Division of Medical Radiation Physics
Beam Production, Characteristics and ShapingDr. Manfred Sassowsky
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 2
Division of Medical Radiation Physics
Outline
X-ray production60Co unitsLinear AcceleratorsBeam characteristicsBeam shaping
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 3
Division of Medical Radiation Physics
Literature
1. E.B. Podgorsak (Technical Editor): Radiation Oncology Physics: A Handbook for Teachers and Students, IAEA, Vienna, 2005, ISBN 92–0–107304–6, http://www.iaea.org/books
2. TRS 398: Absorbed Dose Determination in External Beam Radiotherapy, IAEA, Vienna, 2000
3. H. Reich (Hrsg.): Dosimetrie ionisierender Strahlung, B.G. Teubner, Stuttgart, ISBN 3-519-03067-5 (out of print)
4. H. Krieger: Strahlenphysik, Dosimetrie und Strahlenschutz (2 volumes), B.G. Teubner, 2001, ISBN 3-519-23078-X and ISBN 3-519-43052-5
5. Recommendations of the Swiss Society of Radiobiology and Medical Physics (http://www.sgsmp.ch/recrep-m.htm#rec)
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 4
Division of Medical Radiation Physics
X ray production: Principle
• Heated cathode: AC voltage UH• Thermoelectric emission of electrons• Tube voltage U accelerates electrons
towards anode • Electrons interact with anode material
(See lecture "Basic radiation physics") => X rays
• Kinetic energy of electrons:
• e.g.: U = 100 kV => W = 100 keVUeW ⋅=
e-
e- e-
Cathode (heated filament)
Anode
~ UH
U
Vacuum
X rays
e-
e-e-
e-
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 5
Division of Medical Radiation Physics
X ray production: Focussing
• In particular for diagnostic X ray beams: small spot size needed (higher image resolution)
• Focusing using guard electrode with voltage Uf ; negative with respect to cathode e-
e-e-
e-
e-
Cathode (heated filament)
Anode
~ UH
U
Vacuum
X rays
e-
Uf
Guard electrode
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 6
Division of Medical Radiation Physics
X ray production: Efficiency
• Efficiency:
Z = atomic number of anode materialU = tube voltagek = constant;
• Numerical example: Z = 74 (Wo) U = 100 kV =>i.e.:
- only 1% of electron kinetic energy is transformed into radiation energy- 99% are dissipated in heat
• Technical challenge: cooling of anode => rotating anode
01.0≈η
UZk ⋅⋅=η
1910 −−≈ Vk
Electric
Radiation
PP
=η
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 7
Division of Medical Radiation Physics
X ray production: Rotating anode tube
AnodeCathode
Exit window
Dielectric cooling oil
Focal spot
Stator
Vacuum
Rotor
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 8
Division of Medical Radiation Physics
X ray production: Depth dose curve
• U < 100 kV: maximum dose practically at surface
Relative Dose
Depth in water (cm)
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Division of Medical Radiation Physics
X ray production: Photon energy spectrum
• As discussed in lecture "Basic radiation physics"- Continuous X rays (Bremsstrahlung)- Characteristic X rays (Ionisation/excitation; subsequent photon emission)
Photon energy
Relative Intensity
UeW ⋅=max
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Division of Medical Radiation Physics
X ray production: Filtering
• Diagnostic applications: - Low energy photons to not traverse patient- Do not contribute to diagnostic image- But: lead to dose deposition
• => use of Cu and/or Al filters to decrease intensity of low energy photons
Photon energyUeW ⋅=max
Relative Intensity
Increasing filter thickness
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 11
Division of Medical Radiation Physics
X ray production: Applications, Naming conventions
• Diagnostic- 2D images- Computed Tomography (CT) images
=> see lesson "Imaging for radiotherapy"
• Therapeutic- Superficial lesions- Not suited to treat deep seated tumours
• W = 10 keV ... 100 keV Superficial X rays• W = 100 keV ... 500 keV Orthovoltage X rays• W > 1 MeV Megavoltage X rays
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Division of Medical Radiation Physics
60Co units: 60Co decay
• 60Co decays to 60Ni by beta-decay • Half life t1/2 = 5.26 years• Excited state of 60Ni de-excites to ground
state by two subsequent gamma-decays
Co6027
Ni6028
0.31 MeV
1.17 MeV
1.33 MeV
β
γ
γ
5.26 a
• Specific activity a: Activity per mass
MtN
mAa A
2/1
2ln==
A = ActivityNA = Avogadro‘s numbert1/2 = Half lifeM = Molar mass
• a ≈ 4.2⋅1013 Bq/g (pure 60Co)• a ≈ 1013 Bq/g (technically achievable with 25% 60Co)
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Division of Medical Radiation Physics
60Co units: 60Co sources
• Source shape: pellets or disks• 60Co produced using neutrons
from a nuclear reactor:59Co + n → 60Co + γ
• Self absorption of gamma radiation in source:
( )ll
xl
xeff el
AedxlAedAA μμμ
μ−−− −=⋅=⋅= ∫∫ 1
00
• The longer the source, the higher the self absorption• Typical values: μ ≈ 0.385 cm-1 (disks), l ≈ 10 cm => Aeff ≈ 0.25 A
i.e. about ¾ of gamma radiation is absorbed in source
d = 1 cm ... 2 cm
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 14
Division of Medical Radiation Physics
60Co units: Shutter mechanisms
• Operated by electromotor• Safety mechanism: mechanical spring for emergency retraction
W = WolframUr = UraniumPb = Lead
Rotating cylinder Sliding drawer
source
source
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Division of Medical Radiation Physics
Primarycollimator
Source
Fibre optic
Depleted uranium
Lead
Wolfram
Light bulb
Secondarycollimators
Display of field size
Penumbra trimmer
60Co units:Treatment head
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Division of Medical Radiation Physics
Linear accelerators: Introduction
• Need for higher beam energies to treat deep-seated tumors• Electrostatic acceleration limited to U ≈ 1 MV (discharges)• => Use of particle accelerators• Originally developed for research in elementary particle physics• This lecture: only linear accelerators will be treated • Basic idea: use moderate acceleration voltages many times
to obtain higher total acceleration voltage• Typical modern high energy linear accelerators (Linac):
- Two photon energies (e.g. 6 MV, 18 MV)- Several electron energies (e.g. 6, 9, 12, 16, 20 MeV)
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Division of Medical Radiation Physics
Linear accelerators: Overview• Electron gun: thermionic emission
of electrons• Accelerating waveguide:
accelerates electrons using RF waves
• RF Generator• Beam transport: transfer of
electrons from accelerating waveguide to scatter foil or target
• Scatter foil: spreading of electrons for electron beams
• Target: generates photons from incident electrons
• Filter: flattening of photon beam • Monitor chambers
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 18
Division of Medical Radiation Physics
Linear accelerators : RF Generation
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
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Division of Medical Radiation Physics
Linear accelerators: RF Generation (1)
High voltage power supply
Pulse forming network
Klystron
charging
Low power RF pulse
High power RF pulse
• „RF“ = „Radio Frequency“• Used here as synonym for a high power,
high frequency, electromagnetic wave
• Key element: Klystron• High power RF amplifier
• Typical output: P ≈10 MW during Δt ≈ 5 μs
• fRF ≈ 3 GHz
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 20
Division of Medical Radiation Physics
Linear accelerators: RF Generation (2)
Low power RF High power RF
Drift tube
CathodeAnode
Resonant RF cavities
Klystron:• Low power RF excites
standing waves in 1st cavity• Unbunched electron beam
enters 1st cavity• Bunching of electron beam:
velocity modulation• Drift region: bunch size decreases• Bunched electron beam
excites 2nd resonant cavity• High power RF extracted from cavity
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 21
Division of Medical Radiation Physics
Linear accelerators : Electron gun
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 22
Division of Medical Radiation Physics
Linear accelerators: Electron gun (1)
• Triode gun: pulsed electron „bunches“
• Thermoelectric emission of electrons (heated cathode)
• Anode voltage U positive with respect to cathode (15 kV ... 50 kV)
• Grid voltage Ug negative with respect to cathode (typ. –150 V)
• => Electrons can not pass grid, are kept in region between cathode and grid
Cathode (heated filament)
Anode
~ UH
U
Ug
Grid electrode
Accelerating waveguide
Electrons
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 23
Division of Medical Radiation Physics
Linear accelerators: Electron gun (2)
• Grid voltage Ug set to zero during short time interval
• => Some electrons can pass grid, are accelerated towards anode
• Electron „bunch“
Cathode (heated filament)
Anode
~ UH
U
Ug
Grid electrode
Accelerating waveguide
Electrons
time
Ug
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 24
Division of Medical Radiation Physics
Linear accelerators: Electron gun (3)
• Periodic pulsing of grid voltage• Repeated electron bunches
Cathode (heated filament)
Anode
~ UH
U
Ug
Grid electrode
Accelerating waveguide
Electrons
time
Ug
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 25
Division of Medical Radiation Physics
Linear accelerators: Acceleration
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 26
Division of Medical Radiation Physics
Linear accelerators: Acceleration (1)
• Principle: use moderate acceleration voltages many times• Explained using Wideroe accelerator
(only of historical importance, but convenient to explain principle)• Drift tubes, varying length L, alternating polarities• Driven by RF Generator, frequency fRF, period TRF = 1 / fRF• Acceleration of electron bunch in gaps between drift tubes
~AC generator
Drift tubes
L
Gun
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 27
Division of Medical Radiation Physics
Linear accelerators: Acceleration (2)
~AC generator
0Tt =
~AC generator
RFTTt 21
0 +=
~AC generator
RFTTt += 0
2RFTvL =Synchronicity condition:
L
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 28
Division of Medical Radiation Physics
Linear accelerators: Acceleration (3)
• Modern linac: RF waves in accelerating waveguide
- Travelling wave- Standing wave
• Travelling wave:- Hollow conducting cylinder as
wave guide- Filled with discs (irises)
• Propagation speed (group velocity) of RF wave depends on geometry:
- length of cells- Inner vs. outer diameter of irises
• Matched with electron speed
RF in RF out
5 cm ... 10 cm
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 29
Division of Medical Radiation Physics
Linear accelerators: Acceleration (4)
• Pictorial view: electrons „ride“ close to the crest of the RF wave
• Bunch size is further decreased:- Faster electrons:
◦ advance ◦ experience lower accelerating field◦ are thus brought back to bunch
- Slower electrons:◦ lag behind ◦ experience higher accelerating field◦ are thus brought back to bunch
Vel
Vwave
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 30
Division of Medical Radiation Physics
Acceleration using standing waves
• End of accelerating waveguide is made reflecting for RF
• Forward RF wave is reflected back• Superposition of „forward“ and
„reflected“ RF wave yields a standing wave
• Maxima and minima of standing wave do not travel, but stay at fixed positions
Linear accelerators:Acceleration (6) RFforward
RFreflected
E
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 31
Division of Medical Radiation Physics
Linear accelerators: Acceleration (7)
• More advanced standing wave structure with „side coupled“ cavities
• Advantage: Can be made shorter than travelling wave structure (while obtaining the same electron energy)
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Division of Medical Radiation Physics
Linear accelerators: Acceleration (8)
• Typical time structure of electron beam:
Time
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 33
Division of Medical Radiation Physics
Linear accelerators : Beam transport
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 34
Division of Medical Radiation Physics
• Charged particle in magnetic field:- Lorentz-Force:
- Radius of curvature r :
Linear accelerators: Beam transport (1)
• Bend electron beam onto target• Energy selection
BvqF L ×=
q = Chargev = Velocityp = MomentumB = Magnetic induction
qBpr =
Other magnet geometries are possible – omitted here
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Division of Medical Radiation Physics
Linear accelerators: Beam transport (2)
• Steering coils for fine-tuning beam position and beam angle• Feedback from monitor chambers (explained later)
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 36
Division of Medical Radiation Physics
Linear accelerators : Scatter foil / Target + Filter
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 37
Division of Medical Radiation Physics
Linear accelerators: Scatter foil (1)
• Left: Electron beam from accelerating structure is only a few mm in diameter („Pencil beam“)
• Right: Treatment requires wide beams with a flat transverse beam profile
• S: Scatter foil scatters electrons to transform pencil beam into beam useful for treatment
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Division of Medical Radiation Physics
Linear accelerators: Scatter foil (2)• One scatter foil: limited transverse homogeneity and field size• Energy dependence• =>
Multiple scattering foils for high energy beams
Additional ring shaped foils for low energy beams
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Division of Medical Radiation Physics
Linear accelerators: Scatter foil (3)
• Homogeneous transverse beam profile ...
• ... but:- Energy loss- Energy straggling- Photon contamination from Bremsstrahlung
• Maximize scattering, minimise adverse effects=> materials with high atomic number Z are preferred
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Division of Medical Radiation Physics
Linear accelerators: Target / filter (1)
• Treatment with photons requires wide beams with a flat transverse beam profile
• Target: electrons interact with nuclei and emit photons (Bremsstrahlung); materials with high atomic number (Z)
• Filter: used to homogenise transverse beam profile; preferably materials with high Z
Target
Filter
Collimator
Photons
Inte
nsity
dis
tribu
tion
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Division of Medical Radiation Physics
e- e-Target
Filter
Electron stopper Target
Filter
Primary collimators
Primary collimators
Linear accelerators: Target / filter (2)„Thin“ target „Thick“ target
• Average photon energy Higher Lower• Yield Lower Higher• Electron stopper Yes No• Cooling requirements Low High
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Division of Medical Radiation Physics
Linear accelerators: Target / filter (3)
• Examples for different filter shapes
a) Pb for low energiesb) Pb or Wo for energies up to 15 MeVc) Fe with Pb core (25 MV Photons)d) Low Z (Al or steel) for high energies
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Division of Medical Radiation Physics
Linear accelerators: Target / filter (4)
Effects of filter on photon beam:
• Reduction of beam intenstiy in center of beam• Reduction of total beam intensity• Compton interaction and Bremsstrahlung
=> Decrease of photon energy• Preferred absorption of low energy photons
=> Increase of average beam energy• Contamination of beam with secondary electrons
=> Increase of skin doseModification of depth dose distribution
• Photon energies above ~10 MeV: nuclear photo effect=> Contamination of beam with neutrons
Activation of materials in treatment head
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Division of Medical Radiation Physics
Linear accelerators: Target / filter (5)
• Effects of electron-beam mis-steering
a) Nominal beamb) Beam inclined c) Beam displacedd) Beam divergent
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Division of Medical Radiation Physics
Linear accelerators: Target / filter (6)
New approach: omit flattening filter• Inhomogeneous open field• Beam shaping with dynamic MLC (explained later)• Higher dose rate in central part of beam
10X FFF
10X
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Division of Medical Radiation Physics
Linear accelerators: Monitor chambers
Electron gunRF generator
Stand Gantry
Acceleratingwaveguide
RF
Beamtransport
Scatter foil / Target + FilterMonitor chambers
Treatment couch
Powerconverters
Isocenter
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 47
Division of Medical Radiation Physics
Linear accelerators: Monitor chambers (1)
• Monitor chambers measure:- Beam output in MU (monitor units)- Beam symmetry
• Calibrated so that 1 MU = 1cGy under defined conditions
• Transmission ionisation chamber- Two electrodes on HV (U)- Beam ionises air molecules- Charge separation in electric field- Charge (Q=∫ I dt) => MU
Beam
U
I
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Division of Medical Radiation Physics
Linear accelerators: Monitor chambers (2)
• Located downstream of scatter foil / filter• Two independent systems• Each system: two sectors• Differential signals => Beam symmetry
=> feedback to steering coils (explained earlier: beam transport)
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Division of Medical Radiation Physics
Linear accelerators: Manufacturers
• Elekta• Siemens• Varian
• All manufacturers supply complete systems, i.e.:- Linac itself - Control system- Treatment table- Imgaging for patient positioning- Treatment planning software- Record + Verify software- …
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Division of Medical Radiation Physics
Beam Characteristics: Absorbed dose to water
• Absorbed dose is the deposited energy per mass:
dmdED =
• Water is used as reference material (Properties similar to tissue, availability, physical properties well defined)
• SI unit is the gray (Gy):
GykgJ===
][][][
mED
kgJ1 Gy1 =
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Division of Medical Radiation Physics
Dsurface
Buildup region
Depth in water
Beam Characteristics: Depth dose photons (1)
• Percentage Depth Dose (PDD) curve of photons in water
• Dsurface = Dose at surface• Dex = Dose at exit• dmax = Depth of
dose maximum
• Build-up region
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Division of Medical Radiation Physics
Beam Characteristics: Depth dose photons (2)
• Percentage Depth Dose (PDD) curves of photons in water
• Source to Surface Distance (SSD) = 100 cm
• Photon beams ranging from 60Co to 25 MV
• 10 × 10 cm2 field
Increasing energy
Increasing dmax
e.g.:6 MV beam: dmax ≈ 1.4 cm
18 MV beam: dmax ≈ 3.0 cm
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Division of Medical Radiation Physics
Beam Characteristics: Depth dose photons (3)
• Percentage Depth Dose (PDD) curves of photons in water
• Source to Surface Distance (SSD) = 100 cm
• 6 MV Photon beam• Field size:
3×3 …40×40 cm2
Increasing field size
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Division of Medical Radiation Physics
Beam Characteristics: Depth dose electrons (1)
• Percentage Depth Dose (PDD) curve of electrons in water
• Rth = therapeutic range• Rp = practical range• Rmax = maximum range
• R50 = depth where Drel = 50%(used as beam quality specification for electrons)
• e.g. 6 MeV electrons: R50 ≈ 2 cm
Depth in water
Dsurface
Dskin
Buildupregion
Bremsstrahlung underground
Bremsstrahlung tail
= R50
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Division of Medical Radiation Physics
Beam Characteristics: Depth dose electrons (2)
• Percentage Depth Dose (PDD) curve of electrons in water
• Electron energy ranging from 4 MeV to 30 MeV
Drel
100
50
Depth in water (cm)
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Division of Medical Radiation Physics
Beam Characteristics: Transverse beam profile (1)
• Idealised transverse beam profile• Relative dose as function of a
transverse coordinate • Transverse field size defined by
Collimators / Blocks / MLCs ("beam shaping" - explained later)
Penumbra region
Drel (%)
D0100
50
xField size
20
80
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Division of Medical Radiation Physics
Beam Characteristics: Transverse beam profile (2)
• Real profiles• Source to Surface Distance
(SSD) = 100 cm• 6 MV Photon beam
• Field size = 10 × 10 cm2• Depth = 1.5 cm … 30 cm
Increasing depth
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Division of Medical Radiation Physics
Beam Characteristics: Transverse beam profile (3)
• Real profiles• Source to Surface Distance
(SSD) = 100 cm• 6 MV Photon beam
• Depth = 1.5 cm• Field size =
3×3 cm2 … 27×27 cm2
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Division of Medical Radiation Physics
Beam Characteristics: Beam quality specification
• Superficial and orthovoltage X Rays:Half Value Layer (HVL) thickness
• Megavoltage X rays: Tissue to Phantom Ratio Q = TPR20,10 = D20 / D10Higher TPR20,10 => more penetrating beam
• Electrons:R50
SCD = 100 cm
d = 20 cmd = 10 cm
Field size = 10 × 10 cm2
D20 D10
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Division of Medical Radiation Physics
Beam Characteristics: Isodose distributions
• Isodose distribution: contours of equal relative dose
Photons (18 MV) Photons (6 MV) Electrons (20 MeV) Electrons (9 MeV)
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Division of Medical Radiation Physics
Beam shaping: Collimation (photons)
• Collimation- Primary collimators define maximum field size- Secondary collimators define actual field size in
two transverse directions
Target
Filter
Primary collimator
Secondary collimatorsY direction
Secondary collimatorsX direction
• Beam‘s eye view:
X
Y
0
X1 X2
Y1
Y2
Rotation of collimator assembly Monitor chambers
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Division of Medical Radiation Physics
Beam shaping: Collimation (electrons)
Secondary collimator
Electron Applicator
Scattered electrons used to saturate field edges
With electron applicator
Withoutelectron applicator
• Collimation- Primary collimators - Secondary collimators- Tertiary collimator
(electron applicator)
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Division of Medical Radiation Physics
Beam shaping: Blocks
• Block: device to adjust transverse field shape to target volume• Mold for block is cut from styrofoam• Block is made by casting Pb alloy (low melting point) in the mold• Individually manufactured for each field => time consuming and labour intensive
• Beam‘s eye view: • Cut A-A‘:
x
D
A A‘
B B‘
x
D
• Cut B-B‘:
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Division of Medical Radiation Physics
Beam shaping: Multi Leaf Collimators (MLC)
• MLC: device to adjust transverse field shape to target volume• Made from single small leafs• Each leaf can be moved to its individual position under computer control
• Beam‘s eye view:
A
• Leaf shape:
Cut
Leaf movement
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Beam shaping: Multi Leaf Collimators (MLC)
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Beam shaping: Wedges (1)
• Wedge: device or method to achieve a linear tilt of isodose curves in one transverse direction
• Wedge angle α: angle of isodose curve w.r.t. curve without wedge(at reference depth)
α
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Division of Medical Radiation Physics
Beam shaping: Wedges (2)
• Hard wedge: - Inserts in treatment head- Progressive decrease in intensity
across the beam- Reduction of beam intensity
=> wedge transmission factor- Influence on beam quality
• Dynamic wedge: - Closing motion of one
collimator jaw during irradiation- Modulation of dose rate
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Division of Medical Radiation Physics
Beam shaping: Wedges (3)
• What are wedges used for ?
• Application: 2 crossed fields
Volume to be irradiated
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Division of Medical Radiation Physics
Beam shaping: Wedges (4)
• Simple field setup: 2 crossed fields
+ =
Inhomogeneous PTV coverage
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Beam shaping: Wedges (5)
• Inhomogeneity due to depth dose distribution
Beam 1
Beam 2
Depth dose distribution of beam 1
Transverse profile of beam 2
Sum
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Division of Medical Radiation Physics
Beam shaping: Wedges (6)
• Inhomogeneity due to depth dose distribution
Beam 1
Beam 2
Depth dose distribution of beam 1
Transverse profile of beam 2 (with wedge)
Sum
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 72
Division of Medical Radiation Physics
Beam shaping: Wedges (7)• Application: varying tissue thickness; e.g. treatment of breast cancer• 2 tangential fields
Without wedges With wedges
Homogeneous PTV coverageOverdosage
1
2
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 73
Division of Medical Radiation Physics
Beam shaping: Dynamic beam delivery
• Beam shaping described so far: static• i.e. beam shape dose not change while radiation is on
(except dynamic wedge)• => 3D conformal RT
• Dynamic beam shaping methods / special delivery techniques:- IMRT- Conformal Arc- Rapid Arc- VMAT- Spot scanning with protons- Tomotherapy- …
• Will be described in dedicated talks later during this course• Just as example: IMRT
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 74
Division of Medical Radiation Physics
Beam shaping: IMRT (1)
• IMRT:Intensity Modulated Radio Therapy
• Highly conformal dose application• e.g. head & neck cancer
PTV: 54 Gy
Spinal cord: < 50 Gy
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 75
Division of Medical Radiation Physics
Beam shaping: IMRT (2)
• 7 fields
1
2
3
4
5
6
7
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 76
Division of Medical Radiation Physics
Beam shaping: IMRT (3)
• Each field is fluence modulated
1
2
345
6
7
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 77
Division of Medical Radiation Physics
Beam shaping: IMRT (4)
• Fluence modulation is achieved by moving the MLC while the beam is on
Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 78
Division of Medical Radiation Physics
The end ...
• Thank you for your attention !
• Questions ?
Beam Production, Characteristics and ShapingOutlineLiteratureX ray production:PrincipleX ray production:FocussingX ray production:EfficiencyX ray production:Rotating anode tubeX ray production:Depth dose curve X ray production:Photon energy spectrumX ray production:Filtering X ray production: Applications, Naming conventions60Co units: 60Co decay60Co units: 60Co sources60Co units: Shutter mechanisms60Co units:�Treatment headLinear accelerators:IntroductionLinear accelerators:OverviewLinear accelerators :RF GenerationLinear accelerators:RF Generation (1)Linear accelerators:RF Generation (2)Linear accelerators :Electron gunLinear accelerators:Electron gun (1)Linear accelerators:Electron gun (2)Linear accelerators:Electron gun (3)Linear accelerators:AccelerationLinear accelerators:Acceleration (1)Linear accelerators:Acceleration (2)Linear accelerators:Acceleration (3)Linear accelerators:Acceleration (4)Linear accelerators:�Acceleration (6)Linear accelerators:Acceleration (7)Linear accelerators:Acceleration (8)Linear accelerators :Beam transportLinear accelerators:Beam transport (1)Linear accelerators:Beam transport (2)Linear accelerators :Scatter foil / Target + FilterLinear accelerators:Scatter foil (1)Linear accelerators:Scatter foil (2)Linear accelerators:Scatter foil (3)Linear accelerators:Target / filter (1)Linear accelerators:Target / filter (2)Linear accelerators:Target / filter (3)Linear accelerators:Target / filter (4)Linear accelerators:Target / filter (5)Linear accelerators:Target / filter (6)Linear accelerators:Monitor chambersLinear accelerators:Monitor chambers (1)Linear accelerators:Monitor chambers (2)Linear accelerators:ManufacturersBeam Characteristics:Absorbed dose to waterBeam Characteristics:Depth dose photons (1)Beam Characteristics:Depth dose photons (2)Beam Characteristics:Depth dose photons (3)Beam Characteristics:Depth dose electrons (1)Beam Characteristics:Depth dose electrons (2)Beam Characteristics: Transverse beam profile (1)Beam Characteristics: Transverse beam profile (2)Beam Characteristics: Transverse beam profile (3)Beam Characteristics: Beam quality specificationBeam Characteristics:Isodose distributionsBeam shaping:Collimation (photons)Beam shaping:Collimation (electrons)Beam shaping:BlocksBeam shaping:Multi Leaf Collimators (MLC)Beam shaping:Multi Leaf Collimators (MLC)Beam shaping:Wedges (1)Beam shaping:Wedges (2)Beam shaping:Wedges (3)Beam shaping:Wedges (4)Beam shaping:Wedges (5)Beam shaping:Wedges (6)Beam shaping:Wedges (7)Beam shaping:Dynamic beam deliveryBeam shaping:IMRT (1)Beam shaping:IMRT (2)Beam shaping: IMRT (3)Beam shaping: IMRT (4)The end ...