both readings (readings may have same sign in case where signal to noise is very low and cable/stem contribution does not change sign with change in polarity).
106
where is virtual source position relevant?
-ortho photons, electrons, and MV photons at extended SSDs (TBI)
-
107
why is there a virtual source?
• Scatter (due to air, electron applicator, etc.) increases the penumbral width, decreases the output (i.e., cGy/MU), and makes it appear (according to ISL) that electrons/photons emanate from a virtual source position ≠ nominal source position.
108
air gaps for electrons, virtual source measurement
For electrons, additional air gap correction factor may be needed for extended SSDs to account for deviations from ISL behavior due to large amount of scatter.
• For SSDs up to 110 cm and energies up to 25 MeV, this is < 2% effect.
• Expect large air gap correction factor when using cones designed to improve field flatness (as opposed to cone that is only designed to reduce the penumbra).
• With increasing air gap, penumbra increases ~ in proportion to size of air gap.
• A large number of clinical situations involving electron beams require extended SSD. Therefore, measurements at extended SSD are an important part of the TPS commissioning process.
109
how to measure virtual source position?
-measure with ion chamber at various distances from nominal source position
- Plot reciprocal of the square root of the normalized ionization vs nominal distance. Get virtual source position based on assumption that ISL holds.
• Alternate method: I0/I = (SSDeff + dmax + d)2 / (SSDeff + dmax)2… SSDeff = 1/m – dmax where m is the slope.
110
TBI virtual source determination
For treatments at extended distances, need to measure data at the extended distance since inverse square relationship which is valid in the vicinity of isocenter may not accurately predict the decrease in beam intensity at distances of 3-4 m. Various things (e.g., head scatter, electron contamination) vary with SSD in a way that cannot be dealt with using simple correction factor.
If phantom is not large enough to intercept the full beam then must use factors to correct for loss of scatter (if TPS expects data measured in phantom large enough to intercept full beam)
111
list of measurements we need for wedges
o For both hard and soft wedges, need profiles, PDDs and wedge factors. From the profiles, must verify the wedge angle at the appropriate depth or isodose curve.
112
wedge angles checked for dynamic and universal wedges?
a variety
-also check measurements at various depths
113
why use film or 2D detector for dynamic wedge measurements?
each data pt has to be integrated over whole jaw movement
114
commissioning inhomogeneity corrections?
-have to validate correction factors prior to clinical use (compare with measurements in non-homogeneous phantom)
115
what is included in commissioning report
-proper signature and date
-outline, scope, what was measured, how, any data handling
-PDD and TMR tables for open and wedged x-ray fields, PDD only for electrons
-x-ray Sc, Sp
-wedge factors
-electron cone factors and respective effective source distances
-transmission factor tables
-off-axis tables at selected depths, large FS, open fields, wedges
-isodose curves for references fields from PDD and profiles
-printout of all data
-Validation: Compare data from similar machines within your own department or from different institutions. Comparison to vendor supplied golden data is also acceptable but do not blindly use this data. [can also compare to published data, have independent physicist do spot check, and/or do external audit (especially for commissioning a technique that is new for the clinic]
-Vendor provided data could be used as a reference but it should never be used as a substitute for the commissioned data.
-Backup entire electronic data, analyzed data and spread sheets.
-Write the report with detailed description of how the beam data were collected and conditions of the beam data collection.
116
lasers are aligned to which isocenter?
mechanical
117
tolerance recommended for TG142
• TG-142 recommends that the measurement system and procedure repeatability be such that 2 SD for ≥ 3 repeated consecutive measurements are less than the tolerance value.
118
what makes the choice for daily vs monthly tests?
daily: o Want to include tests of dosimetric and geometric parameters that can affect dose to the patient and which have the possibility of changing based on collisions, upgrades or afterhours work (research, service, etc.)
o In contrast, monthly tests include parameters that have a lower likelihood of changing over a month
119
what to do where CPQR and TG142 differ?
use more stringent
120
can centres adopt different test frequencies than stated in CPQR/TG142?
cpqr says yes, but have to document supporting evidence
-also, may need to amend CNSC license to change test frequency
121
what happens if safety test fails QA?
use of radiation equipment must be suspended. Use of equivalent measures to mitigate risk must be approved by the CNSC.
122
why are there no door interlocks on CTSim room?
• No door interlocks for CT sim because dose to someone in the room but not in the gantry would be low and this would lead to repeat scans if interruptions occurred. Repeat scans are especially problematic with contrast injection.
123
overall aim of QA program
• Quality control program aims to assure that operation standards considered acceptable at time of purchase continue to be maintained. QC tests are periodic repetitions, partial or full, of acceptance and commissioning tests.
124
how must equipment with restrictions (ex cannot rotate 90 degrees because inadequate shielding) be dealt with?
-hardware locks of software admin settings
125
how long to maintain QA records
CNSC- 3 years
CPQR- 10 years
126
how often should QA processes be reviewed?
annually
-ensure redundancy and adequate monitoring
127
field standard
• Non-reference dosimetry refers to detectors used to ensure stability of a device on a routine basis. They can be used to get absolute dose following a cross-calibration process with a local/secondary standard – then they are referred to as “field standards”.
128
local/secondary standard
• A local/secondary standard is a chamber/electrometer combination that has a calibration coefficient in terms of absorbed dose directly traceable to a primary standards laboratory.
129
record and verify system
software that organizes and stores all data fed into and captured from the linac during patient treatment. RV system acts as a conduit between TPS and linac.
130
Winston Lutz test
-coincidence of mechanical and radiation isocenters
o Ball bearing is placed at mechanical isocentre as defined by the lasers ( lasers should coincide with the mechanical isocentre).
o Ball bearing is irradiated and the resulting image is captured (with film or using the EPID).
o Images are obtained with the gantry, couch, and possibly also the collimator changing to different positions in between images (only one parameter changes at a time so that the different error contributions can be isolated
o Apparent movement of the ball with respect to the field aperture from one image to the next indicates misalignment of mechanical and radiation isocentres which can arise due to e.g., gantry flex, couch walkout.
o W-L is not a test of the laser or imaging systems (i.e., image may move relative to EPID panel; this may be due to panel sag; doesn’t tell you anything about mechanical/radiation isocentres of the linac).
131
alternative method to verify radiation isoceter
check that the charge per MU measured by an ion chamber (located at mechanical isocentre as defined by the lasers) is independent of gantry, couch and collimator positions – variation in chamber reading indicates a shift in chamber position (mechanical isocentre) relative to the field (radiation isocentre) – use a small field size for this so that chamber reading is highly sensitive to small shifts.
o This test should be done with the collimator used for the procedure when done before single fraction SRS.
132
kV xray daily QA (CPQR)
kVp and mA indicators, beam off with key, beam interrupt,
-backup timer/MU check-1%/2%
-output-2%/4%
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kV xray weekly QA (CPQR)
Unit motions and motion stops. This is especially important for ceiling mounted units (risk of collapsing on patient).
Couch movements and brakes
Interlocks for filter-kVp choice.
Output-2%/4%
134
kV xray monthly QA (CPQR)
Mechanical stability and safety
Cone & filter integrity, cone indicators [correct cone showing up on console once cone is installed I think]
Physical distance indicators-2mm/3mm
Accuracy of head tilt/rotation readouts-1/1.5 degrees
Light/x-ray field coincidence-2mm/3mm
Light field size-2mm/3mm
X-ray field size indicator-2mm/3mm
Uniformity (can use film)-5%/8%
Output with head tilt/rotation-2%/4%
Timer accuracy-1%/2%
Output vs. dose rate-2%/4%
135
kV xray annual QA (CPQR)
Reference (not relative) dosimetry-1%/2%
Timer and end-effect error-0.05 min
Output linearity-1%
-output reproducibility-2%/3%
Output error associated with beam interrupt-2%/4%
Beam quality (HVL)- 10%/15%
Alignment of focal spots-0.5mm/1mm
Focal spot size- reproducible
• Using resolution tools: can determine by measuring line pair pattern; smallest line pair discernable can be related to spot size. Can also measure star pattern; diameter of zero contrast region can be related to spot size.
• Using pin hole: convention is to place pin hole halfway between source and imaging plane so that magnification factor = 2.
PDD and profiles verification
136
brachy per treatment QA (CPQR)
o Plan data transfer from planning to treatment console (check source strength, dwell positions, dwell times [or subset of these])
o Plan dwell times adjustment at the treatment console [for decayed source]
o Minimum dwell times – positioning reproducibility may be dependent on dwell time
o Catheters planned connection vs. actual connections to remote afterloader
o Complete source retraction – survey room and patient after treatment
o [also check patient name, ID, treatment site, Rx]
137
brachy daily QA (CPQR)
o Treatment interrupt
o Treatment status indicator
o Key switch
o Date, time and source strength
o Source/dummy positional accuracy - 2mm
o Dwell time accuracy -2%
138
brachy quaterly QA (CPQR)
(at source replacement)
o Mechanical integrity of applicators, tubes, connectors
o Internal backup battery test – source retraction, record of treatment delivered so that can resume when power returns
o Source/dummy interlocks – obstruction, incorrect connection
o Dummy positional accuracy – for proper obstruction detection - 3 mm
o Source positional accuracy – can use autoradiograph or fluoro - 1 mm
o Source strength calibration – should agree with manufacturer supplied value within 5%. Use re-entrant (well-type) chamber calibrated every 2 years. Second check required.
o Source homogeneity – relevant for sources containing multiple pellets - baseline
Radioactive material should be evenly distributed in the encapsulated source.
139
brachy annual QA (CPQR)
o Hand crank operation (for emergency source retraction) – use dummy source to test. All physicists should practice this annually.
o Leakage radiation- baseline
o Multi-channel indexer function – check that wire is sent to proper channel
o Dwell time accuracy – use some more rigorous method than is used for daily test (e.g., film it and look at individual frames) - 1%
o Timer linearity - 1%
o Transit time/dose reproducibility – will depend on applicator shape (i.e., depends on path travelled)- baseline
o Length of applicators and guide (transfer) tubes - 1mm
o Applicators and template dimensions – should match dimensions used in treatment planning process- baseline
o Shield integrity of shielded applicators – visual and radiographic inspections- baseline
o X-ray marker positional accuracy- 1mm
o Review emergency response procedures
o Verify PDR pulse sequencing functionality according to manufacturer’s recommendations.
140
differences between CTSim and diagnostic CT
-patient position and immobilization
-may scan more of body for treatment planning purposes compared to DI
-localization on patient skin
-need flat tabletop like Tx machine
141
daily CTSim QA tests (CPQR)
o Laser alignment, spacing and motion: 2 mm
In particular, check wall laser position wrt imaging plane since this is used to set patient-based reference points for localization.
Tolerance should match Tx room lasers
o CT number for water
Mean (accuracy): 0 +/- 4 HU
SD (noise): 10% or 0.2 HU from baseline [whichever is larger]
Mean vs. position (uniformity): 2 HU
o Functionality of respiratory motion monitoring system
o Functionality of A/V coaching software
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monthly CTSim QA tests (CPQR)
o Couch tabletop is level and orthogonal with the imaging plane. Performed radiographically (a level will provide readings relative to a horizontal reference, which we don’t want): 2 mm
Image jig at head and foot of couch
o Laser orthogonality and orientation wrt imaging plane: 2 mm
Line up jig to lasers, then image jig…
o Couch displacement: 1 mm
Check digital indicators compared to long ruler. Perform test with typical patient load of 80 kg.
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quaterly CTSim QA tests (CPQR)
Amplitude and periodicity of surrogate with monitoring software and/or CT console: 1 mm, 0.1 s
• Use programmable respiratory motion phantom (e.g., QUASAR)
• Check various amplitudes and periods. Use option for motion in all 3 dimensions if available.
4DCT reconstruction: functional (appropriate # bins with appropriate # slices in each bin)
Amplitude of moving target in 4DCT dataset: 2 mm
Spatial integrity (measure target diameter in directions parallel and perpendicular to motion) and positioning of moving target at each phase: 2 mm (or more for amplitudes > 2 cm)
Mean CT number and SD of moving target at each phase: 10 HU and 10%
4DCT intensity projection image reconstruction (AIP, MIP, MinIP): 2 mm (or more for amplitudes > 2 cm)
• Measure diameter and expected CT number variation in the direction of motion
4D data import to TPS: functional
-CT number for other material- reproducible
-low contrast res- reproducible
-spatial resolution at 10 and 50% MTF: +/-0.5 lp/cm or +/-15% of baseline
-slice thickness (sensitivity profile): (±1 mm from baseline for
slices ≥2 mm
±50 % from baseline for slices
of 1 to 2 mm
±0.5 mm from baseline for
slices <1 mm)
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annual CTSim QA tests (CPQR)
o Radiation dose: 20% (also do after tube replacement)
TG-66: 20% tolerance on CTDI
o X-ray generation: kVp, mAs linearity & reproducibility, time accuracy, HVL: 2 kVp, 10%
o Gantry tilt: 0.5 degrees
Ideally, a CT dedicated to RO should not allow scans with gantry tilted (this feature is really only useful for diagnostic CT).
o 4D low contrast resolution, high contrast spatial resolution, slice thickness at each phase
Use CT image QA phantom that can be motion driven
o Simulated planning: 2 mm
See description of CT sim process test from TG-66: scan phantom, check orientation, check areas/volumes, import to TPS, check orientation, plan beams, create DRRs, check CT numbers, set up with DRRs on Tx unit, verify positioning.
145
catphan test for geometric distortion
making sure that the distance between four markers in the shape of a square remains constant
146
catphan slice thickness
• Slice thickness is determined by a wire at 23 degrees, running perpendicular to the slices. Depending on how long the wire appears in a particular slice determines the slice thickness (using simple trigonometry).
147
what is spatial integrity
accurate reproduction of true patient dimensions and shape
148
overall CT Sim commissioning steps
o Scan phantom with fiducial marker
o Check orientation of phantom is correct on CT sim workstation
o Check areas and volumes of phantom are correct
o Set iso to fiducial marker
o Move couch to iso
o Mark phantom (i.e., like tattooing patient)
However, note that nowadays we would not shift at tattoo at iso since iso and user origin do not need to coincide (we do couch shifts from user origin as determined by tattoos to get to iso).
o Send data to TPS.
o Check orientation of phantom on TPS.
o Check CT number accuracy
o Set up beams and generate DRRs and setup documentation
o Go to treatment unit to set up and verify phantom positioning.
• The CT number to relative electron density conversion relationship should be determined during initial scanner commissioning (and verified at least annually).
149
TG report for acceptance testing of CT scanners
TG-39
150
items included in CTSim acceptance testing
-radiation shielding survey
-performance of electromechanical components
-image quality
-radiation dose (CTDI)
o Ensure that tabletop does not contain any object that can produce clinically significant image artefacts (e.g., screws)
• Note: CT number to RED calibration curve determined at commissioning.
o Emergency off switches: these can damage CT scanner so should be tested under conditions that will not harm the scanner
151
electromechanical CT acceptances tests
-laser accuracy wrt scan plane
-alignment of table longitudinal axis to gantry rotational axis
-accuracy and reproducibility of longitudinal table motion
-table position and gantry tilt indication accuracy
-slice localization from scout
-collimation (radiation and sensitivity profile widths)
-kVp, mAs, HVL
152
image quality CT acceptance tests
-noise
-artefacts: geometric, aliasing, edge gradient streaks, geometric misalignment, motion artefacts
-reconstruction algorithm effects (blurring)
-attenuation measurement errors
-resolution
153
aliasing artifact CT
when highest frequencies in the scanned objects are under-sampled due to using excessively large intervals between projections or under-sampling within a projection. The former results in faint streaks radiating from sharp edges (such as bone-soft tissue interfaces). The latter results in the windmill artefact (multiple radially oriented light and dark bands)
154
edge gradient streak CT
Beam hardening due to high attenuation object which is most prominent in direction of long edge of high-density object (because there is more attenuation in this direction)
155
ring artifact CT
detector drifting or not working
156
non-linear attenuation errors in CT
detector saturation, beam hardening (higher energy reduced attenuation object seems less attenuating than it actually is), scatter effects (making beam hardening more apparent), cupping artefact (which leads to non-uniformities)
157
annual CVSim QA (CPQR)
o Coincidence of mechanical (gantry, collimator, couch), optical and radiation isocenters. Also isocenter definition- 1mm/2mm
o Crosswire centring at various SAD- 1mm/2mm
o Couch deflection with 70 kg-3mm/5mm
o Focal spot alignment-0.5mm/1mm
o kVp over range of 60-120 kVp-5%/10%
When measured non-invasively (e.g., using Raysafe detector with automatic kVp readout), tolerances should refer to baseline values obtained as acceptance and referenced to invasive measurements (using multimeter with probe touching relevant tube component).
o Reference dosimetry with and without AEC (automatic exposure control)-5%/10%
o Beam quality (HVL)-5%/10%
o AEC – make sure various detectors used for AEC are consistent.-5%/10%
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semi-annual CVSim QA (CPQR)
o Lead apron – visual and radiological inspection
o Focal spot -reproducible
o Contrast and resolution- reproducible
o Fluoro timer-5%/10%
159
monthly CVSim QA (CPQR)
o Gantry, collimator angle readouts - 1 degree
o Orthogonality of field wires; perpendicularity of field wires wrt cross wires- 1 degree
o Verification of automatic setting of focus-axis-distance- 1mm/2mm
o Image amplifier movement
o Couch isocentre (walkout), parallelism to simulator geometry, angle readout, position readouts, relative displacement- 1mm/2mm
o Laser/crosswire isocoentricity wrt radiation isocentre at various gantry angles-1mm/2mm
o ODI – check over clinically relevant range of SSD and gantry angle-1mm/2mm
o Crosswire centring (walkout) – compare optical and radiological images wrt radiation isocentre for a range of collimator angles-1mm/2mm
o Light/radiation coincidence – set up field according to jig with radio-opaque markers. Take image and compare images of field defining wires with images of jig markers. Various gantry angles and FS including half beam block; and also agreement with electronically indicated field size- 1mm/2mm
-field size indicators- 1mm/2mm
-couch angle- 1 degree
-couch position readouts- 1mm/2mm
-couch displacement-1mm/2mm
160
daily CVSim QA (CPQR)
o Collision avoidance system
o Agreement of lasers and crosswires - 1mm/2mm
o ODI accuracy - 1mm/2mm
o Coincidence of crosswires, graticule/reticle and block tray axes- 1mm/2mm
o Light/radiation coincidence of field defining wires; and also agreement with electronically indicated field size. For one FS and gantry angle for daily test.-1mm/2mm
-optical and xray field size indicators- 1mm/2mm
161
typical dose diagnostic fan beam CT
0.1-0.1 cGy
162
typical dose kV CBCT
0.1- 3.5 cGy
163
typical dose MV-CBV=CT
3-10 cGy
164
CT-on rails
diagnostic CT scanner integrated into RT treatment room
-can be used for adaptive planning
-same imager used for planning-good for image registration
165
kV CBCT compared to fan beam kV CT
o Image quality is not as good as diagnostic kV FBCT due to motion blur (due to long acquisition time), scattered radiation due to the volumetric image acquisition and image artefacts (e.g., beam hardening due to large amount of scatter) – resulting in degraded image contrast, noise and uniformity.
o CT number not accurate due to sensitivity of scatter-to-primary x-ray fluence to object and/or field size (CT numbers are very phantom/patient/FS dependent)
166
MV-FBCT
available with helical Tomotherapy based IMRT. Nominal electron beam energy reduced to 3.5 MeV.
o Results in fewer scatter artefacts compared to kV, fewer beam hardening artifacts compared to CBCT and eliminates most of the artefacts cause by high-Z
o However, there is poorer subject contrast
o MV-FBCT typically noisier than kV CT scans.
o Can image soft tissue, bony anatomy or implanted markers
o Can be used for dose calculations unlike kV-CBCT
May be used instead of kV-FBCT for e.g., planning pelvic treatment of patient with hip prosthesis.
o Like CT-on-rails, MV-FBCT offers high CT gantry rigidity/reproducibility (unlike CBCT)
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MV-CBCT
Uses a-Si EPID flat panel attached to linac
o Imaging dose can be accounted for in treatment planning
o The image beam and the treatment beam are the same so this simplifies QA (in terms of imaging isocentre = treatment isocentre always; inherent robustness)
o Can be used for dose calculations unlike kV-CBCT
Although this technique will have more scatter than a FBCT technique, it has less scatter (?) and reduced energy dependence of photon interactions on the phantom medium compared to kV-CBCT. This latter point results in a cupping artefact that can be dealt with using simple nonpatient specific correction methods. Therefore, MV-CBCT is suitable for dose calculations.
o 2 mm localization accuracy (1 mm for other modalities)
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varian isocal phantom
to determine agreement between imaging and treatment isocentres for on-board CBCT (kV and MV).
o Use phantom with implanted BBs throughout and collimator plate
o Images are obtained at various collimator and gantry positions
o Software uses images to find Tx iso and its relation to imaging iso based on location of collimator plate and known phantom geometry (and comparing this to imaged phantom geometry)
o Get correction file (“flexmap”) as a function of gantry angle to adjust imaging panel to correct for sag/flex (this is how it is done with Truebeam)
169
cupping artefact CT
centre of image of uniform object appears darker than at the periphery due to beam hardening through more material in the center
170
CT pitch
L/NT
L in the distance travelled by the couch per helical rotation or between consecutive axial scans; N is the number of simultaneously acquired tomographic sections (i.e., # detectors); T is the nominal slice thickness. Pitch > 1 means that the patient is undersampled; pitch < 1 means that the patient is oversampled (redundant information is acquired).
171
What does TPS dose calculation accuracy depend on?
1) the calculation algorithm (photon and electron transport and energy deposition handled properly under all clinical conditions encountered) and (2) modelling of the actual clinical radiation beams for a particular treatment unit (the commissioning process).
172
TPS quaterly test (CPQR)
o CPU/server
o Digitizer – use on screen ruler to check accuracy of known contour. Digitizer is used to acquire manual contours drawn on simulator films. Typically consists of a backlit tablet with stylus. May be used to create custom block using pantograph to create Styrofoam mould, which is used to cast Cerrobend- 2/3mm
o Electronic plan transfer – check plan data transferred from planning to treatment console
o Plan details – check hard or digital treatment plan printouts.
o Plotter/printer – test by comparing against known contour. Plotter/printer is the output device for text and contour graphics obtained via digitizer- 2/3mm
o Backup recovery
o CT geometry/density- 2mm/0.02 and 3mm/0.03
173
TPS annual tests (CPQR)
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 beam data used for commissioning (PDDs, profiles)
End-to-end test performed as realistically as possible (anthropomorphic phantom; use immobilization devices)
-independent QC review by 2nd physicist
174
picket fence band intervals
2 mm bands at 2 cm intervals
• TG-142 recommends carrying out this test weekly. However, patient-specific QA (dose verifications) are a surrogate for MLC QA (if MLCs were not working properly, then dose verifications would fail)
175
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 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.
176
how do the algorithms model the shape of leaf edges
• 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.
177
how is the MLC penumbra modelled?
• Different manufacturers use different strategies to model penumbra. Note that Varian uses rounded leaf ends that travel in a straight trajectory while Elekta uses square leaf ends that travel in a curved trajectory.
178
how to calculate DLG?
o Sweep a leaf gap across the field, considering a range of different gap values. Record resulting ion chamber charge readings.
o Subtract leaf transmission value from these readings (scale according to amount of field that actually has leaves over it).
o Plot corrected charge readings as a function of gap width.
o DLG is the absolute value of the x-intercept of this linear plot.
179
klystron vs magnetrons
• Klystrons are more stable than magnetrons, and can achieve higher powers.
A klystron is a microwave amplifier used in higher energy (>12MeV) linear accelerators. Because the klystron is an amplifier rather than a generator, low powered microwaves must be provided to the klystron for amplification by a microwave oscillator.
Magnetrons are microwave generators which operate as high power oscillators driven by electrons traveling in interacting magnetic and electric fields.
180
leave water tank filled overnight?
No, in case of leaks
181
water profile scan is jagged/noisy-what can you do?
consider the following: gain of electrometer, PDD scan direction, scan speed (too fast = ripples and/or noise), measurement integration time, repetition rate may be too low, reference chamber position, scan mode (consider step-by-step instead of continuous)
• This will be especially problematic for low energy electron beams at deeper depths.
182
if you only have to measure one of in-plane or cross-plane, which do you choose?
choose the one that results in the chamber moving along the stationary scanning arm; don’t choose the one where the entire arm must move.
183
regions where you expect discrepancies from TPS
-build-up region
-penumbra
184
why is verification of IMRT more complicated than 3DCRT?
have to integrate dose because of temporal nature
185
acceptance testing new ion chambers- list of tests
-linearity with absorbed dose
-response to different dose rates
-beam quality dependence (how does charge collected change for different energies, when the irradiation is carried out such that dose to water is the same).
-orientation dependence
-o Communication with atmospheric conditions (reading should change as expected when temperature and pressure change, according to ideal gas law)
-o Polarity effect
o Charge collection efficiency (ion recombination correction)
o Radiation equilibration (short term stabilization after pre-irradiation)
o Stem/extracameral effect (irradiate at orientations that include and exclude the stem; if the guard electrodes are working properly, then the readings should only differ by a small amount)
o Leakage current (measured with beam off, but every component plugged in, with voltage applied)
Can separate out contributions from the 3 components by unplugging things: look at reading with all 3 things connected, then unplug chamber and look at reading, then unplug cable (just having electrometer on its own) and look at reading.
186
what do you do when you send out an ion chamber for calibration?
• Should perform at least two independent checks before sending a chamber/electrometer out for calibration and repeat the same checks when the chamber is returned to ensure that the equipment has not been damaged during transit.
187
max acceptable change in calibration coefficient when a chamber is sent out
< 0.3%
Calibration is performed when the chamber is first purchased, when repaired, when the redundant checks suggest a need, or once every 2 years.
Calibration of reference chamber (local/secondary standard) every 2 years [c.f. CNSC requiring yearly calibration of survey meters]. HOWEVER, cross-calibration of field standard every year.
188
excellent, average, and bad electrometer drifts
10-15 A, 10-14 A and 10-13 A
189
why use gamma test for verifications?
o Dose difference is overly sensitive in regions of high dose gradient (the doses differ by a considerable amount, but this may really correspond to a relatively small positional shift in the dose distribution).
o Distance to agreement is overly sensitive in regions of low dose gradient (the isodose contours are separated by a considerable distance, but this may really correspond to a relatively small dose difference)
gamma test combines the 2
-highly modulated fields are more likely to pass verification
190
what does selection, installation, and clinical use of modern radiation equipment involve?
-develop specs of equipment
-design and construct facilities to accomodate equipment, including shielding
-install equipment
-acceptance testing
-commission
-train staff
-develop QA program
191
what does physicist check during equipment installation?
-install radiation warning signs
-ensure CCTV installed
-ensure door interlocks in place
-provide appropriate training
etc
192
what does physicist check at first delivery of beam?
-test door interlock
-determine radiation levels for occupied spaces are safe (so that work can continue)
--preliminary calibration of machine output
193
what does physicist check for initial checking of mechanical and radiation systems
-alignment of collimator axis and collimantor jaws
-collimator axis, light localizer axis, cross hairs
-light to rad and readout accuracy
-mechanical isocenter location
-radiation isocenter location
-patient support system
-anticollision system
-beam modifier system (wedges, applicators)
-beam stopper
194
what are console system tests
-mode selection- should not be able to select dangerous combinations of current, target etc
-software validation (test operation and safety specs, backups)
-readouts (must be accurate)
-record and verify systems - ensure recorded info agrees with all actual values
195
tests involved in checking of radiation systems and beam parameters
-beam output (calibration, stability, timer)
-monitor characteristics (linearity and end effect, dose rate accuracy, dose rate dependence, constancy with gantry position, make sure monitor is not tracking P and T, collection efficiency, backup counter)
-flatness, symmetry, OARs
-penumbra
-dmax and %depth
-collimator transmission
-x-ray contamination of electron beams
-jaws move, wedges, transmission through jaws, wedges, beam stopper
-isodose curves
-surface dose
196
special commissioning for TBI
large fields at extended SSD- commission for this
may also need larger phantom to intercept full beam (if not, have to apply factors that correct for the loss of scatter)
197
special commissioning for TSEI
-increased beam currents are often necessary to maintain the desired dose at the extended sources
-have to evaluate dosimetry system: verify linearity, collection efficiency
-have to consider oblique entry of beams and effect on PDD and Dmax
198
overall definition of quality assurance
-planned actions to ensure that the radiation oncology services achieve the quality of care
199
how often should a quality audit be performed?
at least annually
examples of external audits: mailed TLD services, Novalis
200
QA of simulators includes what?
-same as linacs
-also image quality for simulator
201
how can we ensure dose calibration equipment is working well?
-use a check source
202
related QA for positioning and immobilization
-port films
-laser alignment
203
related QA for patient data acquisition
-image quality for MRI, CT, simulator
-accuracy of mechanical contouring
204
QA for data transfer to TPS
QA of entire data transfer process
205
QA for target volume definition
peer review
206
QA related to computation of dose distribution
machine data from commissioning and QA of treatment machines
accuracy and QA of treatment planning system
207
QA related to plan evaluation
-independent check by physicist
-peer review of plan
208
QA related to computation of MUs
independent check (rad calc, verification)
209
QA related to plan implementation
-review of set-up by treatment planning team
-can use record and verify system to ensure parameters are similar each day- but if off on 1st day, all treatments would be off-make sure to take special care 1st day
210
patient QA
-weekly chart review
-in vivo dosimetry
-status check
211
realistic uncertainty level for brachy delivery
15% vs 5 % for linacs
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brachy source: direct traceability
-source was calibrated at NIST or accredited lab
213
brachy source: secondary traceability
-source is calibrated in comparison with source of same design and comparable strength which has direct traceability
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brachy source: remote traceability
-source calibrated by manufacturer. May or may not be traceable to a national standard
215
brachy source- institutional calibration should agree with manufacturer calibration within what?
-if disagreement more than 3 %, investigate
if > 5 %, report
216
what does TG40 recomment for redundancy of brachy calibrators?
at least a 2 component redundant system
A two-component redundant system consists of a calibrator and one long half-life source or a calibrator and the
manufacturer’s source specification
Three components could include a check source, for example
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what is portal image
A portal image is obtained using a relatively sensitive x-ray film exposed to only a small fraction of the daily
treatment dose
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kV CBCT daily tests (CPQR)
-collision and safety inerlocks
-laser/image/treatment iso coincidence OR repositioning with couch shift- 2 mm
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kV CBCT monthly tests (CPQR)
-kV/MV/laser alignment- 1 mm
-E2E with couch shift accuracy - 1 mm
-image quality is reproducible (spatial integrity, uniformity etc)
-high contrast resolution = 2 mm or 5 lp/cm
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kV CBCT annual tests (CPQR)
-radiation dose reproducible
-xray generator performance reproducible
-orientation reproducible
-complete independent QC review
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linac daily tests, CPQR
-door interlock/LPO
-couch brakes
-beam status indicators
-CCTV and intercom
-beam interrupt
-lasers/crosswires- 1mm/2mm
ODI- 1mm/2mm
-Jaws/MLC field definition- 1mm/2mm
-photon and electron output constancy-2%/3%
-wedge factors-2%/3%
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linac monthly tests, CPQR
-emergency off
-wedge/tray/cone interlocks
-gantry and collimator angle readouts- 0.5/1 degree
-cross hair centering/collimator rotation isocentre - 1mm/2mm
-couch position readouts- 1mm/2mm
-couch rotation isocenter-1mm/2mm
-couch isocentric angle- 0.5/1 degree
-ODI- 1mm/2mm
-relative dosimetry-2%/3%
-central axis depth dose reproducibility- 1%/2mm and 2%/3mm
-beam profile constancy -2%/3%
-light to rad- 1mm/2mm
-jaw position accuracy-1mm/2mm
-MLC leaf position accuracy-1mm/2mm
-picket fence test-0.5mm/1mm
-MLC fluence delivery- 95% and =3%/3mm, and 95% and =5%/3mm
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annual linac tests, CPQR
-profile reproducibility- 2%/3%
-depth dose reproducibility- 1%/2%
-reference dosimetry-1%/2%
-relative output factor reproducibility-1%/2%
-wedge transmission factor reproducibility- 1%/2%
-accessory transmission factor reproducibility-1%/2%
-profile and output reproducibility vs gantry angle-1%/2%
-monitor chamber linearity- 1%/1MU and 2%/2MU
-end monitor effect- 0.5MU/1MU
-collimator rotation iso- 1mm/2mm
-gantry rotation iso-1mm/2mm
-couch rotation iso-1mm/2mm
-coincidence of radiation and mechanical isos- 1mm/2mm
-coincidence of axes of rotation- 1mm/2mm
-couch deflection-3mm/5mm
-leaf transmission-1%/2%
-leakage between leaves-2%/3%
-transmission through abutting leaves-2%/3%
-MLC leaf alignment with jaws- 0.5/1 degree
-dosimetric leaf gap - 0.2/0.3 mm
-isocenternumbeersabovearediameter
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CPQR ion chamber QA for initial use and following calibration
-same if chamber calibrated with electrometer
-stem effect (0.5/1%)
-ion collection efficiency
-polarity
-linearity (0.5/1%)
-leakage (0.1/0.2%)
-collection potential reproducibility (1/2%)
225
CPQR ion chamber QA at each use
signal reproducibility (0.2/0.5%)
226
CPQR ion chamber QA for chambers used for relative dosimetry
initial use:
-linearity (0.5/1%)
-stem effect (0.5/1%)
-annual-signal reproducibility- 0.5/1%
227
diode initial use QA, CPQR
-linearity
-energy dependence
228
initial use QA for MOSFETs and OSLDs, CPQR
-energy dependence (MOSFET)
-linearity (OSLD)
-absolute dose calibration
229
QA for thermometers, barometers
-reference ones are calibrated every 2 years
-others are cross-calibrated
230
automated beam scanning devices QA, CPQR
Initial: alighment, hysteresis, orthogonality
-stem effect - 0.5/1%
-linearity- 0.5/1%
-leakage- 0.5/1%
-scan speed insensitivity
-scan mode insensitivity (continuous vs step-by-step)
-agreement with static measurements (1/2%)
annual: positional accuracy- 1mm/2mm
-reproducibility of collection potential- 0.5/1%
231
diode and ion chamber array QA, CPQR
Initial use:
-positional accuracy, DTA - 1mm/2mm
-signal reproducibility
-linearity
-agreement with static measurements - 1/2%
-energy dependence
Annual or biennial:
-relative array calibration
232
diode and ion chamber array for treatment delivery QA, CPQR
Initial use:
-signal reproducibility
-linearity (dose and dose rate)
-agreement with static measurements (%/DTA)- 1%/1mm and 2%/2mm
-orientation of measured dose vs TPS dose map
]-energy dependence
annual or biennial
-agreement of device measurement with TPS- 95% of detectors have 3%/3mm
-relative array calibration
-absolute cross-calibration (1/2%)
233
why do we add laundry detergent to the water tank?
reduce surface tension
234
main issue with TLD
some energy dependence but nowhere near as much as with diodes (LiF vs Si)
supra-lnearity
-sometimes do output factors with micro TLDs
-cross-validation of pt dose in small field
235
detector must be < 1/3 of FS rule is for what condition?
small FS only
236
why not just use micro ion chamber for small and large fields?
-stem effect and leakage are more significant for micro vs standard ion chamber
237
advantage of miucro chamber
more accurately capture penumbra
238
what happens to photon and electron profiles if water tank is shifted?
photon- center is shifted
electron- profile becomes asymmetric
239
why does scanning direction matter for cylindrical chamber?
with A12, would get volume averaging (less resolution) if the chamber was pointing in and out of oage instead of right for a profile going in and out of page
-since CC13 is smaller volume, this effect is less significant, so with CC13 chamber can face either way
-also, stem effect is consistent in field one way whereas inconsistent the other way
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steps for commissioning a detector like octavius
-E2E with a plan with known distribution
-angular dependence
-dose rate dependence
-TG199 plans to test
-compare to former device and backup
-communicate with the software
-export results
-documentation and training for users
-QA- and how often
-physical integrity
-energy dependence
-calibrate- and how often
-set default settings
-check that the software functions work as intended
241
steps for commissioning lasers
-calibrate mechanically with a phantom
-coincident, parallel
-as you move away from isocenter, continue being parallel and straight
-don’t flicker
-thin- high resolution
-laser WL like on CTSim
-QA program –
-bright enough
242
steps for commissioning water tank
-accuracy of motor readout
-moves orthogonally in 3 directions
-hysteresis
-communication with software
-PDI to PDD conversion, EPOM correction, centering, smoothing functions
-levelling function
-mechanical integrity
-sufficient size
-consistent scanning speed across entire thing
-check motor limitors work
-direction of tank is consistent with TPS
243
steps for commissioning CTSim
-table orthogonal with imaging plane
-lasers with plane and table
-table flex
-table motion
-mAs, kV, CTDI as expected, linearity
-RED curve
-uniformitu, contrast, resolution, water HU 0, HU constancy
-4DCT
-readout is consistent with actual motion
-slice thickness
-dose profile
-distortions
-E2E with TPS
