Radiation Risk and Dose Calculation

Question 1

Describe the ICRU Sphere.

Answer 1

The ICRU sphere is 0.3 m in diameter with a density of 1000 kg m−3 and a mass composition equivalent to tissue of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen and 2.6% nitrogen. The field is ‘expanded’ so that it encompasses the sphere and ‘aligned’ so the quantity is independent of the angular distribution of the radiation field. In effect this defines an instrument with a uniform isotropic response.

Question 2

What is the Ambient Dose Equivalent (H*(10))?

Answer 2

This is the absorbed dose which would be generated in the associated oriented and expanded radiation field at a depth of 10 mm on the radius of the ICRU sphere which is oriented opposite to the direction of incident radiation.
An oriented and expanded radiation field is an idealised radiation field which is expanded and in which the radiation is additionally oriented in one direction.

Question 3

When would H’(0.07) be used?

Answer 3

For low energy (i.e. Mammography) X-Rays.

Question 4

How are H*(10) and H’(0.07) converted into Personal Dose equivalences (Hp(10), Hp(0.07))?

Answer 4

Corrected for anticipated backscatter.

Question 5

How is the Effective Dose Equivalent Defined?

Answer 5

The average absorbed dose to each organ multiplied by its radio-sensitivity.

Question 6

How is absorbed dose converted into equivalent dose?

Answer 6

Multiplied by radiation weighting factor.

Question 7

What are the radiation weighting factors according to ICRP-103?

Answer 7

Photons and Electrons (All Energies) - 1
Neutrons (All energies) - Continuous function between 2 and 20.
Protons (>20MeV) - 2
Alpha particles, fission fragments, heavy nuclei - 20

Question 8

What are the radiation weighting factors according to ICRP-60?

Answer 8

Photons and Electrons (All Energies) - 1
Neutrons (20MeV) - 5
Protons (>20MeV) - 5
Alpha particles, fission fragments, heavy nuclei - 20

Question 9

How do stochastic effects change with dose?

Answer 9

Probability of effect is proportional to dose, no threshold.

Evidence from Hiroshima/Nagasaki/Chernobyl survivors, early radiation workers, radium dial painters, Uranium miners.

Question 10

How do deterministic effects change with dose?

Answer 10

No effect below threshold dose.

Severity of effect increases with dose above threshold.

Question 11

In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.12?

Answer 11

Red bone marrow
Colon
Lung
Stomach
Breast
Remainder of organs not mentioned.
(adrenals, small intestine, kidney, muscle, pancreas, spleen, thymus, uterus, extrathoracic region, gall bladder, heart, lymph nodes, oral mucosa, prostate, cervix)

Question 12

In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.04?

Answer 12

Bladder
Liver
Oesophagus
Thyroid

Question 13

In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.08?

Answer 13

Gonads

Question 14

In ICRP-103, which tissues/organs have a Tissue Weighting Factor of 0.01?

Answer 14

Skin
Bone surface
Brain
Salivary glands

Question 15

What are the four components that the Tissue weighting factor depends on?

Answer 15

Probability of fatal cancer.
Probability of severe genetic effects
Relative length of life lost
Weighted probability of non-fatal cancer.

Question 16

What is the average annual radiation dose to a person in the UK from all sources?

Answer 16

2.7mSv

Question 17

What percentage of the annual dose arises from medical exposures?

Answer 17

14%

Question 18

What percentage of the annual dose arises from Radon Gas?

Answer 18

50%

Question 19

What percentage of the annual dose arises from food and drink and other naturally occurring radiation in the body?

Answer 19

10%

Question 20

What is the risk of life-time fatal cancer for the adult population?

Answer 20

5%/Sv

Question 21

What is the risk of lifetime fatal cancer for paediatric examinations?

Answer 21

6%/Sv

Question 22

What is the risk of lifetime fatal cancer for newborns?

Answer 22

15%/Sv

Question 23

What is the threshold lethal dose for of an embryo/foetus?

Answer 23

0.1-1Gy

Question 24

What is the threshold dose for gross malformations in an embryo/foetus?

Answer 24

0. 2Gy at 2-5 weeks

0. 5Gy at 5-7 weeks

Question 25

What is the threshold dose for abnormal brain development in an embryo/foetus?

Answer 25

No threshold at 8-15 weeks

0.6-0.7Gy at 16-25 weeks

Question 26

What are the advantages of measuring patient surface dose?

Answer 26

Easy to measure (TLD on patient)

Easy to calculate from radiographic factors (Tube output, and SSD)

Question 27

What are the disadvantages of measuring patient surface dose?

Answer 27

No indication of volume irradiated

Non-additive if beam position changes.

Question 28

How can patient surface dose be calculated?

Answer 28

Surface Dose = Tube output x mAs x Inverse Square Law Correction x Backscatter factor

Question 29

What are the advantages of patient organ dose measurement?

Answer 29

Can be calculated from surface dose if organ position known.

Tables of normalised organ dose data available.

Question 30

What are the disadvantages of patient organ dose measurement?

Answer 30

Difficult to measure directly

Need to know attenuation of beam at organ depth (PDD)

Question 31

What assumptions are made when using DAP as a patient dose measurement?

Answer 31

All energy is absorbed by patient

DAP-meter is larger than field

Question 32

Define the CTDI?

Answer 32

CTDI = 1/nT * INT(D(z) dz)
n - number of simultanteous slices
T - slice thickness
D(z) - dose profile along z-axis
Usually integrated between -7T and +7T

Question 33

Define the CTDI-100?

Answer 33

CTDI corrected by L/(nT) where L is the chamber length (100mm), n is the number of simultaneous slices, T is the slice width.

Question 34

Define the CTDI-w?

Answer 34

CTDI-w = 1/3 CTDI-100, centre + 2/3 CTDI-100, periphery

Question 35

Define the CTDI-vol?

Answer 35

CTDI-w corrected for pitch (couch increment) and mAs

Question 36

Define the Dose-Length Product?

Answer 36

CTDI-vol x irradiated length.