Codes of Practice

Question 1

Define absorbed dose

Answer 1

A physical dose quantity, D, representing the mean energy imparted to matter per unit mass by ionising radiation.

Question 2

Define calorimetry

Answer 2

The process of measuring the temperature change of a material due to the energy imparted.

Question 3

Define specific heat capacity

Answer 3

The amount of heat that must be added to a material to raise the temperature of 1kg of the substance by 1 ºC.

Question 4

Give the equation for specific heat capacity

Answer 4

∆Q = change in heat energy
m = mass
c = specific heat capacity
∆T - change in temperature

Question 5

Give the equation that links absorbed dose to change in heat energy

Answer 5

D = absorbed dose
∆Q = change in heat energy
m = mass
c = specific heat capacity
∆T = change in temperature

Question 6

What does an ionisation chamber measure?

Answer 6

Charge (in Coulombs) due to ionisations events from radiation

Question 7

What is a calibration chain?

Answer 7

A sequence of comparisons used to link a measurement to a national or international measurement standard, establishing traceability.

Question 8

Why are calibration chains necessary?

Answer 8

They are required to ensure all instruments measure the same dose.

Question 9

Describe the calibration chain followed in the UK

Answer 9

All calibrations in the UK are traceable to the National Physics Laboratory (NPL).

Question 10

Give the equation for the calibrated dose of a hospital field instrument

Answer 10

N_dw and f_ic are calibration factors

Question 11

All calibrations and inter-comparisons must follow a ____ __ ________.

Answer 11

Code of practice

Question 12

How often are secondary standards sent to the NPL for calibration?

Answer 12

Every 2-3 years

Question 13

How often are field instruments calibrated against secondary standards?

Answer 13

Every year

Question 14

Define beam energy

Answer 14

The amount of energy carried by a radiation beam to treat cancer. This is difficult to quantify as clinical beams are not monochromatic.

Question 15

Which measurement is used to determine the beam quality for MV photons?

Answer 15

TPR(20,10)

Question 16

Which measurement is used to determine the beam quality for kV photons?

Answer 16

HVL

Question 17

Which measurement is used to determine the beam quality for MV electrons?

Answer 17

R(50,D)

Question 18

Describe the process followed (for all radiation types/energies) when calculating the dose delivered by a linac

Answer 18

Question 19

What is an intercomparison factor?

Answer 19

The factor applied when comparing a secondary standard with a field instrument. To calculate this factor, both chambers should be exposed to the same radiation and their response should be compared.

Question 20

What is a constancy check?

Answer 20

A quick way to verify chamber functionality without full intercomparison. The chamber’s response under known conditions is measured.

Question 21

Monitor units, the unit of treatment planning, are _________ related to change.

Answer 21

Linearly

Question 22

State the relationship between MU and dose

Answer 22

1 MU = 0.01 Gy at d_max under certain conditions

Question 23

What are the two calibration methods (conditions) for calibrating a linac beam output?

Answer 23

1) Isocentric method
2) Fixed SSD method

Question 24

What is the isocentric method of output calibration?

Answer 24

Measurements are made at the isocentre (100cm SCD, 5cm deep in ‘water’). The SSD is 95cm. This method uses a 10cm² field, and measurements are corrected back to d_max (green).

Question 25

What is the fixed SSD method of output calibration?

Answer 25

Measurements are made at 105cm SCD, 5cm deep in 'water'. The SSD is at the isocentre, 100cm. This method uses a 10cm² field, and measurements are corrected back to d_max (green).

Question 26

Give the equation for the dose to water at the effective measurement point for MV photons

Answer 26

D = dose to water M = measured chamber reading N = NPL calibration factor K = local reference beam quality correction factor

Question 27

Define beam quality

Answer 27

A measure of the energy distribution of a radiation beam. The higher the mean energy, the higher the beam quality.

Question 28

What is TPR(20,10)?

Answer 28

The tissue phantom ratio is the isocentric dose ratio at 10cm and 20cm deep in water. It is used to specify the beam quality of MV photons.

Question 29

What are the 3 categories of kV photons?

Answer 29

1) Medium energy 2) Low energy 3) Very low energy

Question 30

Give the equation for the dose to water at the effective measurement point for medium energy kV photons

Answer 30

D = dose to water at 2cm depth M = instrument reading corrected to standard temperature and pressure N = NPL chamber calibration factor k = chamber response correction water to air [term] = mass energy absorption coefficient ratio

Question 31

Give the equation for the dose to water at the effective measurement point for low energy kV photons

Answer 31

D = dose to water at surface M = instrument reading corrected to standard temperature and pressure N = NPL chamber calibration factor B = backscatter factor [term] = mass energy absorption coefficient ratio

Question 32

State the 4 ion chanber correction factors

Answer 32

1) Temperature and pressure correction factor 2) Ion recombination correction factor 3) Polarity correction factor 4) Electrometer correction factor

Question 33

Give the equation for the dose to water at the effective measurement point for electrons

Answer 33

D = dose to water at depth z_ref M = corrected chamber reading N = NPL absorbed dose calibration factor R(50,D) = electron beam quality

Question 34

Give the equation for the temperature and pressure correction factor

Answer 34

f_tp = temperature and pressure correction factor T = temperaure T_0 = temperature constant = 293.15 P = pressure P_0 = pressure constant = 1013.25

Question 35

Give the equation for the ion recombination correction factor

Answer 35

f_ion = ion recombination correction factor M1 = chamber reading 1 M2 = chamber reading 2 V1 = voltage 1 V2 = voltage 2

Question 36

Give the equation for the polarity correction factor

Answer 36

f_pol = polarity correction factor M+ = positive chamber reading M- = negative chamber reading

Question 37

What is the electrometer correction factor based on?

Answer 37

The calibration of the electrometer used