Dosimetry

Question 1

Define dosimetry

Answer 1

Measurement of ionising radiation in a given place or in the body

Question 2

Define exposure and how it is measured

Answer 2

Quantity used to indicate amount of ionisation in air produced by x-rays
Measured by C/kg

Question 3

Define absorbed dose and how it is measured

Answer 3

Energy absorbed per unit of mass of material
Measured by Gy

Question 4

Define KERMA and how it is measured

Answer 4

Sum of initial kinetic energies of all charged particles liberated by ionising radiation divided by the mass of the sample.​
Measured by Gy

Question 5

Describe the relationship between KERMA and absorbed dose related to high energy photons

Answer 5

Many highly energetic secondary electrons and X-rays escape region of interest before depositing energy.
KERMA is higher than absorbed dose

Question 6

Describe the relationship between KERMA and absorbed dose related to low energy photons

Answer 6

The scattered radiation does not travel significant distances, so nearly all the energy is absorbed in the same location.​
KERMA and absorbed dose similarly

Question 7

What is the electronic equilibrium

Answer 7

Energy lost by secondary scattered radiation away +
Energy gained by secondary scattered radiation into point

Question 8

Define equivalent dose and how it is measured

Answer 8

Absorbed dose x radiation weighting factor
Measured in Sv

Question 9

Define effective dose and how it is measured

Answer 9

Equivalent dose x tissue weighting factor
Measured in Sv

Question 10

What do ALARA and ALARP stand for?

Answer 10

ALARA - as low as reasonably achievable
ALARP - as low as reasonably practicable

Question 11

Describe time with reference to patient care

Answer 11

Patient should be irradiated for as little time as possible

Question 12

Describe shielding with reference to patient care

Answer 12

Protective viewing window
If in surgery, lead aprons
Patient shielding, eg gonad shields

Question 13

Describe distance with reference to patient care

Answer 13

Create as much distance as possible with the machine

Question 14

How do we assess radiation risks for patients?

Answer 14

Dose area product (DAP)
Diagnostic reference levels

Question 15

Define dose area product and how it is measured

Answer 15

Absorbed dose x area irradiated
Measured by Gycm2

Question 16

Describe diagnostic reference levels

Answer 16

Ensure dose is optimised due to different sized patients

Question 17

What impact do DRLs have?

Answer 17

After 20 years, dose reduction was ~10-20%

Question 18

How are DRLs measured?

Answer 18

Entrance Surface Dose - mGy​
Dose area product - Gy cm2

Question 19

Describe the structure of TLDs and how they work

Answer 19

Lithium Fluoride crystals
Chips, pellets, microrods ~5mm
Absorbs radiation, electrons become excited, then lose energy but some become trapped

Question 20

How are TLDs read?

Answer 20

Heated to release electrons
Energy released as light pulses
Photoelectric sensitive plate and photomultiplier tube records light pulse
Number of pulses proportional to dose

Question 21

How are TLDs cleared for reuse?

Answer 21

Heated to 400 degrees for 1.5 hours, 80 degrees for 16 hours
Sensitivity depends on temperature cycle

Question 22

Describe ion chambers

Answer 22

Used for dosimetric calibration of radiotherapy equipment
Used with a water tank to measure doses

Question 23

Describe the process of ion chambers

Answer 23

300 to 500 volts applied
Temp rises, air expands, conc of molecules decreases
If atmosphere pressure rises, air compressed, conc of molecules increases

Question 24

Describe diodes

Answer 24

Silicon doped with impurities
N-type has excess electrons
P-type electron deficient
Thin layers of each placed in contact

Question 25

Describe the process of diodes

Answer 25

Electrons migrate, forming holes
When exposed, ion pairs form
Current flows across junction
Diode connected to electrometer, measures current
Current proportional to amount of ionisation