Linear accelerators

Question 1

what are the radiotherapy techniques?

Answer 1

• 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

Question 2

what is the bunker

Answer 2

a housing system for the linac designed with radiation safety

Question 3

what is the gantry

Answer 3

the moving part of the machine

Question 4

what are the treatment couch movements

Answer 4

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

Question 5

how has safety been prioritised within the linac

Answer 5

• 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

Question 6

what is found within the treatment head:

Answer 6

• 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

Question 7

what form of modality can be used in imaging

Answer 7

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

Question 8

why is double exposure used

Answer 8

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

Question 9

what energies does a linac deliver

Answer 9

6MV or 10MV or electrons

Question 10

what has been incorporated into a linac

Answer 10

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

Question 11

why is kv equipment not used

Answer 11

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

Question 12

what is the usage factor

Answer 12

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

Question 13

what is an isocentre and what does it do

Answer 13

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.

Question 14

what is the standard SSD

Answer 14

100cm

Question 15

what are EM waves produced by

Answer 15

a magnetron or klystron

Question 16

describe MV/MeV

Answer 16

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

Question 17

what is the role of the waveguide?

Answer 17

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

Question 18

what is the isolator WG

Answer 18

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

Question 19

what is the circulator WG

Answer 19

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

Question 20

what is the role of an electron gun

Answer 20

• 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

Question 21

what does an rf wave do

Answer 21

accelerate the electrons generated in the rf generator

Question 22

what is a triode gun

Answer 22

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

Question 23

what is a pulse modulator

Answer 23

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

Question 24

what does the magnetron do?

Answer 24

• 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

Question 25

what does a klystron?

Answer 25

- 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

Question 26

describe the accelerating WG

Answer 26

- 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

Question 27

describe the travelling WG

Answer 27

- 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

Question 28

what is the final electron energy dependent on

Answer 28

- waveguide length - total number of electrons emitted - frequency of rf wave

Question 29

describe the standing WG

Answer 29

- 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

Question 30

what is the role of the vacuum

Answer 30

- maintain low pressure - prevent electron collision - prevent electrical breakdown

Question 31

describe the role of steering coils

Answer 31

- 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

Question 32

why is a cooling system used?

Answer 32

- maintain stable temp - constant water flow - temp control

Question 33

why is there a beam bending material

Answer 33

- 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

Question 34

what is the simplest bending angle

Answer 34

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

Question 35

describe the 270 degree bending system

Answer 35

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

Question 36

why is there two layers of copper within the head

Answer 36

aid with a wide scattered beam

Question 37

what is the role of a primary collimator

Answer 37

- defines the angle of the exiting beam - max field size 40 x 40 - 2% beam transmission - focuses electron beam

Question 38

describe the secondary collimator

Answer 38

- 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

Question 39

what is the role of MLC's

Answer 39

- aid in conformity - prevent transmission as they have a smaller gap

Question 40

what is the role of electron scattering foils

Answer 40

- reduces scatter outside the field - solid alloy

Question 41

describe the target

Answer 41

- 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

Question 42

what does an IC do?

Answer 42

- 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

Question 43

what does the flattening filter do

Answer 43

- 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