Notes Flashcards
steps for selection and purchase process
-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
describe tender process
-RFP (request for proposal)- bid request
-• specifications, capabilities, service, training, warranty, costs and delivery times
example of RFP details for CT sim
-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
what is included in purchase contract
-manufacturer
-make/model
-specification document
-acceptance test procedures, with associated tolerances
what is verified during acceptance testing
-performance of equipment must meed or exceed contract agreement
-environment free of radiation and electrical hazards
who is present during acceptance testing
vendor
physicist
how are acceptance tests designed?
-any deficiencies discovered and rectified have minimal impact on tests previously done- order matters
define commissioning
-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)
should commissioning data always be independently double checked?
yes
-external audits like IROC are recommended when commissioning new equipment that the centre hasn’t dealt with before, and for special techniques
is vendor present during commissioning?
No
time required for commissioning
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
what is required to bring unit into clinical operation
-acceptance testing
-commissioning
why is commissioning important?
RT is directly related to accuracy in delivered dose to patient which is dependent on accuracy of beam data
define golden beam data
-attempt by vendors to standardize machines to have identical characteristics
-can purchase golden beam data which includes most of data required for TPS commissioning
cons of using golden beam data
-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)
pros of golden beam data
-less risk of catastrophic error
-faster commissioning because only a spot check is required
2 additional steps once commissioning is done
-perform independent audiot of collected data and report
-backup whole dataset (raw and processed)
survey equipment required
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)
dosimetry equipment required
-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.
what equipment require calibration certificate?
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).
list of acceptance testing safety tests
-interlocks
-warning lights
-patient monitoring
-training
-radiation survey
-collimator and head leakage
goal of safety testing
-eliminate possibility of unplanned/inappropriate irradiation of people
list of interlocks
-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)
describe the radiation sruvet steps with order of regions surveyed
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.
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
collimator axis of rotation acceptance level
radius < 1 mm
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.
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
acceptance limit for symmetry of collimator jaw images
< 1 mm
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.
gantry axis of rotation acceptance limit
radius < 1 mm
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.
mechanical isocentre acceptance limit
-collimator, gantry, and couch should intersect in a sphere of radius < 1 mm (0.5 mm for stereo)
is radiation isocenter determined for each energy?
yes
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.
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.
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.
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??
increment distances checked for ODI at acceptance testing
5 cm
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)
table flex?
Check that flex in longitudinal and lateral travel with and without load is within specification
Max load typically 550 lbs
• 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.
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
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.
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.
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
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.
where do you do flatness/symmetry test for electreons
at some particular depth, probably near Dmax
for what planes do you do flatness/symmetry tests
principal planes and diagonals
uniformity index
area enclosed by the 90% isodose divided by the area enclosed by the 50% isodose
-can be measured along with flatness, symmetry
where are penumbra defined?
-usually get 80-20% penumbra
-10 cm depth for photons, near dmax for electrons
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]
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
should the linac monitor chamber vary with T and P?
No, this would indicate it is not sealed properly
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.
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
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
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
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
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).
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.
photon PDD measurements for commissioning
-various FS, to depths of 40 cm
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.
describe volume averaging
-when measuring small field with large volume chamber, volume averaging will make dose reading appear too low
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
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.
how do you measure PDD for soft wedge?
must integrate dose at each depth over the entire jaw sequence.
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.
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).
where is EPOM for PP chamber
inside surface of entrance window
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.
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.
where do you measure output factors for commissioning?
-dmax or 10 cm depth
ie calibration condition
how do you approximate output factors for rectangular fields?
square root of product of output factors for the 2 square fields
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
collimator exchange effect
affects output factors due to two sets of jaws being at different heights
Sc is due to ?
air
flattening filter
jaws
monitor chamber
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).
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»_space; 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
Does AAA need Sc?
Nope
how to determine Sp?
divide output factor by Sc
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.
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
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
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)
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.
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.
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!)
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.
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
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 %
transmission through cerrobend blocks
3.5-5%
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)
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%.
where is wedge angle measured
between isodose and line perpendicular to CAX
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).
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.
