1. Ch 4. Clinical Radiation Generators
The physics of Radiation Therapy, pp
Ch 4. Clinical Radiation Generators

2. Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

3. 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

5. 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

6. 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

7. 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

9. 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

10. Kilovoltage Units Resonant transformer units
Used to generate x-rays from 300 to 2000 kV
At resonant frequency
Oscillating potential attains very high amplitude
Peak voltage across the x-ray tube becomes very large

11. 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)

12. Van de Graaff Generator
Kilovoltage Units
Van de Graaff Generator
Linear Accelerator
Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

13. 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)

14. Linear Accelerator Kilovoltage Units Van de Graaff Generator Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

15. 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

16. Linear Accelerator
Fig 4.5. A block diagram of typical medical linear accelerator

17. 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

24. 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)

25. 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

26. The Klystron Not a generator of microwaves Microwave amplifier
Needs to be driven by a low-power microwave oscillator

27. 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
Electrons arrive catcher cavity
Generate a retarding E-field
Electrons suffer deceleration
KE of electrons converted into high-power microwaves
Velocity of e- is altered by the action of E-field (velocity modulation)
Some e- are speed up
Other are slowed down

28. Auxiliary system
The linac auxiliary system consists of several services that are not directly involved with electron acceleration, yet make the acceleration possible and the linac viable for clinical operation.
● A vacuum pumping system producing a vacuum pressure of ~10–6 torr in the accelerating guide and the RF generator;
● A water cooling system used for cooling the accelerating guide, target, circulator and RF generator;
● An optional air pressure system for pneumatic movement of the target and other beam shaping components;
● Shielding against leakage radiation.

30. 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)

31. linac treatment head 31

32. Lead or tungsten
Opening from 0 x 0 to 40 x 40 cm at SSD 100 cm

33. 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

34. The Effect of Flattening Filter34

35. Vertical and Horizental Chambers

36. 36

37. Linac generations
● Low energy photons (4–8 MV): straight-through beam; fixed flattening
filter; external wedges; symmetric jaws; single transmission ionization
chamber; isocentric mounting.
● Medium energy photons (10–15 MV) and electrons: bent beam; movable
target and flattening filter; scattering foils; dual transmission ionization
chamber; electron cones.
● High energy photons (18–25 MV) and electrons: dual photon energy and
multiple electron energies; achromatic bending magnet; dual scattering
foils or scanned electron pencil beam; motorized wedge; asymmetric or
independent collimator jaws.
● High energy photons and electrons: computer controlled operation;
dynamic wedge; electronic portal imaging device (EPID); multileaf
collimator (MLC).
● High energy photons and electrons: photon beam intensity modulation
with MLC; full dynamic conformal dose delivery with intensity
modulated beams produced with an MLC.

38. 38

39. Sections of a Linac
The linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine:
(I) gantry
(2) gantry stand or support
(3) modulator cabinet
(4) patient support assembly, i.e., treatment couch
(5) control console 39

41. Betatron Kilovoltage Units Van de Graaff Generator Linear Accelerator
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

42. 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

43. Microtron Kilovoltage Units Van de Graaff Generator Linear Accelerator
Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

44. 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

45. Cyclotron Kilovoltage Units Van de Graaff Generator Linear Accelerator
Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

46. 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

47. 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

48. 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

49. Machine Using Radionuclides
Kilovoltage Units
Van de Graaff Generator
Linear Accelerator
Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

50. 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
Half-Life
(Years)
γRay Energy
MeV
I- Value
( Rm2_)
Ci – h
Specific Activity Achieved in Practice (Ci/g)
Radium-226 (filtered by 0.5 mm Pt)
1622
0.83 (avg.)
0.825
~ 0.98
Cesium-137
30.0
0.66
0.326
~ 50
Cobalt-60
5.26
1.17, 1.33
1/30
~ 200

51. 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)

52. Cobalt-60 teletherapy machine
Theratron-780, AECL (now MDS Nordion), Ottawa, Canada

53. Methods for moving the teletherapy source from the BEAM-OFF into the BEAM-ON position and back:
Source on a sliding drawer Source on a rotating cylinder

54. 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

55. Transmission penumbra
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
The inner surface of the blocks remains always parallel to the edge of the beam
Collimator
SDD
SSD

56. Radiation source: not a point source
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

57. 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

58. Heavy Particle Beams Kilovoltage Units Van de Graaff Generator
Linear Accelerator
Betatron
Microtron
Cyclotron
Machine Using Radionuclides
Heavy Particle Beams

59. 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

60. Neutrons Sources of high energy neutron beams
D-T generator, cyclotrons, or linear accelerators
D-T generators
2H + 3H → 4He + 1n 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

61. Mostly in forward direction
Cyclotron
Stripping reaction
2H + 9Be → 10Be + 1n
Mostly in forward direction
Spectrum of energies (40% - 50% of deuteron energy)
Fig Neutron spectra produced by deuterons on beryllium target

62. Protons and Heavy Ions Energy of therapeutic proton beams
150 – 250 MeV
Sources: produced by cyclotron or linear accelerator
Major advantage
Characteristic distribution of
dose with depth
Bragg peak

63. 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

64. 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

65. 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