Download Kilovoltage Units

The physics of Radiation Therapy, pp. 45 - 70
Chapter 4
Clinical Radiation Generators
1
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
2
X-ray machines for radiotherapy
The main components of a
radiotherapy x-ray
machine are:
• X-ray tube
• Ceiling or floor mount
for the x-ray
tube
• Target cooling system
• Control console
• X-ray power generator
3
The components of a
radiotherapy x-ray
machine:
• X-ray tube
• Applicators
4
The main components of
a typical therapy x-ray
tube are:
• Water or oil cooled
target (anode)
• Heated filament
(cathode)
5
X-ray machines for radiotherapy
With x-ray tubes the patient dose is delivered using a
timer and the treatment time must incorporate a shutter
correction time.
In comparison with diagnostic radiology x-ray tubes, a
therapy x-ray tube operates:
• At about 10% of instantaneous current.
• At about 10 times average energy input.
• With significantly larger focal spot and a fixed rather than
rotating anode.
6
Kilovoltage Units
• Up to above 1950
• X-rays generated at voltages up to 300 kVps
• Still some use in the present era, esp. treatment of
superficial skin lesions
• Kilovoltage Therapy
–
–
–
–
–
Grenz-Ray Therapy
Contact Therapy
Superficial Therapy
Orthovoltage Therapy or Deep Therapy
Supervoltage Therapy
7
Kilovoltage Units
• Grenz-Ray Therapy
– Energy : < 20 kV
– Very low depth of penetration
– No longer used in R/T
• Contact Therapy
–
–
–
–
–
Energy: 40 – 50 kV
Short SSD (< 2 cm)
Produces a very rapidly decreasing depth dose
Max irradiated tissue : skin surface
Application: Tumor not deeper than 1 – 2 mm
8
Kilovoltage Units
• Superficial Therapy
–
–
–
–
–
–
Energy: 50 – 150 kV
HVLs: 1.0- – 8.0-mm Al
Applicator or cone attached to the diaphragm
SSD: 15 – 20 cm
Tube current: 5 – 8 mA
Application: tumors confined to about 5-mm depth
9
Kilovoltage Units
• Orthovoltage Therapy or
Deep Therapy
–
–
–
–
Energy: 200 – 300 kV
Tube current: 10 – 20 mA
HVLs: 1 – 4 mm Cu
Cones or movable diaphragm
(continuous adjustable field size)
– SSD: 50 cm
– Application: tumor located <
2 –3 cm in depth
– Limitation of the treatment:
•
•
•
•
skin dose
Depth dose distribution
Increase absorbed dose in bone
Increase scattering
10
Kilovoltage Units
• Supervoltage Therapy
– Energy: 500 – 1000 kV
– Technical problem
• Insulating the high-voltage transformer
• Conventional transformer systems were not suitable
for producing potential > 300 kVp
– The problem solved by invention of resonant
transformer
11
Kilovoltage Units
• Resonant transformer units
– Used to generate x-rays from 300 to 2000 kV
At resonant frequency
1. Oscillating potential attains very high amplitude
2. Peak voltage across the x-ray tube becomes very large
12
Megavoltage Therapy
• X-ray beams of energy > 1 MV
• Accelerators or γray produced by radionuclides
• Examples of clinical megavoltage machines
–
–
–
–
–
Van de Graaff generator
Linear accelerator
Betatron
Microtron
Teletherapy γray units (e.g. cobalt-60)
13
Clinical x-ray beams
• In the diagnostic energy range (10 - 150 kVp) most
photons are produced at 90 from the direction of
electrons striking the target (x-ray tube).
• In the megavoltage energy range (1 - 50 MV)
most photons are produced in the direction of the
electron beam striking the target (linac).
14
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
15
Van de Graaff Generator
• Electrostatic accelerator
• Energy of x-rays: 2 MV
(typical), up to 10 MV
• Limiation:
– size
– high-voltage insulation
• No longer produced
commercially
– Technically better machine
(e.g. Co-60 units & linear
accelerators)
16
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
17
LINACS
Medical linacs are cyclic accelerators that accelerate
electrons to kinetic energies from 4 to 25 MeV using
microwave radiofrequency fields:
• 103 MHz : L band
• 2856 MHz: S band
• 104 MHz: X band
In a linac the electrons are accelerated following straight
trajectories in special evacuated structures called
accelerating waveguides.
18
Linac generations
During the past 40 years medical linacs have gone
through five distinct generations, each one increasingly
more sophisticated:
(1) Low energy x rays (4-6 MV)
(2) Medium energy x rays (10-15 MV) and electrons
(3) High energy x rays (18-25 MV) and electrons
(4) Computer controlled dual energy linac with electrons
(5) Computer controlled dual energy linac with electrons
combined with intensity modulation
19
Linear Accelerator
• Use high frequency electromagnetic waves to
acelerate charged particles (e.g. electrons) to high
energies through a linear tube
• High-energy electron beam – treating superficial
tumors
• X-rays – treating deep-seated tumors
20
Linear Accelerator
• Types of EM wave
1. Traveling EM wave
•
•
Required a terminating (“dummy”) load to absorb the
residual power at the end of the structure
Prevent backward reflection wave
2. Standing EM wave
•
•
•
Combination of forward and reverse traveling waves
More efficiency
– Axial beam transport cavities and the side cavities can
be independently optimized
More expensive
– Requires installation of a circulator (or insulator)
between the power source
– the structure prevent reflections from reaching the
power source
21
Linear Accelerator
Fig 4.5. A block diagram of typical medical linear accelerator
22
Accelerating waveguide
In the standing wave accelerating structure each end of the accelerating waveguide is terminated with
a conducting disk to reflect the microwave power producing a standing wave in the waveguide. Every
second cavity carries no electric field and thus produces no energy gain for the electron (coupling
cavities
In the travelling wave accelerating structure the microwaves enter on the gun side and
23
propagate toward the high energy end of the waveguide. Only one in four cavities is at any
given moment suitable for acceleration
Microwave power transmission
The microwave power produced by the RF generator is
carried to the accelerating waveguide through rectangular
uniform waveguides usually pressurized with a dielectric
gas (freon or sulphur hexafluoride SF6).
Between the RF generator and the accelerating waveguide
is a circulator (isolator) which transmits the RF power from
the RF generator to the accelerating waveguide but does
not transmit microwaves in the opposite direction.
24
25
The Magnetron
• A device that produces microwaves
• Functions as a high-power oscillator
• Generating microwave pulses of several
microseconds with repetition rate of several
hundred pulses per second
• Frequency of microwave within each pulse is
about 3000 MHz
• Peak power output:
– 2 MW (for low-energy linacs, 6MV or less)
– 5 MW (for higher-energy linacs, mostly use klystrons)
26
The Magnetron
The cathode is heated by an inner filament
Electrons are generated by
thermionic emission
Pulse E-field between cathode & anode
Electron accelerated toward the anode
Static B-field perpendicular to the plane of cavities
Electron move in complex spirals toward the resonant cavities
Radiating energy in form of microwave
27
The Klystron
• Not a generator of microwaves
• Microwave amplifier
– Needs to be driven by a low-power microwave
oscillator
28
The Klystron
Electrons produced by the cathode
Passed in the drift tube
(field-free space)
Electrons are accelerated by –ve pulse
into buncher cavity
Lower level microwave set up an
alternating E field across the buncher
cavity
Velocity of e- is altered by the action
of E-field (velocity modulation)
1. Some e- are speed up
2. Other are slowed down
Electrons arrive catcher cavity
1. Generate a retarding E-field
2. Electrons suffer
deceleration
3. KE of electrons converted
into high-power
microwaves
29
Schematic diagram of a modern
fifth generation linac
30
Electron beam transport
Three systems for electron
beam bending have been
developed:
• 90o bending
• 270o bending
• 112.5o (slalom) bending
31
The Linac X-Ray Beam
• Production of x-rays
– Electrons are incident on a target of a high-Z material
(e.g. tungsten)
– Target – need water cooled & thick enough to absorb
most of the incident electrons
– Bremsstrahlung interactions
• Electrons energy is converted into a spectrum of x-rays energies
• Max energy of x-rays = energy of incident energy of electrons
• Average photon energy = 1/3 of max energy of x-rays
• Designation of energy of electron beam and x-rays
– Electron beam - MeV (million electron volts,
monoenergetic)
– X-ray beam – MV (megavolts, voltage across an x-ray
tube, hetergeneous in energy)
32
Linac treatment head
Components of a modern linac
treatment head:
• Several retractable x-ray targets (one for
each x-ray beam energy).
• Flattening filters (one for each x-ray
beam energy).
• Scattering foils for production of clinical
electron beams.
• Primary collimator.
• Adjustable secondary collimator with
independent jaw motion.
• Dual transmission ionization chamber.
• Field defining light and range finder.
• Retractable wedges.
• Multileaf collimator (MLC).
33
Physical Wedge Beamline
34
Virtual Wedge Beamline
Dose Rate Control
MU/min
Jaw Speed Constant
mm/sec
35
Virtual Wedge Beamline
Dose Rate Control
MU/min
Jaw Speed Constant
mm/sec
36
Virtual Wedge Beamline
Dose Rate Control
MU/min
Jaw Speed Constant
mm/sec
37
Multi Leaf Collimator (MLC)
38
39
Siemens
Source
Source
MLC
MLC
Y
Jaw
X1
Varian
Elekta
Source
Y
Y
Jaw
YY
X2
39.2 cm
55.0 cm
Jaw
X
X
MLC
X
X
X
X
57.6 cm
Accessory
Accessory
Holder
Holder
Holder
Accessory
100 cm
Isocenter
1.0 cm
Resolution
29.2 cm
32 cm
43 cm
1.0 cm
1.0 cm
Resolution
Resolution
40
41
Lead or tungsten
Opening from 0 x 0
to 40 x 40 cm at SSD
100 cm
42
Production of clinical x-ray beams
Typical electron pulses arriving on the x-ray
target of a linac.
Typical values:
Pulse height: 50 mA
Pulse duration: 2 μs
Repetition rate: 100 pps
Period: 104 μs
43
44
Collimation System
In modern linacs the x-ray beam collimation is achieved
with
three collimation devices:
• Primary collimator.
• Secondary adjustable beam defining collimator
(independent jaws).
• Multileaf collimator (MLC).
The electron beam collimation is achieved with:
• Primary collimator.
• Secondary collimator.
• Electron applicator (cone).
• Multileaf collimator (under development).
45
46
47
Production of clinical electron beam
To activate the electron mode
the x-ray target and flattening
filter are removed from the
electron pencil beam.
Two techniques for producing
clinical electron beams from the
pencil electron beam:
• Pencil beam scattering with a
scattering foil (thin foil of lead).
• Pencil beam scanning with two
computer controlled magnets
48
49
Narrow pencil about 3
mm in diameter
Uniform electron
fluence across the
treatment field
e.g. lead
Electron scatter
readily in air
Beam collimator
must be achieved
close to the skin
surface
50
Dose monitoring system
Transmission ionization chambers, permanently
embedded in the linac clinical x-ray and electron
beams, are the most common dose monitors in
linacs.
Transmission ionization chambers consist of two
separately sealed ionization chambers with
completely independent biasing power supplies
and readout electrometers for increased patient
safety.
51
Dose monitoring system
Most linac transmission ionization chambers are
permanently sealed, so that their response is not
affected by ambient air temperature and pressure.
The customary position for the transmission
ionization chamber is between the flattening filter
(for x-ray beams) or scattering foil (for electron
beams) and the secondary
collimator.
52
Dose monitoring system
The primary transmission ionization chamber measures
the monitor units (MUs).
Typically, the sensitivity of the primary chamber
electrometer is adjusted in such a way that:
• 1 MU corresponds to a dose of 1 cGy
• delivered in a water phantom at the depth of dose maximum
• on the central beam axis
• for a 10x10 cm2 field
• at a source-surface distance (SSD) of 100 cm.
53
Dose monitoring system
Once the operator preset number of MUs has
been reached, the primary ionization chamber
circuitry:
• Shuts the linac down.
• Terminates the dose delivery to the patient.
Before a new irradiation can be initiated:
• MU display must be reset to zero.
• Irradiation is not possible until a new selection of
MUs and beam mode has been made.
54
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
55
Betatron
Betatron is a cyclic accelerator in which the electrons are
made to circulate in a toroidal vacuum chamber (doughnut)
that is placed into a gap between two magnet poles.
Conceptually, the betatron may be considered an analog of
a transformer:
• Primary current is the alternating current exciting the
magnet.
• Secondary current is the electron current circulating in the
doughnut.
56
Betatron
• Electron in a changing magnetic field
experiences acceleration in a circular orbit
Energy of x-rays:
6 – 40 MV
Disadvantage:
low dose rate
Small field size
57
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
58
Microtron
is an electron accelerator that combines the
features of a linac and a cyclotron.
The electron gains energy from a resonant wave guide
cavity and describes circular orbits of increasing radius
in a uniform magnetic field.
After each passage through the
wave guide the electrons gain an
energy increment resulting in a
larger radius for the next pass
through the wave guide cavity.
59
Microtron
• Electron accelerator which combines the principles
of both linear accelerator and the cyclotron
Advantage:
Easy energy selection, small beam energy spread
and small size
60
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
61
Cyclotron
• Charged particle accelerator
• Mainly used for nuclear physics research
• As a source of high-energy protons for proton
beam therapy
• Have been adopted for generating neutron beams
recently
62
Cyclotron
Structures
•
Short metallic cylinder divided into two section (Ds)
•
Highly evacuated
•
Placed between the poles of a direct current magnet
•
Alternating potential is applied between two Ds
63
Cyclotron
• In a cyclotron the particles are accelerated
along a spiral trajectory guided inside two
evacuated half-cylindrical electrodes (dees)
by a uniform magnetic field produced
between the pole pieces of a large magnet
(1 T).
64
Cyclotron
Positive charged particles (e.g. protons or deuterons) are
injected at the center of the two Ds
Under B-field, the particles travel in a circular orbit
Accelerated by E-field while passing from one D to the other
Received an increment of energy
Radius of its orbit increases
65
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
66
The important characteristics of radionuclides useful for
external beam radiotherapy are:
• High gamma ray energy (of the order of 1 MeV).
• High specific activity (of the order of 100 Ci/g).
• Relatively long half life (of the order of several years).
• Large specific air kerma rate constant.
Of over 3000 radionuclides known only 3 meet the
required characteristics and essentially only cobalt-60 is
currently used for external beam radiotherapy.
67
Machines Using
Radionuclides
• Radionuclides have been used as source of γrays
for teletherapy
• Radium-226, Cesium-137, Cobalt-60
•
60Co
has proved to be most suitable for external
beam R/T
• Higher possible specific activity
• Greater radiation output
• Higher average photon energy
Radionuclide
Radium-226 (filtered
by 0.5 mm Pt)
Cesium-137
Cobalt-60
I- Value
Half-Life
(Years)
γRay Energy
MeV
( Rm2_)
Specific Activity Achieved in
Practice (Ci/g)
1622
0.83 (avg.)
0.825
~ 0.98
30.0
5.26
0.66
1.17, 1.33
0.326
1/30
~ 80
~ 300
Ci – h
68
Teletherapy sources
Teletherapy radionuclides: cobalt-60 and cesium-137
• Both decay through beta minus decay
• Half-life of cobalt-60 is 5.26 y; of cesium-137 is 30 y
• The beta particles (electrons) are absorbed in the source capsule.
69
Teletherapy machines
Treatment machines used for external beam radiotherapy
with gamma ray sources are called teletherapy machines.
They are most often mounted isocentrically with SAD of
80 cm or 100 cm.
The main components of a teletherapy machine are:
• Radioactive source
• Source housing, including beam collimator and source movement
mechanism.
• Gantry and stand.
• Patient support assembly.
• Machine control console
70
Teletherapy machines
Cobalt-60 teletherapy machine, Theratron-780, AECL (now MDS Nordion), Ottawa, Canada
71
Cobalt-60 Unit
• Source
– From 59Co(n, γ) nuclear reactor
– Stable 59Co → radioactive 60Co
– In form of solid cylinder, discs, or pallets
• Treatment beam
60Co →60Ni + 0β(0.32 MeV) + γ(1.17 & 1.33 MeV)
• Heterogeneity of the beam
– Secondary interactions
– βabsorbed by capsule → bremsstrahlung x-rays (0.1MeV)
– scattering from the surrounding capsule, the source
housing and the collimation system (eletron contamination)
72
Teletherapy sources
To facilitate interchange of sources from one teletherapy
machine to another and from one radionuclide production
facility to another, standard source capsules have been
developed.
Teletherapy sources are cylinders with height of 2.5 cm
and diameter of 1, 1.5, or 2 cm.
• The smaller is the source diameter, the smaller is the physical
beam penumbra and the more expensive is the source.
• Often a diameter of 1.5 cm is chosen as a compromise between
the cost and penumbra.
73
Teletherapy sources
Typical source activity: of the order of 5 000 - 10 000 Ci
(185 - 370 TBq).
Typical dose rates at 80 cm from source: of the order of
100 - 200 cGy/min
Teletherapy source is usually replaced within one half-life
after it is installed. Financial considerations often result in
longer source usage.
74
Teletherapy source housing
The source head consists of:
• Steel shell with lead for shielding purposes
• Mechanism for bringing the source in front of the collimator
opening to produce the clinical gamma ray beam.
Currently, two methods are used for moving the
teletherapy source from the BEAM-OFF into the BEAMON position and back:
• Source on a sliding drawer
• Source on a rotating cylinder
75
Teletherapy source housing
Both methods (source-on-drawer and source-on-cylinder)
incorporate a safety feature in which the beam is terminated
automatically in the event of power failure or emergency.
When the source is in the BEAM-OFF position, a light
source appears in the BEAM-ON position above the
collimator opening, allowing an optical visualization of the
radiation field, as defined by the machine collimator.
76
Teletherapy source housing
Some radiation (leakage radiation) will escape from the
teletherapy machine even when the source is in the
BEAM-OFF position.
Head leakage typically amounts to less than 1 mR/h
(0.01 mSv/h) at 1 m from the source.
International regulations require that the average
leakage of a teletherapy machine head be less than 2
mR/h (0.02 mSv/h).
77
Collimator and penumbra
Collimators of teletherapy machines
provide square and rectangular radiation
fields typically ranging from 5x5 to 35x35
cm2 at 80 cm from the source.
78
Penumbra
•
The region, at the edge of a radiation beam, over
which the dose rate changes rapidly as function
of distance from the beam axis
1. Transmission penumbra
2. Geometric penumbra
79
Source
• Transmission penumbra
– The region irradiated by photons
which are transmitted through the
edge of the collimator block
– The inner surface of the blocks is
made parallel to the central axis of
the beam
– The extent of this penumbra will be
more pronounced for larger
collimator opening
– Minimizing the effect
Collimator
SDD SSD
• The inner surface of the blocks remains
always parallel to the edge of the beam
80
• Geometric penumbra
• Radiation source: not a point source
– e.g. 60 Co teletherapy → cylinder of diameter ranging
from 1.0 to 2.0 cm
From considering similar
triangles ABC and DEC
DE = CE = CD = MN = OF + FN – OM
AB CA CB OM
OM
AB = s (source diameter)
OF = SSD
DE = Pd ( penumbra)
Pd = s (SSD + d – SDD)
SDD
Parameters determine the width of
penumbra
81
• Geometric penumbra (con’t)
– Solutions
• Extendable penumbra trimmer
– Heavy metal bars to attenuate the beam in the penumbra
region
• Secondary blocks
– Placed closed to the patient for redifining the field
– Should not be placed < 15 – 20 cm, excessive electron
contaminants
– Definition of physical penumbra in dosimetry
• Lateral distance between two specified isodose curves
at a specified depth
– At a depth in the patient, dose variation at the field border
– Geometric, transmission penumbras + scattered radiation
produced in the patient
82
1. Kilovoltage Units
2. Van de Graaff Generator
3. Linear Accelerator
4. Betatron
5. Microtron
6. Cyclotron
7. Machine Using Radionuclides
8. Heavy Particle Beams
83
Heavy Particle Beams
• Advantage
– Dose localization
– Therapeutic gain (greater effect on tumor than on
normal tissue)
• Including
– neutrons, protons, deuterons, αparticles, negative pions,
and heavy ions
• Still experimental
• Few institutions because of the enormous cost
84
Neutrons
• Sources of high energy neutron beams
– D-T generator, cyclotrons, or linear accelerators
• D-T generators
2H
1
+ 31H → 24He + 01n + 17.6 MeV
– Monoenergetic (14 MeV)
– Isotropic (same yield in all directions)
– Major problem
• Lack of sufficient dose rate at the treatment distance
• 15 cGy/min at 1 m
– Advantage
• Its size is small enough to allow isocentric mounting on gantry
85
• Cyclotron
– Stripping reaction
2
H + 9Be → 10Be + 1n
1
4
5
0
– Mostly in forward direction
– Spectrum of energies (40% - 50% of deuteron energy)
Fig 4.15. Neutron spectra produced by deuterons on beryllium target
86
Comparative beam characteristics
n’s vs. Co-60
87
Protons and Heavy Ions
• Energy of therapeutic proton beams
– 150 – 250 MeV
• Sources: produced by cyclotron or linear
accelerator
Bragg peak
• Major advantage
– Characteristic distribution of
dose with depth
88
Comparative beam characteristics
Heavy charged particle beams vs. n’s
89
Main Interactions of Protons
p
p
• Electronic (a)
– ionization
– excitation
• Nuclear (b-d)
e
(a)
p
(b)
– Multiple Coulomb scattering
(b), small q
p
– Elastic nuclear collision (c), (c)
p’
large q
– Nonelastic nuclear
p
interaction (d)
(d)
p’
q
p
’
nucleu
es
g, n
nucleu90
s
Comparative beam characteristics
•Electron beams & protons
91
Why Protons are advantageous
Relatively low entrance dose
•
•
•
•
•
•
•
(plateau)
Maximum dose at depth
(Bragg peak)
Rapid distal dose fall-off
Energy modulation
(Spread-out Bragg peak)
RBE close to unity
10 MeV X-rays
Relative Dose
•
Modulated
Proton Beam
Unmodulated
Proton Beam
Depth in Tissue
92
1 mm
4 mm
93
• Range energy relationship for
protons
• Use to calculate the range for other
particles with the same initial
velocity
R1/R2 = (M1/M2) · (Z2/Z1)2
R1, R2 — particle range
M1,M2 — Masses
Z2, Z1 — the charges of the two particle
– e.g. Protons (150 MeV), Deuterons
(300 MeV), Helium ions (600 MeV)
have same range of about 16 cm
water
– Range  A/Z2
94
Negative Pions
• Pi meson (pion, π)
– Protons and neutrons are held together by a mutual
exchange of pi mesons
– Mass : 237x of electron
– Charge : π+, π-, π0
– Decay: π+ → μ+ + ν (mean life: 2.54 x 10-18)
π- → μ- + ν (mean life: 2.54 x 10-18)
π0 → hν1 + hν2(mean life: 2.54 x 10-18)
μ— mesons; ν — neutrinos
95
Negative Pions
• Pi meson (pion, π) (con’t)
– Sources:
• nuclear reactor
• Cyclotron or linear accelerator with protons (400 – 800 MeV)
and beryllium as target material
– Energy range of pion interest in R/T — 100 MeV
– Range in water about — 24 cm
– Problems
• Low dose rates
• Beam contamination
• High cost
96