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Linear accelerators Flashcards

1
Q

what are the radiotherapy techniques?

A
  • IMRT (intensity modulated radiotherapy): static gantry angle with moving MLC’s
  • VMAT (volumetric modulated arc therapy): moving gantry and MLC’s
  • SBRT (stereotactic body radiotherapy): smaller fields, higher doses (3-5 fractions)
  • conventional/ static beam
  • IGRT (image guided radiotherapy); SRS (stereotactic radio surgery): single fraction
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2
Q

what is the bunker

A

a housing system for the linac designed with radiation safety

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3
Q

what is the gantry

A

the moving part of the machine

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4
Q

what are the treatment couch movements

A
  • ant and post (YAW)
  • sup and inf (ROLL)
  • left to right (PITCH)
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5
Q

how has safety been prioritised within the linac

A
  • interlocking systems
  • beam uniform intensity is checked
  • within the linac there is systems which won’t allow a higher dose to be delivered than the prescribed dose
  • internal checking systems
  • imaging allows for verification
  • high density concrete walls, so high energy x-rays are shielded
  • only emits radiation when energised
  • plans are checked to make sure if MLC’s are needed
  • first three days of treatment require imaging
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6
Q

what is found within the treatment head:

A
  • tungsten target
  • beam bending material
  • primary collimators: initial beam shaping and determines the field
  • secondary collimators
  • MLC’s: motor powered, made from tungsten they are positioned at a 90 degree angle from the patient allowing for beam conformity, only a 5% transmission is allowed
  • scattering foil
  • beam monitoring devices
  • shielding material
  • beam flattening filter
  • field light: projects outside the head, demonstrates the path of the beam, aiding in patient set up, aligned at the isocentres
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7
Q

what form of modality can be used in imaging

A

CBCT
- rotating gantry with kv source, images are captured by fan beam images, which provides better quality than MV as MV has poor resolution

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8
Q

why is double exposure used

A

even though it adds a dose it allows for images to be contrasted

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9
Q

what energies does a linac deliver

A

6MV or 10MV or electrons

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10
Q

what has been incorporated into a linac

A
  • an X-ray tube for better image quality
  • its job is to produce high energy x-ray beams which can penetrate the body
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11
Q

why is kv equipment not used

A

as it produces low energy x-rays which are unable to destroy tumour cells

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12
Q

what is the usage factor

A

it is a fraction of time in which the beam is projected at a particular barrier

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13
Q

what is an isocentre and what does it do

A

it is a fixed point in space, which is checked when imaging. It is based on the trajectory of the x-ray source, in which the radiation isocentre is the beam intersection, all beams converge. A treatment of two isocentres will have a simultaneous boost.

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14
Q

what is the standard SSD

A

100cm

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15
Q

what are EM waves produced by

A

a magnetron or klystron

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16
Q

describe MV/MeV

A
  • 6MV is heterogeneous (MAX = 6MeV, AVG = 2MeV)
  • electron energy is mono-energetic (all have the same energy)
  • photons from radioactive decay will have MeV
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17
Q

what is the role of the waveguide?

A
  • prevents the back flow of the RF pulse back to the RF generator
  • it is either isolator or circulator
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18
Q

what is the isolator WG

A

allows microwave power to be fed to acerbating waveguide and electron gun. High impedance to lower amplitude reflected waves

19
Q

what is the circulator WG

A

directs the microwave power along one path as its fed into an accelerator and along a series of different paths

20
Q

what is the role of an electron gun

A
  • it is an electron source which accelerates electrons through the WG
  • heated tungsten filament
  • electrons are given off by thermionic emission, and are focused at the CA
  • electrons emitted at a pulse will coincide with the RF wave
21
Q

what does an rf wave do

A

accelerate the electrons generated in the rf generator

22
Q

what is a triode gun

A

a cathode, anode and a specialised grid
- the cathode has a constant potential
- there is a pulsation across the grid preventing the electrons from moving to the anode
- removing a - potential causes the electrons to accelerate so can be controlled.
- a - potential prevents the electrons from reaching the anode
- anode focuses the electrons as it contains a hole which causes the electrons to pass through and enter the accelerating WG

23
Q

what is a pulse modulator

A

when high voltage pulses are to the microwave source and the electron gun, which enables the power output and dose rate to be controlled

24
Q

what does the magnetron do?

A
  • cathode releases electrons via thermionic emission
  • there is a permanent magnet above and below the cathode producing MF perpendicular to the initial field
  • cavities are found in the anode, these free electrons become excited which move in a random motion generating an rf field, which are picked up by the arial
25
what does a klystron?
- found at the back of gantry - rf amplifier - an oscillator produces an rf wave which is passed to the klystron. The electrons produced from the gun are found in a cluster due to the rf wave in a pulse - each electron which passes over the outer cavity, each chamber excites a voltage ionising KE - rf exits and travels to the wave guide - absorption of electrons occurs at the electron catcher
26
describe the accelerating WG
- RF wave travels at SOL - RF wave accelerated electrons at a high velocity - electron gun is synchronised to rf wave - two types: travelling and standing
27
describe the travelling WG
- it is made of a long series of coils, with a middle hole focusing the electrons down the central channel - rf wave and electron gun have a - voltage which allows electrons to enter the WG with the wave - if iris' are close together they reduce the velocity, allowing for the electrons to be in the correct position - electrons enter the RF wave - bunched iris' slows the wave, with wider spaces causing acceleration - electrons can join at different positions, but want to travel with the middle electron - electrons below the middle have a greater acceleration compared to the middle electrons which will merge together - electrons above the middle will have a slower acceleration to the middle electrons - an electrical field induces a charged region, in which electrons are accelerated through cavities changing its polarity, continuing until it reaches the end - the WG length determines the final electron energy
28
what is the final electron energy dependent on
- waveguide length - total number of electrons emitted - frequency of rf wave
29
describe the standing WG
- it is closed at both ends causing reflection once the wave hits the WG - a standing wave is the interference of two waves - NODES = areas which undergo displacement as a result of the restrictive interference of two waves (cancel each out) - ANTINODES = undergo a max displacement caused by a mix of constructive and destructive inference - the braking is due to the interaction between the electrons and the electrostatic field - there is no net movement as the nodes and antinodes cancel each other out - an antinode has twice the displacement as the travelling WG - as it moves along it creates + and - areas - alternate chambers to max acceleration has no charge - shortened waveguides have the same acceleration - nodes are fixed so don't contribute to the acceleration
30
what is the role of the vacuum
- maintain low pressure - prevent electron collision - prevent electrical breakdown
31
describe the role of steering coils
- runs alongside the WG - prevents divergence of the electron beam - produces magnetic fields - it centres the electrons at the gun end and centres the beam at the target - magnets help the centre alignment to the WG
32
why is a cooling system used?
- maintain stable temp - constant water flow - temp control
33
why is there a beam bending material
- the electron must bend once outside the head, so requires magnets - no acceleration occurs - high energy electrons bend at a greater degree allowing more electrons to focus to one spot
34
what is the simplest bending angle
90 degrees - producing a wide focus onto the target, chronic deflection, electrons have different energies - high energy electrons are deflected more
35
describe the 270 degree bending system
- less deflection at higher energies: convergence - lower energies are deflected more - ACHROMATIC DEFLECTION = focused at a spot - need a larger couch
36
why is there two layers of copper within the head
aid with a wide scattered beam
37
what is the role of a primary collimator
- defines the angle of the exiting beam - max field size 40 x 40 - 2% beam transmission - focuses electron beam
38
describe the secondary collimator
- 2 adjustable lead blocks - half beam blocking - square or rectangle field - used as a vertical wedge - can move symmetrically and independently - parallel to beam edge minimising transmission penumbra
39
what is the role of MLC's
- aid in conformity - prevent transmission as they have a smaller gap
40
what is the role of electron scattering foils
- reduces scatter outside the field - solid alloy
41
describe the target
- made from tungsten due to the high Z - use all photons - if too thick, photons are lost by attenuation - an increase in energy, causes a greater forward direction - at lower energies a transmission target is used
42
what does an IC do?
- allows for the monitoring of alignment and external factors - closed system - dose output surfaces are terminated if the tolerance is accelerated - feedback if the beam is flat and symmetrical
43
what does the flattening filter do
- flattens the x-ray beam, so there is a more uniform distribution - homogenous distribution at depth - attenuation at CA, less towards the edges - slight increase in TT - copper coil can increase dose rate, so TT is faster

Physics (25 decks)

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