aapm tg106, linac beam data commissioning
TRANSCRIPT
Samir Laoui, Ph.D. 3/09/2016
Scope & Purpose
AAPM Task Group Report 106 has been prepared
to
“ facilitate accelerator beam data commissioning
by describing specific setup and measurement
techniques, reviewing different types of radiation
phantoms and detectors, discussing possible
sources of error, and recommending procedures
for acquiring specific photon and electron beam
parameters. ”
Introduction
The data obtained during commissioning is
treated as the standard for clinical use and should
be verified periodically
At minimum, the following should be collected
during commissioning:
Photons: PDD, profiles (in and/or cross plane) at
various depths for open and wedged fields, data related
to MLC… etc.
Electrons: PDD, profiles, cone factors, insert factors,
and virtual source positions
Issues
Data can vary with the choice of phantoms and
detectors and methods used
Data
Phantoms, methods and detectors
Phantoms Scanned data must be carried out with a water
phantom (40x40x40 min). Point dose may be
carried out in solid water phantom
Scanning in both in-planes and cross planes
Distilled water, with algae growth prevention
Room temperature
Evaporation/Surface should be verified
75x75 cm2 is optimum recommended size
Scanning system setup
Water tank needs to be calibrated
Check the free movement of each arm, and the x, y , z,
and diagonal motion
Accuracy and linearity, leakage, cracks, mechanical
stability, cables quality.
Phantoms, methods and detectors
Detectors Ion chambers: Can provide direct dose measurement.
Cylindrical, spherical and parallel plate
Diodes: Energy independence of mass collision stopping
powers (Silicon to Water). Attractive for electron dosimetry
Detector array: Profile measurements.
TLD: Point dose measurements. Strong energy dependence
and nonlinear dose response
Film: silver halide and Gafchromic. planar dose maps in small
fields measurements.
Scanning and reference detectors
High voltage requirement: Ion Chambers (300-
400 V), diodes: 0 V
Polarity: Agreement between positive and negative
Connections and Cables Most cables used in radiation dosimetry and with
the scanning system have triaxial adapter
Some manufacturers market unusual looking
triaxial ends nonstandard that may not fit standard
ion chambers. PTW is one such vendor that has
different triaxial adapter ends
Scanning Water Phantom
UCI MC makes use of original Wellhöfer
scanning water phantom
Scanning Water Phantom
The scanning tanks should never be placed on
the machine treatment table (weight)
The tank origin should be close to the machine
isocenter
The detector should be level with the water
surface in all four corners of the tank
The z-direction movement of the detector
should be parallel and should follow the
central axis of the machine at 0° gantry angle
Positioning
The detector position should be set such that the center
of the detector splits the water surface: 0 position
Arm tilt, gantry tilt
Premeasurement test
Position the scanning detector at isocenter:
Perform an in-air scan of a 20x20 cm2 field,
allowing the scan to run from −20 to +20 cm (40
cm total). Also perform a scan with beam off
Analyze for the following:
Noise, STNR, response time of the system, leakage,
polarity
Saturation test: Repeat dry run with 20x20 field at
max dose rate
Data Acquisition
Data acquisition should be conducted in an organized
fashion to avoid confusion
Data acquisition speed: critical for small fields
Sampling time should be long enough
Photon beam data
PDD 100 SSD, long tradition of such setup
Scaling of data taken from a different SSD should only
be used as QA checks to ensure consistency for the
following reasons:
Electron contamination: Depend on SSD
Primary dose: can be scaled for different SSDs just by
applying the inverse square law, except for small field sizes
Scatter dose: SSD dependent
Penumbra: It cannot be scaled from one SSD to another
Photon beam data
Beam Profiles The choice of detector orientation is critical for
profile measurements for small fields and high
gradient regions
Small volume detector is preferred for profile
measurements
Beam profiles: Data collection
Maximum of 1 mm spacing in the penumbra region and
preferably no more than 2 mm spacing in the remainder of the
field\
Profiles at 5–7 depths are sufficient for each 1 cm spaced field
size up to 6x6 cm2, and then 5 cm spacing for field sizes
10x10 cm2 and greater is sufficient
Beam profiles
Star pattern: Some TPS require BP with respect to the
collimator angle
Physical wedge: Data collection at smaller spacing in the high
gradient area
Electronic wedge: Standard scanning cannot be used. Profiler
may be used.
MLC Data
These parameters, as described below, should be quantified for
each photon energy and a minimum of four gantry angles (0°,
90°, 180°, 270°) to examine the effect of gravity on leaf
motion
Light and radiation field congruence
Interleaf leakage (leakage between two leaves)/ Portal imager
Intraleaf leakage (transmission though a leaf)/Portal imager
Penumbra
Photon point dose data
For manual dosimetry calculations, the following should be
collected
Scp :
Large water phantom ensures full lateral buildup
The deepest point of measurement should be at least 10 cm to
ensure full backscatter
In-air output ratio Sc: Ratio of primary collision water KERMA in
free-space, Kp, per monitor unit M between an arbitrary collimator
setting and the reference collimator setting at the same location.
Measured with IC with a buildup cap
Photon point dose data
Sp: is defined as the ratio of the scatter factors between the
actual field size, s, in the phantom and that of the reference
field size, sref Where SF= D/Dprimary
Wedge factors, tray factors
Small field considerations: Small-field dosimetry is
challenging due to lack of lateral electronic equilibrium
Electron Beam
Diode, parallel plate ion chamber, cylindrical ion chamber and
films
Percent depth ionization curves should be scanned for all
energies for the reference cone to a depth of Rp+10 cm with
depth increment of 0.1 cm
When IC is used for measuring depth ionization curves in
water, reading should be converted to dose using stopping
power ratios
Small volume electron diode is ideal since it does not require
ionization to dose conversion
Profiles
Attention should be given at depths greater than dmax,
especially for low energies
Electron point dose data
Cone factors: Ratio of dose at dmax for a given cone to the dose
at dmax for the reference cone
Cutout factors: Ratio of the dose with and without the cutout at
their respective dmax
TrueBeam Biannual QA
Photons:
5 energies * 8 fs = 40 PDD scans
5 energies * 4 depths (dmax, 4, 10, 15) = 20 Diagonals
5 energies * 4 depth (dmax, 4, 10, 15) * (IPxCP)= 40 profiles
Wedge and accessory TF:
3 energies (4 physical, 4 EDW) = 24 measurements
Electrons:
6 energies * 5 cones = 30 PDD/PDI scans
6 energies x 1 cone (15x15) = 12 profiles
TrueBeam Biannual QA- TG 51
5 Photons
TrueBeam Biannual QA- TG 51
6 Electrons
Miscellaneous
MU Linearity
Output constancy Vs. gantry angle
Mechanicals
The end