Dosimetry 2

Question 1

What is absolute dosimetry?

Answer 1

• The direct measurement of dose
• Performed in standards laboratories
• Measurements which define the unit Gy

Question 2

What is reference dosimetry?

Answer 2

• Transfer of dose standards to the hospital
• Measurements taken using hospital equipment so it can be calibrated
• Performed in line with IRR 17
• Only certain dosemeters can be used to calibrate the linac

Question 3

What is relative dosimetry?

Answer 3

• Measure dose but not to stringent standards of reference dose
• Data is relative

Question 4

What is In-Vivo dosimetry?

Answer 4

• Measurements take on/near the patient during treatment
• Verifies treatment plan
• Can be relative or can measure a dose

Question 5

Why measure in a phantom?

Answer 5

• An object that can be used to make measurements
• It is robust and can be easily set up
• Must mimic as closely as possible, attenuation and scattering that occurs in the human body

Question 6

Measuring radiation deposition using ionisation?

Answer 6

• measuring the amount of ionisation will give a measure of the energy
• This is principal method of dosimetry in the hospital

Question 7

What is charged particle equilibrium?

Answer 7

• Charged particle equilibrium (electrons entering = same as electrons leaving)
• will adding the device, change the behaviour of dose within the phantom

Question 8

What is the Bragg-Gray Cavity theory?

Answer 8

• If the hole is small enough (cavity) then it will not alter the number or distribution of the electrons in the material that would exist if cavity was not there

Question 9

What is required of a dosemeter?

Answer 9

• Accurate across a range of doses
• Have high precision to resolve small doses
• Be sensitive
• Be linear across working range
• Be independent of dose rate
• Can measure large and small doses equally well
• Can measure equally well different energies of photons, electrons etc
• Mut be able to represent dose in tissues
• Be small enough to measure high dose gradients

Question 10

How does the ionisation chamber work?

Answer 10

• Amount of ionisation produced is proportional to the amount of energy absorbed per unit mass of material
• separate positive and negative charges before they recombine
• High voltage is used and this allows the positive and negative charges to be collected individually

Question 11

What is a parallel plate chamber?

Answer 11

• Have thin (thinner than thimble) measurement space in the direction of the beam for better spatial resolution
• Particularly useful for measurements in electron beams and in the build up region of x-ray and electron beams

Question 12

What is an electrometer for?

Answer 12

• a device to produce the high voltage and measure the charge