Linac design and function

Question 1

Two most frequently used Mega voltage X-rays used in radiotherapy and Why?

Answer 1

6MV and 10MV

Good beam penetration and fast dose rate

Question 2

What type of EMR is used in a linac?

Answer 2

Microwaves

Question 3

Where are the microwaves produced?

Answer 3

Magnetron (Elekta) and Klystron (Varian)

Question 4

What is the waveguide?

Answer 4

The series of pipes that carry the microwaves from the EMR source to the accelerating structure.

Question 5

What gas is contained within the waveguide and why?

Answer 5

Sulphur hexafluoride (SF6) - prevent arcing (interaction of EMR with the walls of the pipes)

Question 6

What is the accelerating structure (sometimes known as the waveguide)?

Answer 6

Horizontal tube within the gantry, where electron acceleration takes place. Contains a vacuum and a series of cavities in which the EMR travels through.

Question 7

What are the two types of wave guide and briefly explain the difference?

Answer 7

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.

Question 8

Which type of waveguide is most commonly used in linacs?

Answer 8

Standing waveguide

Question 9

What is the electron gun?

Answer 9

Contains a tungsten filament which is heated to high temperatures to produce electrons via thermoionic emission.

Question 10

What end of the accelerating structure are the electrons and EMR loaded on?

Answer 10

Bunching section

Question 11

Explain in terms of E=mc2 what happens to the electrons as they travel through the accelerating structure?

Answer 11

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.

Question 12

Name two components surrounding the accelerator structure that ensure the electrons travel correctly through the waveguide?

Answer 12

Focussing coils

Steering coils

Question 13

What do focussing coils do?

Answer 13

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.

Question 14

What do steering coils do?

Answer 14

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.

Question 15

What is the flight tube?

Answer 15

an evacuated tube where no further acceleration takes place and they enter the bending cavity

Question 16

What are the 2 functions of the bending magnets in the bending cavity?

Answer 16

1. 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.

Question 17

What type of target is used in a linac

Answer 17

Transmission

Question 18

What are the advantages of a tranmission target?

Answer 18

They are more effective than reflective targets as more energy is converted to X-rays and less energy converted to heat loss (30:70)

Question 19

Primary Collimator

Answer 19

Is close to the target so it can create a small beam. This shapes the beam by defining the maximum angle of the beam.

Question 20

Flattening filter

Answer 20

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

Question 21

Ion chamber

Answer 21

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.

Question 22

Secondary collimator

Answer 22

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.

Question 23

Multi-leaf Collimators (MLCs)

Answer 23

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.

Question 24

Wedges

Answer 24

These are used to allow different doses to be delivered within a specific field size.

Question 25

Advantages of MLC’s

Answer 25

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

Question 26

ISOCENTER

Answer 26

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.