Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Yr 1: Radiotherapy > External beam equipment > Flashcards

External beam equipment Flashcards

1
Q

What differences are there between kV and MV RT equipment? (4 marks)

A
  • kV are usually fixed emissions such as Co-60 sources (used in stereotactic systems e.g. GammaKnife) 1.17 and 1.33 MeV. Used for small tumours e.g. brain. Photons only produced.
  • MV systems, such linacs, are multi-energy and are generally used for deeper treatments, generally more expensive and more common. Photons and electrons produced.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What differences are there between kV diagnostic and kV RT treatment machines? (2 marks)

A
  • The angle of the anode is more shallow in RT treatment machines to reduce the anode-heel affect as this causes a variation in the dose distribution across the treatment field.
  • The larger focal spot size is less of an issue in RT treatments as image quality is not the aim of treatment. Whereas in diagnostic a larger focal spot is a problem as it increases the geometric penumbra produced.
  • For RT equipment there are changeable applicators and filters.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

For MV beams, what options are there in terms of equipment? (6 marks)

A
  • Linacs, multi-modal (photons and electrons, deeper and more complex treatments possible (VMAT), more expensive and more common.
  • Fixed emission systems such as Co-60 sources used in GammaKnife, 1.33 and 1.17 keV. More specialised tratments.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the isocentre? (3 marks)

A
  • It is a point in space which all axes rotate around (gantry, head, couch etc)
  • In reality it forms a sphere with a small diameter (0-2 mm)
  • Generally set at 100 cm from the source…
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Sketch the basic components of a linac. (6 marks)

A
  • Modulator (PFN and thyratron)
  • Microwave generator
  • Electron gun
  • Waveguide
  • Bending magnet
  • Head
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are the main components of a modulator and an electron gun (sketch it)? (5 marks)

A

Modulator = PFN and thyratron

e gun = cathode (concave and at 20 kV), heater (1000 C), anode (0 V), grid for rapid switching (matches magnetron, more negative than cathode to focus e.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

How does a waveguide work? What types are there and how do they differ? (6 marks)

A
  • Basic operation is that electrons are injected with microwaves, energy from the microwaves is transferred to the electrons accelerating them to close to the speed of light by the time the electrons exit the WG. The electrons bunch up due to the microwaves and gain energy and mass as they travel down the WG.
  • Two types are standing or travelling waveguide.
  • Travelling WG are longer and more simple in design, the microwaves must be injected at the end.
  • Standing WG are more complex, but shorter and the microwaves can be injected at any point.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe the purpose of the bending magnet? (2 marks)

A
  • Used for the practical reason that the waveguide would be too long to be positioned vertically.
  • Also used as an energy discriminator as electrons which are travelling too slowly are bent too much, and fast electrons are not bent enough, so they do not strike the target.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Sketch and explain how a magnetron works. (5 marks)

A
  • Electrons generated via thermionic emission from cathode
  • e’s follow spiral path due to action of applied B field and pulsed E-field
  • Generates E-field across cavities
  • e’s resonant frequency set by the cavities amplifying the power of the microwaves produced via a feedback mechanism…
  • e’s then injected into the WG
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Sketch and explain how a klystron works? (5 marks)

A
  • Low power microwaves enter the klystron into the buncher cavity
  • Here electrons, generated from a cathode via thermionic emission, bunch at the frequency of the microwaves
  • This causes an increase in the E-field density
  • As electrons and the microwaves travel across the drift tube and into the catcher cavity energy is imparted from the electrons to the microwaves
  • The catcher cavity is tuned to resonate at the buncher frequency
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Sketch the main head components for photons and electron beams. (7 marks)

A
  • Target
  • Primary collimator
  • Flattening filter or scattering foils for e’s
  • MU chamber
  • Secondary collimators (jaws and MLCs)
  • Accessory holder (applicators, wedges etc)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the purpose of the target and how does it work? (4 marks)

A
  • Used as a transmission target
  • forward peaked, to produce bremstrahlung x-rays
  • High Z material with a high melting point, such as tungsten or steel
  • Water cooled
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the purpose of the primary collimators? (3 marks)

A
  • Determines the maximum extent of the treatment field
  • Usually fixed in position
  • High Z and density
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the purpose of the flattening filter and how is it designed? (4 marks)

A
  • Conical, high Z material used to flatten the beam profile
  • Used for simplification of treatment calculations
  • Not really required anymore with VMAT treatments and computation calculations
  • The height and thickness is energy specific
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How do electron foils work and what is their purpose? (4 marks)

A
  • Low Z (Al-13), used to broaden (primary) and flatten electron beam (secondary)
  • Initially the electron beam is a narrow pencil beam (1-3 mm)
  • Designed to maximally scatter but minimally attenuate the beam
  • Energy specific filters are mounted on a carousel
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What purpose does an MU chamber serve and how is it constructed? (4 marks)

A
  • Used to control when the beam should terminate (i.e when a set MU number has been reached)
  • There is a primary and secondary chamber
  • Primary is for beam control and radial flatness and symmetry measurements
  • Secondary is for redundancy (back-up) and for transverse flatness and symmetry
17
Q

Why might an MU chamber for electrons differ from that used for photons? (3 marks)

A
  • Photons are more penetrative than electron beams, therefore the walls can be thicker. This allows temperature and pressure corrections not to have to be applied.
  • For electron beams, the walls must be thin as the beam is easily attenuated. A sealed chamber is more prone to distortions in shape, therefore, the chamber must not be sealed…
  • Not always the case for all manufacturers…
18
Q

Explain the purpose of secondary collimation and how it works. (3 marks)

A
  • Secondary collimators consist of thick, high Z, jaws which determine the extent of the treatment field
  • One set of the jaws can be replaced by MLCs
  • The MLCs can also sometimes have back-up jaws to minimise inter-leaf leakage
19
Q

What are MLCs for and how are they designed? (3 marks)

A
  • Used for greater beam conformance
  • Typically 30-80 leafs in a MLC around 0.5 cm in width
  • Angled or designed to minimise inter-leaf leakage

Yr 1: Radiotherapy (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions