Radiotherapy Equipment Flashcards
What are the differences between diagnostic and therapeutic X-Ray systems?
Diagnostic -
- Rotating anode at step angle (6-15deg)
- Adjustable light-beam diaphragm/collimators
- Target at positive voltage
- Typically fixed filtration
Therapeutic:
- Fixed target at shallow angle (30-40deg)
- Interchangeable applicators
- Target at ground (0V)
- Interchangeable filter
Why is Tungsten used as the cathode in a therapeutic X-Ray set?
High melting point.
High atomic number - low binding energy of outer shell electrons.
By which process are electrons released from a tungsten filament?
Thermonic Emission
How does the filament current affect the intensity of the x-ray beam?
As the filament current increase numer of electrons accelerated per unit time increases.
X-Ray intensity is the number of photons of a given energy per unit area per unit time.
More electrons make more photons, more photons give more total-energy to beam.
How is the intensity of a beam proportional to the voltage across the tube?
Proportional to kV^2
Why is a non-rotating anode feasible in a therapeutic X-Ray set?
Small focal spot (required for sharp images) not needed. Energy can be deposited over a larger area.
What percentage of electrons produce either characteristic X-Rays or bremsstrahlung at therapeutic energies?
1%
Why are additional filters added to the end of the X-Ray set?
Harden the beam by removing unwanted low-energy “soft X-rays” that only contribute to skin dose.
What are two sources of inherent filtration in a therapeutic X-Ray set?
Filtration from the anode itself.
Filtration through Beryllium window.
What functions do the applicators provide?
Collimation of the therapeutic beam.
Keeps Fixed SSD.
What are the differences between at Co-60 Unit and a Linac?
Co-60: - Fixed energy (1.17 & 1.33MeV) - Single modality - Cheaper - Specialised Systems - Gammaknife Linac: - L-Shaped gantry - Multi-Modality and varying energy - Better penetration than Co-60 - Specialised systems - Tomotherapy, Cyberknife.
What are the main components of a Linac gantry?
- Modulator
- Electron gun
- Microwave Generator
- Accelerating Waveguide
- Bending Magnet
What does a modulator primarily consist of?
- Pulse forming Network
- Thyratron (high-speed switch)
What are the main components of the electron (triode) gun?
- Heater (element)
- Cathode
- Grid
- Anode
What shape is the cathode and what is the reason for this?
Concave shape focusses electron emissions
What is the function of the grid?
High-speed switching of +/- potential
Pulses adjusted to phase match electron injection with microwaves
What are the two devices used to generate microwaves in a linac?
Magnetrons and Klystrons
How does a magnetron generate a microwave pulse?
Cathode releases electrons through thermonic emission
Electrons follow a spiral path due to a combination of a pulse E-field and a static B-field
This creates an electric field across the cavities
Width of cavities determines the resonant frequency
At resonant frequency electrons bunch up and in circulating create feedback loop which amplifies power
How does a klystron generate an amplified microwave signal?
- Low power microwaves and electrons injected into first cavity
- Microwaves bunch electrons together at input frequency
- Increases electric field density, amplifies e-field of microwaves
- “Catcher cavity” tuned to resonate at bunch frequency funnels high powered microwaved into the waveguide
What are the differences between a travelling and a standing wave guide?
Travelling waveguide has a simpler design but is longer.
A standing waveguide can have the microwave injected at any point, not just the gun end
At what speed do the electrons leave the buncher section of an accelerating waveguide?
0.99c
Why are the cavities in the buncher section shorter?
Reduces microwave phase velocity to 0.4c similar velocity to that of electrons
Once the cavity diameter becomes constant, how do the electrons gain energy?
Through mass increase
What cavity parameter does the resonant frequency depend on?
Cavity diameter
How does a bending magnet act as an energy discrimiator?
Electrons with too high an energy are bent too much, and those with too lottle energy aren’t bent enough to reach the linac head
What are three common types of bending magnet?
90 degrees
112.5 degree slalom
270 degree doubly achromatic
What advantage does a 270deg bending magnet have over a 90deg?
270 degree ensures that electrons exit the system at the same point and at the same angle ensuring a small focal spot size
90 degree would require excessive control over electron energy to avoid focal spot broadening
What are the main components of a Linac head when using photons?
- Primary, secondary and tertiary collimators
- Photon target
- Flattening filter
- Monitor chamber
- Physical wedge
- Accessory holder
What are the main components of a Linac head when using electrons?
- primary, secondary and tertiary collimator
- scattering foils
- monitor chamber
- applications/cut-outs
What is the purpose of the primary collimator?
Defines the maximum extend of the treatment field
What are important properties for a Linac photon target?
- High Z
- High density
- High melting point
What radiation is produced from a Linac target?
Bremsstrahlung
What is the purpose of flattening filter?
Creates equal intensity across beam profile
What is the purpose of the two electron foils?
First broadens out the pencil beam from the gantry
Second flattens the broadened beam
What are important qualities for an electron foil?
- Low Z(Al)
- Maximise scatter, minimise attenuation
- Minimal x-ray production
What are the differences between a photon and an electron monitoring chamber?
Electron chambers require thin walls to minimse attenuation, this could lead to buckling due to changes in atmospheric temperature and pressure, so are unsealed
Photon chambers can have thick walls without much attenuation, so are sealed
Why are there two monitor chambers in a Linac?
Primary chamber measures dose/dose rate, and radial symmetry
Secondary monitor measures dose/dose rate (redundancy) and transverse symmetry and flatness
What is the purpose of the secondary collimators?
Defines the 2-D treatment field
What is the pulse repetition frequency of a modulator?
200-300Hz
What is the function of a modulator?
Supplies 1us pulses to electron gun and Microwave generating system cathodes.
In the linac treatment head:
electron scattering foils are always made of low atomic number materials?
for a dual energy machine, the same flattening filter is used for each MV photon energy?
the beam spectrum produced in the target depends only on the kinetic energy of the incident electrons?
electron beam currents to produce electron beams are 2-3 orders of magnitude higher than for photons?
the majority of incident electron energy is dissipated as heat in the MV transmission target?
the majority of incident electron energy is dissipated as heat in the MV transmission target
Concerning the construction of a linear accelerator:
The primary collimator has a fixed aperture and is made of high atomic number material?
X-ray production is primarily achieved by photoelectric interactions in the target?
Targets are usually made of low atomic number materials?
To achieve the same energy, the accelerating structure of a standing wave guide is longer than that of a travelling wave guide?
Flattening filters are shaped to preferentially attenuate radiation at the edges of the treatment bea
The primary collimator has a fixed aperture and is made of high atomic number material
When changing from photon to electron mode in a linear accelerator
A flattening filter is required to improve beam homogeneity?
The vacuum inside the waveguide is lost when the x-ray target is retracted?
The electron gun filament current is decreased?
Monitor chambers within the treatment head are no longer required?
Failure to retract the x-ray target would result in an extremely high x-ray dose?
The electron gun filament current is decreased
Adding aluminium filtration to a 100 kVp x-ray beam will
Marginally decrease the percentage depth dose at any given point?
Change the spectrum of the therapy beam?
Degrade the beam quality as demonstrated by a decreased half value thickness?
Increase the dose rate by acting as a scattering foil?
Differentially remove high energy photons by photoelectric absorption?
Change the spectrum of the therapy beam
