Radiation Detectors, Types, and Clinical Uses

Question 1

What is a radiation detector?

Answer 1

A device that, via a calibration factor, can measure the radiation dose delivered to it. Different dosimeters measure this in different ways.

Question 2

Why are accurate dose measurements important for successful radiotherapy?

Answer 2

Because the dose delivered strongly relates to tumour control and healthy tissue damage. Even a small percentage difference in dose can lead to serious healthy tissue damage or tumour underdose. An accurate dose measurement ensures safe, effective radiotherapy.

Question 3

State 7 characteristics of a good radiation detector

Answer 3

1) Small volume
2) High signal-to-noise ratio (SNR)
3) High sensitivity
4) Long-term stability
5) Small energy dependence
6) Small beam perturbation effect
7) Practical

Question 4

Why is it beneficial for a radiation detector to have a small volume?

Answer 4

Because a detector with a small volume has a good spatial resolution, meaning that it can pinpoint exactly where the dose was delivered.

Question 5

Why is it beneficial for a radiation detector to have a high signal to noise ratio?

Answer 5

Because it is important for the detector to distinguish the signal from background radiation.

Question 6

Why is it beneficial for a radiation detector to have a high sensitivity?

Answer 6

Because resolution is very vital to accuracy.

Question 7

Why is it beneficial for a radiation detector to have long-term stability?

Answer 7

A long drifting period is required to ensure that dose measurements are consistent.

Question 8

Why is it beneficial for a radiation detector to have a small energy dependence?

Answer 8

So that the readings aren’t energy dependant.

Question 9

Why is it beneficial for a radiation detector to have a small beam perturbation effect?

Answer 9

So that the dosimeter doesn’t significantly impact the beam characteristics.

Question 10

Why is it beneficial for a radiation detector to be practical?

Answer 10

Because daily/weekly/annual checks are more straightforward if the detector is easy to read out.

Question 11

State 2 characteristics of a practical radiation detector

Answer 11

1) Easy to read out
2) Wireless

Question 12

What is an ionisation chamber?

Answer 12

A chamber filled with gas, in which ionisation events occur.

Question 13

What are the 2 most common types of ionisation chamber?

Answer 13

1) Farmer chamber
2) Parallel plate chamber

Question 14

What is the typical bias voltage of an ionisation chamber?

Answer 14

~ 300V

Question 15

Why is a voltage of ~ 300V chosen for ionisation chambers?

Answer 15

Because this minimises ion recombination without promoting further ionisations.

Question 16

Why are farmer chambers well suited to MV photon beams?

Answer 16

They have:
- Long term stability
- High sensitivity
- Very small energy dependence
- Very good spatial resolution

Question 17

Why are ROOS (parallel plate) chambers well suited to electron beams?

Answer 17

They:
- Minimise perturbation effects
- Have good spatial resolution
- Have a well-known effective point of measurement

Question 18

Why are large volume ionisation chambers used in radiation safety and nuclear medicine?

Answer 18

Because their increased signal is particularly useful with low dose rates. However, this comes with a lower spatial resolution.

Question 19

Give one example of a large volume ionisation chamber

Answer 19

A Geiger counter

Question 20

What is a diode?

Answer 20

A radiation detector made of silicon that is extremely sensitive in the kV range due to its high atomic number. This sensitivity means they can be very small. Diodes provide real-time responses to radiation.

Question 21

Why do diodes have build-up caps?

Answer 21

To reduce over-sensitivity

Question 22

Why do diodes need to be calibrated regularly?

Answer 22

Because they are very sensitive to radiation damage

Question 23

Why do diodes need a temperature calibration factor?

Answer 23

Because they are very temperature dependent

Question 24

State 2 clinical uses of diodes

Answer 24

1) Patient-specific QA (lots of diodes in an array)
2) Measuring beam profiles

Question 25

What is a thermoluminsecent dosimeter (TLD)?

Answer 25

Imperfect crystals with tissue equivalent properties that absorb energy. This energy is released as photons when heated.

Question 26

How are TLDs calibrated?

Answer 26

By performing measurements simultaneously with absolute dosimeters.

Question 27

State the pros and cons of TLDs

Answer 27

+ Virtually dose rate independent + Read out temperature is high, so they aren't dependent on atmospheric temperatures + No directional dependence - Destructive read out - Readings aren't real time

Question 28

What are the 2 most common uses for TLDs?

Answer 28

- Personal dosimetry - Measurements of lens doses

Question 29

What is a MOSFET?

Answer 29

Metal Oxide Semiconductor Field Effect Transistors. They are semiconductor devices that record dose by measuring the 'radiation damage' that the device experiences.

Question 30

True or false: MOSFETs are regularly used in clinical settings.

Answer 30

FALSE

Question 31

What is optically stimulated luminescence (OSL)?

Answer 31

Crystals that store energy. The energy is released by light irradiation. They can be re-read multiple times.

Question 32

What is the most common use of OSLs?

Answer 32

Personal dosimetry

Question 33

What is radiochromic film?

Answer 33

A thin sheet that discolours when exposed to radiation. They are used to measure exit dose.

Question 34

Why is radiochromic film good for QA?

Answer 34

Because it has a very good spatial resolution.

Question 35

What is electronic portal imaging dosimetry (EPID)?

Answer 35

The radiation detector used in an MV photon panel to measure the dose transmitted through radiotherapy patients.