TG106 Flashcards
what is TG-106
Accelerator beam data commissioning equipment and procedures
max data variation among different people taking measurements
< 1 %
issues with using golden beam data
-variations in beam parameters between beams with same nominal energies
-on-site changes such as steering won’t be modelled
-jaw velocity onsite for wedges could affect wedge model
-although acceptable agreement with the golden data set may be found in individual checks, it may be that some clinical setups will have multiple errors, which combine to produce unaccept-able results
difference between acceptance testing and commissioning
acceptance testing- verification that the machine meets standards agreed to with vendor
commissioning- obtain dataset for patient treatment
task report wants measurement error to be within what?
+/- 1 %
time for commissioning of a linac
-1.5 weeks photon beam scanning
-pt data collection 1 week
-electrons 1-2 weeks
-analysis 1-2 weeks
4-6 weeks total
additional time to validate TPS etc
when can you cut down on commissioning if you have 2 identical machines?
-proper analysis of sample data set is done and agreement is within 1 %
2 types of data acquired during commissioning
-scanned data
-pt data
what to do if algae starts growing in water tank
-add detergent
why do you periodically check the water tank level?
water evaporates
how big does scanning water tank need to be?
1.6 X max field size
-75x75 cm2 is optimal
how to get diagonal profile info
-rotate tank not collimator
-collimator rotated will not give FF info that diagonal profiles are intended to provide
-potentially need to take half-scans at deeper depths (large FS required)
acceptable asymmetry for using half-scans
< 0.5%
what can you use solid water for at commissioning?
-output factors
-surface dose
-leakage/transmission
-wedge factors
how to measure Sc
volume of standard vs mini vs micro ion chamber
standard = 0.6 cm3
mini = 0.05 cm3
micro = 0.007 cm3
pros of diodes
-quick reponse time
-excellent spatial resolution
-high sensitivity
-no external bias
-energy independenc eof mass collision stopping power ratios
-however, temperature, dose rate, energy, and angular dependent
what detector do you use for dynamic wedge measurement?
-either diode or ion chamber array
pros of diamond detectors
-no directional dependence
-tissue equivalent
-small (1-6 mm3)
-small dependence on dose rate
accuracy of TLD
+/- 5 %
cons of TLDs
-some energy dependence
-fading
-non-linear dose response
-these effects are small in MV beams
what does film dosimetry depend on?
-field size
-depth
-beam energy
-processing
why use film for electron beam?
Silver halide films exhibit strong energydependence for photon beams but their response is relativelyindependent in megavoltage electron beams.
pros of MOSFETS
-small size
-similar to conventional dosimeters in reproducibility, linearity, energy, and angular reponse
-total dose it can measure is small
-cannot be used for commissioning… just pt doses
BANG gels
-3D dose map
-tissue equivalent
-energy dependent
-need to be imaged with MRI, CT, or optical CT (not practical)
detectors used in scanning water tank
-ion chamber, diode, diamond
issues with small fields
-need small volume chambers
Small volume chambers and diodes tend to have different characteristics for large fields compared to small field and should not be used for all field sizes
-if truly cannot find small chamber, can use complicated deconvolution method to account for volume averaging
why do we have reference detector when tank scanning?
removes the instantaneous fluctua-tions or drifts in the incident beam output.
where should reference detector be placed?
anywhere in the beam where it does not shadow the field detector for the entire area of programmed positions
For very small fields,where the reference detector may shadow the field detector, a time integration method could be used instead of the refer-ence chamber
voltage for diamond detector and diode
diode is none
diamond is 100 V
acceptable polarity (difference between + and - bias voltage)
0.5%
how to investigate diode energy response
compared PDD in large field with ion chamber measurement
If the diode curve does not drop off as rapidly as the ion chamber PDD, then this is an indication of energy response. Generally, diodes should not be used for PDD measurements in large x-ray fields
BNC connection
-twist-on attachment
-both co-axial and triaxial cables
TNC is threaded version of BNC
acceptable leakageforelectrometer
10^-13 to 10^-14 A
what range do electromers read
10^-6 to 10^-14 C
good SNR for tank scanning
100
can adjust gain; make sure not to saturate detector
differences between x and y profiles
+/- 1 %
how to check if probe “walks” when going from surface to depth
-close the jaws to 1 mm, follow the crosshair and jaw definition as you move probe up and down
how many methods must be used to verify SSD?
2
examples:
front pointer
ODI
lasers
why does orientation of water tank matter?
must make sure is is entered properly for TPS…
For example, if the TPS reads scan data as if a 45° wedge scan was performed from the end of table to the gantry with the toe of the wedge facing the gantry, but in fact, the scan was really performed along the non wedged direction in the transverse plane, this would seriously compromise data entry
gantry or tank tilt causes what?
PDD doesn’t follow central axis with depth
how to tell if there is hysteresis in the scanner?
They can be tested by scanning the same field at a moderate speed in one direction and then reverse the scan-ning direction. If these two profiles do not align and match perfectly, there is a hysteresis in the scanning movement.Such scanners should be sent to the scanning system manu-facturer for repair and should not be used for scanning
how to test if the scanning system is responding as fast as the probe is moving?
scan across 40 cm with a 20 cm field at the highest and lowest speed. Compare the two profiles for alignment. If the relative shape agrees but there is a shift, then there may be a limit as to how fast one can scan
what parameters do you measure during tank scanning dry run in air?
-noise: standard dev. in flat region
-SNR: coefficient of variation in flat region
-time constant: when beam is turned off, how long it takes for scan values to settle
leakage: after scan values settle, mean and std dev of nonradiation value
-electrometer offset: mean of standard dev and non-irradiation mean- subtract this from all measurements on the same gain
-repeat these same tests with water
-repeat also at max scanning depth (will have lowest SNR)
saturation test for water tank scans
do the dry run procedure with 20x20 cm2 field with max dose rate and moderate dose rate - compare the profiles
what is extracameral effect
non-thimble area, connector, and cable irradiated either with scat-ter or primary radiation produce ionization contributing to the scan signal
energy response test for diode
-6 MV in 40x40 field
-measure with large volume ion chamber and diode
-if diode does not drop off as fast as ion chamber PDD, then this is indication of energy response variation
issues with high speed scan
-noise
-ripples due to movement of scanning arm
impact of delay time between measurements at 2 consecutive pts
advantageous for electron beam scanning since small ripples in water could change the data significantly.
should you scale data from a different SSD?
-only to check measurements, not to skip measurements at different SSD
different components scale differently with SSD:
-electron contamination
-off-axis softening
-penumbra
what components do scale well with SSD?
-primary beam (IS)
-head scatter (IS)
TBI
collect data at extended SSD
how do you measure TMR or TPR?
easiest from pt dose measurements
what affects surface dose?
-machine
-field size
-SSD
-beam modifiers
-angle of beam incidence
what should you use for surface dose measurement
-not scanning device
-small detector
-extrapolation chambers are best choice but uncommon
-PP chambers- but because of large separation, show over-response in build-up region and surface- use PP with small separation and large guard ring
-TLD, MOSFET, OSLD also used
if you only do in-plane or cross-plane, which should you do?
-crossplane- stable
-steering of electrons in some machines only possible crossplane
-in plane is y
-cross plane is x
obtain profiles at what depths?
Typi-cally, profiles at 5–7 depths are sufficient for each 1 cm spaced field size up to 6x6cm2, and then 5 cm spacing for field sizes 10x10 cm2 and greater is sufficient.
-also get dmax
-for wedge, use smaller intervals in high gradient area
star pattern profiles
profiles at 10 degree angle increments
-don’t rotate colli- doesn’t model FF
what do you use for dynamic wedge measurements
-need 2D array since pt measurement won’t characterize the beam
-often a 2D array with solid water
-waterproof arrays that go onto scanning tank are less common
what to quantify for MLC
-light and radiation field congruence
-interleaf leakage
-intraleaf leakage
-tongue and groove effect
-penumbra
-for each photon energy and at least at each cardinal angle
-also positional accuracy
-use film dosimetry except for penumbra
-use reference field with film and ion chamber to calibrate film
what depth of water should be included beyond deepest pt of measurement?
10 cm
How to determine Scp
measure it
use Monte Carlo
how to measure Sc
-mini-phantom- thick enough to get CPE- need > 1 cm around miniphantom (may have to go to longer SSDs for smaller field sizes and then measure output at 10x10 for that SSD- merger the output factors)
-ion chamber with build-up cap
what does wedge factor depend on?
-wedge angle
-depth
-xray energy
-field size
do wedge factors for dynamic wedge depend on depth?
No, because beam quality is not changed
what is universal wedge
Elekta
60 degree wedge and open field to get different wedge angles
small fields- what is a red flag you need to switch to a smaller detector
-if scan through field center varies more than 1% over the range of the detector diameter
how do you know if you set the 0 point for cylindrical ion chamber in electron beam properly?
A quick depth ionization scan for a low energye.g., 6 MeV electron beam can be used to check if the zero depth is set correctly.The resultant curve will have a well-defined dmax, with an average value of 1.10.2 cm for 6 MeV, regardless of the vendors.
at what dephts do you measure for electron PDD
all energies for reference cone for depth of Rp+ 10 cm
-0.1 cm increments
how to characterize Brems tail for electrons
Bremsstrahlung is divided into three componentshead, cerrobend, and waterand mea-sured by eliminating different components.
can you use same diode for electrons and photons?
usually no
ugly profile scan- what can you do?
-gain of electrometer
-placement of reference probe
-direction of scan motion
-probe motion rate or sampling time
-repetition rate on machine
-turn off dose servo
-slow down scan rate
electron applicator factors
ratio of dose at dmax to dose at dmax for reference cone
-remember dmax depth can change!
cutout factors for cutouts at extended SSD
-measure or calculate based on virtual SSD
virtual source for electron beams
-take measurements at dmax at various air gaps between electron cone and water surface
-plot square root(Io/I) vs gap
-virtual source is calculated from slope
smoothing and filtering routines
-low pass filter
-eliminates high frequencies
data processing examples
-smoothing
-centering
-making beam symmetrical
what must be included on commissioning report
-scope, what was measured, how, results
-PDD and TMR
-wedge factors with edged PDD and TMR
-output factors
-OARs (also for edged fields)
-electron applicatior factors
-electron effective SSDs
-selected isodose curves for reference fields
-compare with data from other machines or golden data
volume of PTW Markus chamber
0.02 cc
how to account for polarity of PP chamber when doing dose measurements
-read at both polarities and average them
can TLD and OSLD be used for oblique beams?
yes because they are the same material throughout
no angular dependence
how accurate is film?
within 2-3% of extrapolation chamber reading
do diamonds have angular dependence?
a little bit
-also have some temperature dependence
-have to pre-irradiate them
at what angle do you measure output factors at?
-90 degrees to reduce scatter from floor
-TG74
how small can mini phantom be
need at least 1 cm of field on each side of it
-achieve tCPE
material for build-up cap in small field
brass
where do you measure output factors?
-at calibration condition, usually iso
diode response compared to 10x10 cm2 FS
-under-responds at small FS
-over-responds at large FS
collimator exchange effect
-MU chamber feedback is different depending on size of x and y jaws
-should be < 2 % difference
Ppol range
-unlikely to exceed 0.03%
typical 0.9998
Pion range
-must be < 1.02
-for us: 1.003, 1.006
water tank commissioning tests
-position accuracy and linearity
-mortion orthogonal in 3 axes
-no obliquity
-communication of components
-mechanical integrity
-interpolation
-conversion of PDI to PDD
-EPOM
-smoothing, centering
-output of files is compatible with planning system
CAP
Customer Acceptance Performance
typical energy spec. on company’s acceptance
within 2 %
tests to try on an unknown dosimeter
-linearity with dose
-saturation
-instanteous dose rate dependence
-energy, angular dependence
-type of radiation
-signal is same for MUs at different dose rate
when must dose verification be done?
-before 3rd fraction or 10% of treatment is done
