Linac design and function Flashcards
Two most frequently used Mega voltage X-rays used in radiotherapy and Why?
6MV and 10MV
Good beam penetration and fast dose rate
What type of EMR is used in a linac?
Microwaves
Where are the microwaves produced?
Magnetron (Elekta) and Klystron (Varian)
What is the waveguide?
The series of pipes that carry the microwaves from the EMR source to the accelerating structure.
What gas is contained within the waveguide and why?
Sulphur hexafluoride (SF6) - prevent arcing (interaction of EMR with the walls of the pipes)
What is the accelerating structure (sometimes known as the waveguide)?
Horizontal tube within the gantry, where electron acceleration takes place. Contains a vacuum and a series of cavities in which the EMR travels through.
What are the two types of wave guide and briefly explain the difference?
Travelling waveguide - Microwaves propagate through the accelerator tube and carry the electrons along on the waves.
Standing waveguide - The electric component of the stationary wave exerts an accelerating force on the electrons.
Which type of waveguide is most commonly used in linacs?
Standing waveguide
What is the electron gun?
Contains a tungsten filament which is heated to high temperatures to produce electrons via thermoionic emission.
What end of the accelerating structure are the electrons and EMR loaded on?
Bunching section
Explain in terms of E=mc2 what happens to the electrons as they travel through the accelerating structure?
Accelerated electrons gain kinetic energy as the move through the tube travelling at the speed of light. Due to this they experience a relativistic change in mass towards the end of the tube.
Name two components surrounding the accelerator structure that ensure the electrons travel correctly through the waveguide?
Focussing coils
Steering coils
What do focussing coils do?
Produce a magnetic field flux parallel to the direction of the electron beam to Prevent dispersion of the electron beam travelling through the accelerator tube.
What do steering coils do?
Produce a magnetic field flux to ensure electrons travel in their optimum trajectory down the centre of the waveguide minimising any losses from incorrect exit from the waveguide.
What is the flight tube?
an evacuated tube where no further acceleration takes place and they enter the bending cavity
What are the 2 functions of the bending magnets in the bending cavity?
- changes the direction of travel of the electrons so they are aimed at the patient
2.Unifies the energy of the electrons leaving the accelerating tube so they hit the target with the same energies and provide mono-energy X-ray beams.
What type of target is used in a linac
Transmission
What are the advantages of a tranmission target?
They are more effective than reflective targets as more energy is converted to X-rays and less energy converted to heat loss (30:70)
Primary Collimator
Is close to the target so it can create a small beam. This shapes the beam by defining the maximum angle of the beam.
Flattening filter
Metal conical shaped device used to ensure X-rays are equally distributed throughout the beam and allows a uniform dose to be given to patients
Ion chamber
Dual chamber containing a primary dosimeter and redundancy dosimeter that measures the dose of radiation being delivered to patient. Terminates exposure when the required dose is achieved. Also has feedback systems to prior components of the linac (electron gun, bending magnet) to ensure correct beam profile characteristics.
Secondary collimator
This consists of two adjustable lead blocks which can be positioned to manipulate the field size of the radiation beam for the tumour of interest. These are square or rectangle in shape.
Multi-leaf Collimators (MLCs)
Collimation device consisting of multiple leaves of tungsten which can move independently to shape the beam. This allows formation of irregular field sizes which are more specific to tumour shape and size. Allows conformal radiotherapy.
Wedges
These are used to allow different doses to be delivered within a specific field size.
Advantages of MLC’s
1) Reduce radiation dose to organs at risk
2) allows complex tumour shapes to be treated minimising radiation to surrounding issues
3) no need for flattening filters or wedges
4) produces different dose intensities throughout the field
5) Has allowed development of IMRT therapy
ISOCENTER
Point in space at which the gantry, radiation head and patient couch all rotate about. Lasers are used to indicate this point and this is where centre of the tumour is positioned.
