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Linear accelerators Flashcards

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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
Q

what does a klystron?

A
  • 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
Q

describe the accelerating WG

A
  • 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
Q

describe the travelling WG

A
  • 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
Q

what is the final electron energy dependent on

A
  • waveguide length
  • total number of electrons emitted
  • frequency of rf wave
29
Q

describe the standing WG

A
  • 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
Q

what is the role of the vacuum

A
  • maintain low pressure
  • prevent electron collision
  • prevent electrical breakdown
31
Q

describe the role of steering coils

A
  • 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
Q

why is a cooling system used?

A
  • maintain stable temp
  • constant water flow
  • temp control
33
Q

why is there a beam bending material

A
  • 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
Q

what is the simplest bending angle

A

90 degrees
- producing a wide focus onto the target, chronic deflection, electrons have different energies
- high energy electrons are deflected more

35
Q

describe the 270 degree bending system

A
  • less deflection at higher energies: convergence
  • lower energies are deflected more
  • ACHROMATIC DEFLECTION = focused at a spot
  • need a larger couch
36
Q

why is there two layers of copper within the head

A

aid with a wide scattered beam

37
Q

what is the role of a primary collimator

A
  • defines the angle of the exiting beam
  • max field size 40 x 40
  • 2% beam transmission
  • focuses electron beam
38
Q

describe the secondary collimator

A
  • 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
Q

what is the role of MLC’s

A
  • aid in conformity
  • prevent transmission as they have a smaller gap
40
Q

what is the role of electron scattering foils

A
  • reduces scatter outside the field
  • solid alloy
41
Q

describe the target

A
  • 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
Q

what does an IC do?

A
  • 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
Q

what does the flattening filter do

A
  • 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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