1. Ch 4. Clinical Radiation Generators

2. 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 Therapys
Grenz-Ray Therapy
Contact Therapy
Superficial Therapy
Orthovoltage Therapy or Deep Therapy
Supervoltage Therapy

3. Grenz-Ray Therapy Contact 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

4. 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

5. 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
Limitations of the treatment:
skin dose
Depth dose distribution
Increase absorbed dose in bone
Increase scattering

6. Supervoltage Therapy Energy: 500 – 1000 kV Technical problems
Insulating the high-voltage transformer
Conventional transformer systems were not suitable for producing potential > 300 kVp
The problem solved by invention of resonant transformer

7. 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

8. 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)

9. Van de Graaff Generator
Electrostatic accelerator
Energy of x-rays: 2 MV (typical), up to 10 MV
Limitations:
size
high-voltage insulation
No longer produced commercially
Technically better machine (e.g. Co-60 units & linear accelerators)

10. Linear Accelerator
Use high frequency electromagnetic waves to accelerate 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

11. 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

12. 行波式與駐波式加速管的比較

13. Fig 4.5. A block diagram of typical medical linear accelerator

14. 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)

15. 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
Electrons move in complex spirals toward the resonant cavities
Radiating energy in form of microwave

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

17. The Klystron Passed in the drift tube (field-free space)
Electrons produced by the cathode
Electrons are accelerated by –ve pulse into buncher cavity
Electrons arrive catcher cavity
Generate a retarding E-field
Electrons suffer deceleration
KE of electrons converted into high-power microwaves
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)
Some e- are speed up
Other are slowed down

18. 磁控管與調速管的比較

19. 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)

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

21. 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

22. Flattening(X-ray)：intensity uniform
Scattering(electron)：add scattering

23. Ion chamber monitor
1.dose rate
2.integrated dose
3.filed symmetry

24. Light field and Radiation field

25. Betatron Energy of x-rays: 6 – 40 MV Disadvantages: low dose rate
Small field size

26. Microtron
Electron accelerator which combines the principles of both linear accelerator and the cyclotron
Advantage:
1.Easy energy selection, small beam energy spread and small size.
2.Single Microtron to supply a beam to several treatment room.

27. 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

28. 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

29. 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

30. 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 (Ci/g)
Greater radiation output per curie
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

31. Cobalt-60 Unit Source Treatment beam Heterogeneity of the beam
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)

32. 鈷六十治療機

33. 鈷六十治療機

34. 直線加速器與鈷六十的比較

35. 直線加速器與鈷六十的比較

36. 直線加速器與鈷六十的比較

37. 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

38. 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

39. 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

40. 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 redefining the field
Should not be placed < 15 – 20 cm, excessive electron contaminants produced by the block carrying tray
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

41. THANKS