Radiation Detector Types and Clinical Uses

Question 1

Describe the principle of operation of a thimble or cylindrical ionisation chamber

Answer 1

Cylindrical shaped outer electrode with a straight inner Aluminium electrode

Filled with gas

Bias voltage applied across the electrodes creating a potential difference and electric field
300V ideal due to minimised recombination

Incident radiation ionises the gas and creates an electron/hole pair which experience a force in the electric field produced by the bias voltage across the electrodes

This creates a current which can be measured and converted into a meaningful reading

Amount of charge measured (by an electrometer) is related to the amount of incident radiation

Question 2

What is a radiation detector?

Answer 2

A device that will, via a calibration factor, measure the dose to its own sensitive material via a quantity of charge, light, film blackening etc.

Question 3

Why are radiation detectors used?

Answer 3

Accurate dose determination is crucial in radiotherapy success due to the steep dependence on both tumour control and healthy-tissue damage, a difference of a few percent may separate success with complications (either over or under dose)

Question 4

What characteristics should a RT radiation detector have?

Answer 4

1. Small volume (good spatial resolution)
2. High signal to noise ratio
3. High sensitivity
4. Long term stability
5. Small energy dependence
6. Small beam perturbation effect (doesn’t significantly impact the beam)
7. Practical (easy to use/wireless/easy read out - use dependent)

Question 5

What are the different types of detectors?

Answer 5

1. Ionisation chambers
2. Diodes
3. TLDs
4. MOSFETs
5. OSL
6. Gafchromic film
7. Portal imager

Question 6

Why are farmer chambers commonly used for photon RT?

Answer 6

Well known effective point of measurement, dose can be measured via a traceable calibration factor ad it is recommended by the MV CoP

Question 7

What are the advantages of using an ionisation chamber in a photon beam?

Answer 7

1. Long term stability
2. High sensitivity
3. Very small energy dependence
4. Very good spatial resolution

Question 8

What are the advantages and disadvantages of using an ionisation chamber in an electron beam?

Answer 8

1. Good spatial resolution
2. High energy dependence
3. Can have significant perturbation effects

Question 9

What type of ionisation chamber is used in a photon beam and an electron beam?

Answer 9

Photon: Farmer
Electron: ROOS

Question 10

What is a ROOS chamber?

Answer 10

Ionisation chamber with a parallel plate configuration. Irradiated with axis parallel to beam direction. 1mm window

Question 11

How does the response of an unsealed ionisation chamber vary with temperature? Why is this the case?

Answer 11

If the temperature goes up, the response of the chamber will go down. This is because the density of air molecules in the chamber has decreased. So the number of ion pairs created decreases, and the chamber response decreases.

Answer 22

Can be used alone for intermediate patients or as a boost for high-risk patients.