Clinical Radiation Generators
Yoichi Watanabe, Ph.D.
Masonic Cancer Center M10-M
(612)626-6708
watan016@umn.edu
http://www.tc.umn.edu/~watan016/Teaching.htm
RTT510, Fall Semester
Energy unit
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1 eV is the energy that an electron or a particle
with one electron charge gains in an electric
potential of 1 V.
1 eV = 1.602x10-19 J.
1 MeV = 106 eV.
Mass can be represented by eV (<= E=mc2).
Electron mass = 511.003 keV
Proton mass = 938.280 MeV
Neutron mass = 939.573 MeV
π0-meson (pion) mass = 134.96 MeV
1 amu = 931.502 MeV
Absorbed Dose
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Unit of dose is Gy (Gray).
1 Gy = 1 J /kg.
1 calorie (cal.) / 1 g of water raises the water
temperature by 1 degree.
1 cal = 4.18 J.
1 cGy = 0.01 Gy.
1 Gy = 1/4.18 cal/kg = 0.24 x10-3 cal/g. =>
0.00024 degree.
To kill cells, one needs 10 Gy (=1,000 cGy).
Depth dose of kVp X-rays
a) Grenz rays, HVL=0.04mm Al,
φ=33cm, SSD=10cm
b) Contact therapy, HLV=1.5mm Al,
φ=2cm, SSD=2cm
c) Superficial therapy, HVL=3mm Al,
φ=3.6cm, SSD=20cm
d) Orthovoltage, HVL=2mm Cu,
10x10cm2, SSD=50cm
e) Co-60 γ-rays, 10x10cm2, SSD=80cm
Requirements of
clinical radiation generators
1.
2.
3.
4.
5.
6.
7.
8.
High particle energy for penetration
High particle flux for sufficient dose rate
Energy efficient
Compact
Not too expensive
Reliable
Simple to operate
Safe
Brief History
1895
1913
1931
1932
1939
1946
1952
1956
1958
1959
1976
1990
K.Roentogen discovers X-rays.
W.E.Coolidge develops vacuum X-ray tube.
E.O.Lawrence develops a cyclotron.
1MV Van de Graaff accelerator installed, Boston (USA).
First medical cyclotron, Crocker (USA).
20MeV electron beam therapy with a Betatron, Urbana (USA).
First Co-60 teletherapy units , Saskatoon (Canada).
First 6MeV linear accelerator, Stanford (USA).
First proton beam therapy (Sweden).
First scanning electron beam therapy, Chicago (USA).
First pion beam therapy, LAMPF (USA).
First hospital based proton therapy, Loma Linda (USA).
Bremsstrahlung Process
Force
F=k
ze ⋅ Ze
r
r
2
F = ma
Acceleration
Ze ⋅ ze
a∝
m
X-ray intensity
Z 2 z 2e6
2
I rad ∝ (a ⋅ e ) =
m2
Principle of x-ray emission
Bremsstrahlung
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The process of bremsstrahlung (braking
radiation) is the result of radiative collision
between a high speed electron and a nucleus.
The probability of bremsstrahlung production
varies with Z2, where Z is the atomic number of
the target.
The efficiency (the ratio of output energy emitted
and to the input electron energy) is proportional
to Z and E (the energy of the electron).
Bremsstrahlung Process
Photon Angular Distribution
Bremsstrahlung (2)
X-rays are emitted more or less equally in
all directions at electron energy below
about 100 keV.
 As electron energy increases, the direction
of x-ray emission becomes increasingly
forward => transmission-type target.

Principle of electron acceleration
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Electrons (or charged particle) can be
accelerated in electric field.
F = qE
q
F
V
E
Basic components
Power supply
 Strong electric field generator
 Vacuum tube
 Particle (or electron) generator/source
 Target, to produce particles for therapy
 Beam collimator
 Beam modulator
 Beam controller
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Acceleration Methods
High voltage linear method
1.
Transformer => alternate current (AC).
Van de Graaff electrostatic generator.
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
3.
Linear RF method
Betatron <= magnetic induction
4.
Circular RF method
2.
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Microtron
Cyclotron (=> Synchrotron)
Types of radiation generators
kVp therapy machines
 Van de Graaff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

Kilovoltage Therapy Units
Photons are generated by hitting a solid
target with electrons via Bremsstrahlung
interactions.
 The electron energy is below 300 keV or
the acceleration potential is smaller than
300 kV.
 The photon energy spectrum is broad.
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Type of kV therapy
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Grenz-ray therapy: < 20 kV
Contact therapy: 40 kV ~ 50 kV
Superficial therapy: 50 kV ~ 150 kV
Orthovoltage (or deep) therapy: 150 kV ~ 500 kV
 Supervoltage therapy: 500 kV ~ 1000 kV
 Megavoltage therapy: 1 MV
Contact Therapy
Accelerating potential = 40 to 50 kV.
 Tube current = 2 mA
 SSD = 2 cm
 Beam hardening by 0.5- to 1.0-mm
aluminum filter.
 Useful for tumors not deeper than 1 to 2
mm.
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Superficial therapy
Accelerating potential = 50 to 150 kV.
 1- to 6-mm aluminum for beam hardening.
 Half-value layer (HVL) = 1- to 8-mm Al.
 Applicators or cones for field collimation.
 SSD = 15 to 20 cm.
 Useful for tumors confined to about 5-mm
depth.
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Orthovoltage therapy
Potential = 150 to 500 kV.
 Beam current = 10 to 20 mA.
 HVL = 1 to 4 mm Copper (Cu).
 SSD = 50 cm.

Philips RT250
Depth dose of kVp X-ray
100 kVp
300 kVp
Supervoltage therapy
Potential = 500 to 1000 kV.
 Resonant transformer is used.

Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

Van de Graaff generator
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An electrostatic accelerator
Can reach 35 MV.
Can produce beam current
of about 1 mA.
Voltage is limited by
insulation.
Not used for therapy now.
http://www.answers.com/topic/van-de-graaff-generator
Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

Betatron principle
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Faraday’s law of induction
Betatron in 1940
Siemens Betatron in 1952
15 MeV electron beam
Betatron
Betatron was the first electron accelerator
used for radiotherapy in early 1950s.
 An electron in a changing magnetic field
experiences accelerator in a circular orbit.
 Can produce 6 MeV to 40 MeV electrons.
 Electron beam current is low for photon
therapy.

Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

Microtron Principle
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Electrons are accelerated by the electric
field every time the electrons pass through
the resonator.
Microtron
Microtron is used to accelerate electrons.
 Electron energy spread is small.
 Easy to select energy.
 Simple.

Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

Modern Linear Accelerators
Siemens Primus
Varian Triology
Elekta Synergy
New Generation
Tomotherapy HiART
Accuray Cyberknife
Components of Linac
Power supply
 Electron source or gun
 RF (radio-frequency) wave generator
 Accelerator tube
 Treatment head (or gantry)
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Bending magnets
Target
Filters: flattening and electron foil
Primary collimator
Secondary collimators or primary jaws
Beam monitor ionization chamber
Linear accelerator (linac)
Karzmark and Pering, 1973
Siemens Accelerator
Varian HDX
Modulator
A power supply provides DC power to the
modulator.
 The modulator includes the pulse-forming
network (PFN) and hydrogen thyratron (a
switch tube).
 The modulator generates high-voltage
pulses, which are flat-topped DC pulses of
a few microseconds in duration.
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Structure of electron guns
Diode
Triode with control grid
Microwave Oven
Scientific American, Oct 2008
Magnetron
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Generates microwave pulses (3 GHz, one pulse
every 2 ms).
Provide microwaves to accelerating structures.
Used with low-energy linacs (6MV or lower).
Klystron
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Microwave amplifier.
(x100,000)
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Needs a low-power microwave
oscillator.
Basic ideas of linear accelerator
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Charged particles (or electrons) are
accelerated by the electric field of
radiofrequency (RF) waves.
Electrons
50 keV
E
5 – 20 MeV/m
~ 400 mA for photon beam
Principle of Acceleration
Person on a surfboard riding an ocean wave
and a fish below the waves.
Temporal structure of electron beam
3 ms
Time
3 GHz
Accelerator structure:
Traveling-wave type
Microwave power is fed to the structure.
 Waves travel through the accelerating
structure.
 The residual power is absorbed at the
distal end of the structure.
 Electrons injected at one end of the
structure from an electron gun travel to the
other end of the structure.

Traveling-wave
Karzmark and Pering, 1973
Traveling-wave guide
Accelerator structure
Standing-wave type
Microwave power can be fed anywhere
along the length of the structure.
 Combination of forward and reverse
traveling waves gives rise to standing
waves.
 The half of cavities have essentially zero
EM field; hence, those are moved off-axis.
So there are accelerating cavity and
coupling cavity (or bimodal design).
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Standing-wave
Karzmark and Pering, 1973
Standing-wave guide
Karzmark and Pering (1973)
TW vs. SW
The efficiency of conversion of RF power
to electron energy is about twice as high
as in SW as in TW.
 For the same beam energy, SW guide is
much shorter than TW guide.
 SW is many times more stable than TW.
 TW is less cost than SW.
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Accelerator head
Photon beam
For electron beam
 No target
 Scattering foil
 Electron cone
Kim et al, Med Phys 28,2457 (2001)
Monte Carlo Model of Linac Head
Monitor Ionization Chamber
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Counts the number of ion pairs produced in a
sealed air cavity by photons or electrons. (The
number of ion pairs is proportional to the particle
flux.)
Serves as beam-on-timer. Beam turns off when
a pre-set number of ion pairs (or monitor unit)
produced in the chamber. There are two monitor
chambers for safety.
Consists of four sectors to monitor symmetry
and flatness of the beam.
Multileaf Collimator
Depth dose characteristics of
electron and photon beams
Auxiliary Systems
Vacuum system generates vacuum in
klystron, electron gun, accelerator guide,
and the beam transport section.
 Water cooling system is used to cool the
accelerator structures, RF power source,
and electromagnet systems in the gantry.
 Dielectric gas, SF6 (w/ Klystron) or Freon
(w/ Magnetron), fills wave guides to
prevent electric discharge or arcing.

Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units
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Lorentz Force
𝐹𝐹⃗ = 𝑞𝑞𝐸𝐸 + 𝑞𝑞𝑣𝑣⃗ × 𝐵𝐵
Cyclotron
Charged particles are accelerated by
electric field cyclically.
 Charged particles fly in circular orbits in
magnetic field.
 The radius of the orbits increases as the
particle speed increases.
 Synchrotron was invented to overcome the
energy limit.
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Diagram of Cyclotron
Dee
High voltage
High frequency
oscillator
Target
http://www.physics.rutgers.edu/cyclotron/theory_of_oper.shtml
Medical applications of cyclotron
Used to generate high energy protons and
heavy ions for therapy.
 Used to accelerate deuterons to produce
neutrons.
 Used for the production of radionuclides.
i.e. for PET.
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Synchrotron
Synchrotron (2)
Heavy particle therapy
Particles heavier than electrons have
radiobiological advantages, or higher LET.
 Depth dose of heavy charged particle
shows Bragg peak.
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Neutrons
Protons
Heavy ions: 12C
Pions
Neutron generators

D-T generator -> 14 MeV neutrons
2
1
H + H → He + n
3
1
4
2
100 to 300 keV

Cyclotron -> 40 MeV neutrons
2
1H
(or
15 to 50 MeV
9
10
p )+ 4 Be→ 5 B
+n
Depth dose characteristics of
electron and photon beams
Depth dose of charged particles
Bragg peak
Spread Out Bragg Peak
(SOBP)
π
Siemens Medical Solutions, Inc.
−
A.R.Smith, Med Phys (1977)
Range of heavy charged particles
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The range of heavy charged particles is proportional to
the mass and inverse proportional to the square of the
electric charge for the same particle energy (i.e. energy
per nucleon).
The range of 160 MeV proton is 18 cm.
For the same initial velocity,
2
R1  M 1  Z 2 
 
= 
R2  M 2  Z1 
Particles with the same kinetic energy per nucleon
(MeV/u) have the same velocity.
150 MeV protons, 300 MeV deuterons, and 600 MeV
helium ions all have the velocity.
Negative pion therapy
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The mass of pion is 273 times of electron.
Negative pions are generated by colliding
protons (400 to 800 MeV) with a beryllium target.
Star formation takes place near the end of pion
passage via pion capture. Hence, the Bragg
peak is more pronounced.
Pion therapy is not currently pursued for clinical
uses because of low dose rates, beam
contamination, and high cost.
Proton accelerator

150 MeV to 250 MeV
Heavy ion therapy facility
Heavy Ion Medical Accelerator in Chiba
(HIMAC)
Linac
Synchrotron
Types of radiation generators
kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units
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Machines using radionuclides
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Gamma rays from Cs-137, Co-60, Ra-226.
Co-60 units are commonly used for therapy.
Radionuclides
Half-life
[years]
γ energy
[MeV]
Γ value
[Rm2/Ci hr]
[Ci/g]
Ra-226
0.5mm Pt
1622
0.83
0.825
0.98
Cs-137
30.0
0.66
0.326
50
Co-60
5.26
1.17, 1.33
1.30
200
Co-60 unit
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60
Co is generated from 59Co via 59Co(n,γ) 60Co
reactions.
The half-life for 60Co is 5.26 years.
60
Co decays to 60Ni with the emission of β
particles and two photons (1,17 MeV and 1.33
MeV) per disintegration.
The Co source is a cylindrical capsule of 1 cm
diameter and 1 cm long, causing relative large
penumbra.
β particles are absorbed in the Co-metal and
the stainless steel capsule.
Theratron
Sourcehead
Dose the source size matter?
W =D
SSD − SDD
SDD
Source diameter, D (= 1 to 2 cm)
SDD
Jaw
SSD
Penumbra width, W
CAX
Gamma Knife
Gamma Knife side view