Linear accelerators Flashcards
what are the radiotherapy techniques?
- 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
what is the bunker
a housing system for the linac designed with radiation safety
what is the gantry
the moving part of the machine
what are the treatment couch movements
- ant and post (YAW)
- sup and inf (ROLL)
- left to right (PITCH)
how has safety been prioritised within the linac
- 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
what is found within the treatment head:
- 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
what form of modality can be used in imaging
CBCT
- rotating gantry with kv source, images are captured by fan beam images, which provides better quality than MV as MV has poor resolution
why is double exposure used
even though it adds a dose it allows for images to be contrasted
what energies does a linac deliver
6MV or 10MV or electrons
what has been incorporated into a linac
- an X-ray tube for better image quality
- its job is to produce high energy x-ray beams which can penetrate the body
why is kv equipment not used
as it produces low energy x-rays which are unable to destroy tumour cells
what is the usage factor
it is a fraction of time in which the beam is projected at a particular barrier
what is an isocentre and what does it do
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.
what is the standard SSD
100cm
what are EM waves produced by
a magnetron or klystron
describe MV/MeV
- 6MV is heterogeneous (MAX = 6MeV, AVG = 2MeV)
- electron energy is mono-energetic (all have the same energy)
- photons from radioactive decay will have MeV
what is the role of the waveguide?
- prevents the back flow of the RF pulse back to the RF generator
- it is either isolator or circulator
what is the isolator WG
allows microwave power to be fed to acerbating waveguide and electron gun. High impedance to lower amplitude reflected waves
what is the circulator WG
directs the microwave power along one path as its fed into an accelerator and along a series of different paths
what is the role of an electron gun
- 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
what does an rf wave do
accelerate the electrons generated in the rf generator
what is a triode gun
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
what is a pulse modulator
when high voltage pulses are to the microwave source and the electron gun, which enables the power output and dose rate to be controlled
what does the magnetron do?
- 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
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
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
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
what is the final electron energy dependent on
- waveguide length
- total number of electrons emitted
- frequency of rf wave
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
what is the role of the vacuum
- maintain low pressure
- prevent electron collision
- prevent electrical breakdown
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
why is a cooling system used?
- maintain stable temp
- constant water flow
- temp control
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
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
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
why is there two layers of copper within the head
aid with a wide scattered beam
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
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
what is the role of MLC’s
- aid in conformity
- prevent transmission as they have a smaller gap
what is the role of electron scattering foils
- reduces scatter outside the field
- solid alloy
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
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
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
