Dosimetry equipment II

Question 1

How does a calorimeter work? (4 marks)

Answer 1

• based on first principles
• measures small temperature changes in graphite (E= mcT)
• used in primary standard labs
• not used clinically due to size and cost

Question 2

Can ionisation chambers be used to measure: absolute dose, PDDs, output factors and penumbra? (4 marks)

Answer 2

• absolute dose: good as Z_eff is close to water and there is minimum pertubation to the beam fluence
• PDDs: ideal there is a flat energy response but may lack resolution in the build-up region (can use a parallel plate chamber)
• output factors: there is no spectral variation but size may compomise small field measurements
• penumbra: relatively poor resolution may affect accuracy of measurements

Question 3

Can diodes be used to measure: absolute dose, PDDs, output factors and penumbra? (4 marks)

Answer 3

• absolute dose: no as higher Z_eff than water causes significant perturbation to energy fluence
• PDDs: good for build-up region but response varies with energy
• output factors: small size makes them ideal for small fields but there is an energy response which needs consideration
• penumbra: ideal due to high res

Question 4

How do TLDs work? (4 marks)

Answer 4

• They have electron traps which capture electrons when the material is irradiated
• Traps hold electrons until they are heat, which releases the elctrons and emits visible light
• Light is proportional to dose
• Commonly LiFl is used

Question 5

Give three pros and cons of using TLDs? (6 marks)

Answer 5

Pros: good spatial resolution, response does not vary much over a wide range of energies, small size is convenient.

Cons: require processing and calibration, output fades over-time (10% per year), results can be affected by radiation and thermal history

Question 6

When is radiographic film used? (4 marks)

Answer 6

Linac commissioning and QA such as focal spot size, leakage and beam uniformity.

Question 7

What are the pros and cons of using film? (5 marks)

Answer 7

Pros: cheap, geometry makes it useful for leakage and beam uniformity, good spatial resolution.

Cons: wet processing required, energy dependence.

Question 8

What is film made of? (2 marks)

Answer 8

Silver halide in an emulsion.

Question 9

What two types of chemical dosimetry are there and when are they used? (6 marks)

Answer 9

Gels = electron beams and 3D dose distributions

Alanine = linac commissioning and patient dosimetry