1. Physics of Radiation Therapy
Lecture III: X-ray Production/Clinical Radiation Generators

2. Production of X Rays The X-Ray Tube Components
Glass tube – maintains vacuum necessary to minimize electron interactions outside of the target area
Cathode – contains filament and focusing cup
Anode – contains x-ray target
Figure 3.1 from Faiz Khan

3. The X-Ray Tube The Cathode
Tungsten filament (high melting point – 3370 ° C)
Thermionic emission – electron production as a consequence of heating
Focusing cup – “directs” electrons to anode
Dual filaments (diagnostic tubes) – necessary to balance small focal spots and larger tube currents

4. The X-Ray Tube The Anode Tungsten target Heat dissipation
High melting point
High Z (74) – preferred since bremsstrahlung production a Z2
Heat dissipation
Copper anode – heat conducted outside glass into oil / water / air
Rotating anode (diagnostic tubes) – larger dissipation area
Anode hood – copper and tungsten shields intercept stray electrons and x rays

5. 0.1 – 2.0 mm in diagnostic tubes,
The Anode
Focal-spot size
Line focus principle
apparent focal spot (a) is smaller then actual (A) due to target angle q :
a = A sin q
Apparent focal spots
0.1 – 2.0 mm in diagnostic tubes,
5 – 7 mm in therapy tubes
Figure 3.2 from Faiz Khan

6. The Anode
Heel effect
Since the x-rays are produced at various depths in the target, they suffer varying amounts of attenuation in the target
There is greater attenuation for x-rays coming from greater depths than those from near the surface of the target.
Consequently, the intensity of the x-ray beam decreases from the cathode to the anode direction of the beam.
This variation across the x-ray beam is called the heel effect.

7. Basic X-Ray Circuit Consists of two parts:
High-voltage circuit – provides x-ray tube accelerating potential
Filament circuit – provides filament current
Figure 3.3 from Faiz Khan

8. Basic X-Ray Circuit
Voltages can be increased or decreased using “step-up” or “step-down” transformers
Wire coils wound around iron rings
Coils create a magnetic field within the ring
Current and voltages (V) on opposite sides (primary and secondary) of the ring are proportional to the number of “turns” (N):
VP / VS = NP / NS
Stepwise and continuous voltage adjustments can be made with autotransformers and rheostats

9. Basic X-Ray Circuit Voltage Rectification
Alternating current is used to energize x-ray tubes
X-ray tubes are designed to operate at a polarity where the anode is positive
Rectifiers permit current flow in only one direction
Rectifiers placed in series in HV circuit provide half-wave rectification

10. Basic X-Ray Circuit
Rectifiers can be arranged in a “diamond” arrangement to provide full-wave rectification – conduction during both halves of alternating (AC) voltage
Figure 3.4 from Faiz Khan

11. X-Ray Production Bremsstrahlung (“braking” radiation)
Electromagnetic radiation emitted when an electron losses energy as a consequence of coulomb interaction with the nucleus of an atom
Figure 3.6 from Faiz Khan

12. X-Ray Energy Spectrum
The bremsstrahlung photon’s energy is equal to the difference between electron’s incident and final energies
This leads to an “energy spectrum” (the Kramer’s spectrum):
IE = K Z (Em- E)

13. X-Ray Energy Spectrum Characteristic X Rays
Ka x rays – most “intense” transition to K-shell (most commonly by transitions from adjacent (L) shell), Kb x rays – second most intense, etc.
For tungsten: Ka ≈ 60 keV, Kb ≈ 70 keV
Figure 3.8 from Faiz Khan

14. X-Ray Angular Distribution
Angular distribution of x rays
Angular distribution becomes more “forward peaked” as the electron energy increases
Figure 3.7 from Faiz Khan

15. X-Ray Spectrum X-Ray Spectrum
Composite of Kramer’s spectrum and characteristic x rays
Filtration reduces lower-energy component
Rule of thumb is average energy ≈ 1/3 maximum energy
Half Value Layer (HVL) is a common descriptor
Figure 3.9 from Faiz Khan

16. Clinical Radiation Generators
Treatment Unit
Kilovoltage Range
Half-Value Layer
Clinical Use
Grenz Ray
» 20 kVp
≈ mm Al
Dermatological Use
Contact Therapy
40-50 kVp
≈ 1-2 mm Al
Few mm
Superficial Therapy
kVp
≈ 1-8 mm Al
Skin Lesions
Orthovoltage
kVp
≈ 1-4 mm Cu
Deep Therapy
Supervoltage
kVp
≈ 10 mm Pb
Replaced by Co-60

17. Clinical Radiation Generators

18. The Linear Accelerator

19. The Linear Accelerator Components
Microwave Power Section
DC Power Section
Treatment Head
Accelerator Guide

20. The Linear Accelerator
DC Power Section
Produces properly-shaped pulses of DC power; these pulses are shaped in the pulse-forming network of the modulator and delivered to the electron gun and microwave power section at the proper frequency through a high-voltage switching device (the thyratron).

21. The Linear Accelerator
Microwave Power Section – Provides microwave power amplification (utilizing either a magnetron or klystron) and transmits the amplified microwaves to the accelerator guide.
Accelerator Guide – A cylindrical tube in which electrons, injected by the electron gun, are accelerated by the amplified microwaves. The accelerated electrons exit the waveguide and enter the treatment head.

22. The Linear Accelerator
Treatment Head
Contains the beam shaping, steering, and control components of the linear accelerator. These components are: the bending magnet, x-ray target, electron scattering foils (most accelerators), x-ray flattening filter, dose monitoring chambers, and beam collimation system.

23. Microwave Power
Electrons gain energy through continued exposure to an increasing electric field
The process is analogous to a “surfer riding a wave”
Microwave cavities create that environment – they are machined to dimensions that resonate at microwave frequencies (S-Band accelerators – 3000 MHz)

24. Microwave Power
In a fashion similar to that by which electrons gain energy from microwaves, microwave power can be amplified by the deposition of electron kinetic energy if electrons (grouped in “buncher” cavities) arrive in “catcher” cavities at the cavities’ resonant frequencies
Figure 4.7 from Faiz Khan

25. Electron Acceleration
Electrons can be accelerated in microwave cavities that are arranged in a linear configuration such that the microwaves’ “phase velocity” is matched to the velocity of the electrons traveling through the guide
(Note that energy gain will be proportional to waveguide length)
Figure 1-11 from Hendee

26. The Treatment Head
Shielded Housing – lead shielding reduces unwanted radiation
Bending Magnet – provides electron energy selection
the magnetic field intensity B is set such that electrons possessing the appropriate energy (momentum mev) are bent through the radius r that allows passage through the magnet’s exit port:
mev µ B r

27. The Treatment Head

28. The Treatment Head
X-Ray Target – transmission-type tungsten target in which electron produce bremsstrahlung radiation; inserted only during x-ray beam production, removed during electron-beam production
Flattening Filter – (photon beams) metal filter placed in the x-ray beam to compensate for the “forward peaked” photon distribution and produce a “flat” beam

29. The Treatment Head
Scattering Foils – (electron beams) thin metallic foils inserted in the electron beam to spread the beam and obtain a uniform electron fluence
Monitoring Chambers – transmission ionization chambers used to monitor dose rate, total integrated dose and beam symmetry.
Collimation System – fixed and movable beam limiting devices, normally made of lead, used to shape and size the beam

30. Cobalt-60 (Megavoltage unit)
Co-60 is most commonly used because of its superior specific activity per gram, its greater beam intensity per curie, and higher average photon energy.
A typical Co-60 source is 1 to 2 cm in diameter, and possesses an activity of about 3000 to 5000 Curies.
Co-60
The source activity is limited by the physical size of the source.
The large size of the Co-60 source produces a significant geometric penumbra (will be discussed later on)
Does not give appropriate dose profile for deep seated tumors (in thoracic area)
Replaced by linear accelerators