Beam Production, Characteristics and Shaping...> 1 MeV Megavoltage X rays Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 12 Division of Medical Radiation

Division of Medical Radiation Physics

Beam Production, Characteristics and ShapingDr. Manfred Sassowsky

Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 2


Outline

X-ray production60Co unitsLinear AcceleratorsBeam characteristicsBeam shaping



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)



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-



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-


Anode

~ UH

U

Vacuum

X rays

e-

Uf

Guard electrode



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



X ray production: Rotating anode tube

AnodeCathode

Exit window

Dielectric cooling oil

Focal spot

Stator

Vacuum

Rotor



X ray production: Depth dose curve

• U < 100 kV: maximum dose practically at surface

Relative Dose

Depth in water (cm)



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



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



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



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)



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



60Co units: Shutter mechanisms

• Operated by electromotor• Safety mechanism: mechanical spring for emergency retraction

W = WolframUr = UraniumPb = Lead

Rotating cylinder Sliding drawer

source

source



Primarycollimator

Source

Fibre optic

Depleted uranium

Lead

Wolfram

Light bulb

Secondarycollimators

Display of field size

Penumbra trimmer

60Co units:Treatment head



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)



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



Linear accelerators : RF Generation


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



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



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



Linear accelerators : Electron gun


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



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


Anode

~ UH

U

Ug

Grid electrode

Accelerating waveguide

Electrons




• Grid voltage Ug set to zero during short time interval

• => Some electrons can pass grid, are accelerated towards anode

• Electron „bunch“


Anode

~ UH

U

Ug

Grid electrode


Electrons

time

Ug




• Periodic pulsing of grid voltage• Repeated electron bunches


Anode

~ UH

U

Ug

Grid electrode


Electrons

time

Ug



Linear accelerators: Acceleration


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



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




~AC generator

0Tt =

~AC generator

RFTTt 21

0 +=

~AC generator

RFTTt += 0

2RFTvL =Synchronicity condition:

L




• 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




• 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



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




• More advanced standing wave structure with „side coupled“ cavities

• Advantage: Can be made shorter than travelling wave structure (while obtaining the same electron energy)




• Typical time structure of electron beam:

Time



Linear accelerators : Beam transport


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



• 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



Linear accelerators: Beam transport (2)

• Steering coils for fine-tuning beam position and beam angle• Feedback from monitor chambers (explained later)



Linear accelerators : Scatter foil / Target + Filter


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



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



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



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



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



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




• 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




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




• Effects of electron-beam mis-steering

a) Nominal beamb) Beam inclined c) Beam displacedd) Beam divergent




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



Linear accelerators: Monitor chambers


Stand Gantry


RF

Beamtransport


Treatment couch

Powerconverters

Isocenter



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



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)



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- …



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 =



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




• 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




• 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



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



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)



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




• 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




• 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



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



Beam Characteristics: Isodose distributions

• Isodose distribution: contours of equal relative dose

Photons (18 MV) Photons (6 MV) Electrons (20 MeV) Electrons (9 MeV)



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



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)



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‘:



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



Beam shaping: Multi Leaf Collimators (MLC)



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




• 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




• What are wedges used for ?

• Application: 2 crossed fields

Volume to be irradiated




• Simple field setup: 2 crossed fields

+ =

Inhomogeneous PTV coverage




• Inhomogeneity due to depth dose distribution

Beam 1

Beam 2

Depth dose distribution of beam 1

Transverse profile of beam 2

Sum




• 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 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 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 shaping: IMRT (1)

• IMRT:Intensity Modulated Radio Therapy

• Highly conformal dose application• e.g. head & neck cancer

PTV: 54 Gy

Spinal cord: < 50 Gy




• 7 fields

1

2

3

4

5

6

7




• Each field is fluence modulated

1

2

345

6

7




• Fluence modulation is achieved by moving the MLC while the beam is on



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 ...

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Beam Production, Characteristics and Shaping...> 1 MeV Megavoltage X rays Beam Production, Characteristics and Shaping / 25.10.2011 / M. Sassowsky 12 Division of Medical Radiation