Introduction to Radiotherapy Equipment

Question 1

What are the 4 types of kV X-rays?

Answer 1

• Grenz
• Contact
• Superficial
• Orthovoltage

Question 2

What is the kVp of Grenz X-rays?

Answer 2

10 - 20 kVp

Question 3

What is the kVp of Contact X-rays?

Answer 3

40 -50 kVp

Question 4

What is the kVp of Superficial X-rays?

Answer 4

50 -150 kVp

Question 5

What is the kVp of Orthovoltage X-rays?

Answer 5

150 - 500 kVp

Question 6

Rank the 4 types of kV X-rays from least to most penetrating

Answer 6

1) Grenz
2) Contact
3) Superficial
4) Orthovoltage

Question 7

Describe the structure of a kV X-ray tube

Answer 7

Key components:
- X-ray beam
- Filament (cathode)
- Target (anode)
- Tungsten shielding
- Beryllium window

An X-ray tube is evacuated

Question 8

Describe how a kV X-ray tube generates X-rays

Answer 8

X-ray tubes emit electrons from the heated filament via thermionic emission. They are focused on the target with shielding and accelerated between the cathode and anode due to the applied voltage. Once the electrons hit the target, they decelerate (bremsstrahlung), which produces X-rays.

Question 9

Why is the anode in a kV X-ray tube hooded?

Answer 9

To absorb stray X-rays and secondary electrons

Question 10

Why does a kV X-ray tube have a beryllium window?

Answer 10

To remove low energy X-rays as the beam passes through the window

Question 11

Describe the Heel effect

Answer 11

The phenomenon where the beam profile (intensity) of X-rays produced by a kV X-ray tube is not uniform along the anode-cathode axis due to varying levels of attenuation within the target.

Question 12

What are the three interactions that occur when accelerated electrons interact with a target?

Answer 12

1) Heat
2) Bremsstrahlung
3) Characteristic radiation

Question 13

What is the most significant interaction when electrons collide with a target in the kV range?

Answer 13

Heat (> 99%)

Question 14

Describe the energy spectrum of kV photons (X-rays)

Answer 14

The low energies are filtered out as they are clinically useless

Question 15

What is characteristic radiation?

Answer 15

Discrete X-ray energies related to a target material, equal to the difference in 2 atomic energy levels. These energies are released when an atomic electron is ejected.

Question 16

Why are clinical X-ray beams filtered?

Answer 16

To remove the low-dose X-rays that would be attenuated within a patient and increase their received dose.

Question 17

What materials are used to filter clinical X-ray beams?

Answer 17

• Tin
• Copper
• Aluminium

Question 18

How are clinical kV X-ray beams collimated?

Answer 18

Using applicators and lead cutouts

Question 19

Why are clinical kV X-ray beams collimated?

Answer 19

To shape the clinical radiation fields to the appropriate size and shield health tissue.

Question 20

What is the primary use of clinical kV X-ray beams?

Answer 20

To treat suprtficial (skin surface) lesions

Question 21

What is a linear accelerator?

Answer 21

The primary treatment unit for radiotherapy. It uses a waveguide to accelerate electrons to very high speeds, producing MV photons as a result.

Question 22

What are the 2 typical energies of a linear accelerator?

Answer 22

1) 6 MV
2) 10 MV

Question 23

What is isocentric mounting?

Answer 23

A feature of linear accelerators where the couch, collimator, and gantry rotate around a fixed point, the isocentre. This gives the beam a focus point.

Question 24

Describe the percentage depth dose curve of MV photons compared to kV photons

Answer 24

MV photons require a buildup region and have a shallow falloff
kV photons mostly deposit surface dose

Question 25

Which beam types can linear accelerators deliver?

Answer 25

Photons and electrons

Question 26

Name 3 common modes of a linear accelerator

Answer 26

1) Photon target 2) Photon flattening filter 3) Electron scattering foil

Question 27

Describe how a linear accelerator works

Answer 27

1) The RF power generator produces high-power microwaves 2) The feed guide carries the microwaves from the power source to the accelerator structure 3) The electron gun produces the electrons via thermionic emission 4) The accelerating waveguide uses the microwaves to accelerate the electrons to high energies 5) The bending magnet (270º) deflects the electrons into the treatment head 6) Electrons hit the X-ray target, producing X-rays that pass through the beam shaping and monitoring equipment 7) The beam leaves the linear accelerator and is directed towards the patient

Question 28

What are the 2 types of RF power generators used in linear accelerator?

Answer 28

- Klystron - Magnetron

Question 29

Describe the design of an accelerating waveguide

Answer 29

It is a vacuum-sealed structure made of a conducting material that contains 2 sections to accelerate the electrons: the bunching section and the relativistic section.

Question 30

What is the purpsose of beam steering/bending magnets in a linac?

Answer 30

To control the path of the electron beam and ensure that it is uniform

Question 31

Linacs use a ____ or ______ beam bend in order to direct the electron beam at the X-ray target.

Answer 31

270º Slalom

Question 32

Why don't beam steering magnets bend the linac beam by 90º?

Answer 32

Because this will cause the beam energies to spread out, unlike the 270º and slalom configurations where all energies re-converge at a single point.

Question 33

Name the 6 components of a linac head (for photons)

Answer 33

- An X-ray target - Primary collimators - A flattening filter - A dual-channel ionisation chamber - A field-defining light system - Secondary collimators

Question 34

What is a flattening filter?

Answer 34

A filter that alters an X-ray dose profile to be flat and a fixed depth in water (compared to the typical pointed profile)

Question 35

True or false: flattening filters are used for both X-ray and electron beams.

Answer 35

FALSE: they are only used for X-ray beams

Question 36

What is flattening filter free?

Answer 36

A beam without a filter in place, generating a slightly softer beam. This allows a higher dose rate to be delivered, increasing patient throughput due to quicker treatment times.

Question 37

What is a monitor chamber?

Answer 37

An ionisation chamber in the head of a linac that measures the charge generated by the X-ray beam. This allows the beam intensity to be controlled.

Question 38

What is a monitor unit (MU)?

Answer 38

A definition of the charge collected by the monitor chamber in a linac. The value of one monitor unit is set a commissioning to give a standard relationship between dose and MU under standard conditions.

Question 39

Why are there 2 ionisation chambers in a linac?

Answer 39

Patient dose must be measured very precisely, so multiple mechanisms are needed to control the delivery of MU.

Question 40

State the 3 mechanisms used to control the delivery of MU within a linac head and their function

Answer 40

1) Primary chamber: set to stop the beam when the prescribed MU is reached 2) Backup chamber: set to stop the beam when the prescribed MU+10 is reached 3) Beam timer: stops the beam if delivery is taking much longer than expected

Question 41

Other than MU, what can ionisation chambers measure?

Answer 41

- Beam steering - Beam energy

Question 42

What is a multi-leaf collimator?

Answer 42

MLCs are movable collimators that absorb the beam outside the desired field. They contain steps in each leaf to prevent any radiation leaks.

Question 43

Name the 6 components of a linac head (for electrons)

Answer 43

- Electron scattering foils - Primary collimators - A dual-channel ionisation chamber - A field-defining light system - Secondary collimators - An external applicator