Radiation Risk

Question 1

What is meant by equivalent dose? (3 marks)

Answer 1

• It is the absorbed dose, D, multiplied by the radiological weighting factor, W_R.
• H_T = D * W_R
• Takes into consideration the different amount of damage done by the same amount of dose for different types of ionising radiation, e.g. photons is = 1 and alpha is = 20.

Question 2

What is meant by stochastic and deterministic effects? (4 marks)

Answer 2

Stochastic = probability of effect increases with dose but the severity is the same, e.g. secondary cancer induction.

Deterministic = above a particular dose the effect with occur and the severity increases with higher doses e.g. erythema.

Question 3

Give four threshold doses for deterministic effects? (4 marks)

Answer 3

• cataracts = 0.5 Gy
• erythema = 2 Gy
• infertility ovaries = 3 Gy
• infertility testes = 6 Gy

Question 4

When can genetic damage occur which is passed on to offspring? What causes it and why may it be difficult to detect? (3 marks)

Answer 4

• Genetic damage only occurs when the gonads are irradiated.
• Caused by faulty chromosome re-joining
• Defects caused my be recessive and therefore hard to measure

Question 5

What is effective dose? (3 marks)

Answer 5

E = sum(H_T * W_T)

Is the equivalent dose multiplied by the tissue weighting factors and summed.

Takes into account the radiosensitivity differences of different tissues.

Can be used to give an indication of total detriment.

Question 6

Give four weighting factors, three which have changed from ICRP 60 -> 103. (12 marks)

Answer 6

• Testes 0.20 to 0.08
• Ovaries 0.05 to 0.12
• Remainder 0.05 to 0.12
• Skin 0.05 to 0.05

Question 7

What is meant by detriment and what does it depend on? (5 marks)

Answer 7

Detriment is a measure of total harm caused by ionising radiation.

1: increase in probability of fatal cancer, which depends on age etc.
2: Probability of non-fatal cancer.
3: Other side effects such as loss of fertility, painful erythema
4: Length of life lost
5: Probability of genetic defects

Question 8

What sources of radiation exposure are there in the UK? What is the total average per capita and what percentage is from medical exposures? (3 marks)

Answer 8

Radon gas, medical, cosmic background.

Total is 2.7 mSv, 14 % is medical (0.4 mSV).

Question 9

What is the risk of death from radiation and how does age affect this? (2 marks)

Answer 9

Roughly 5% per Sv increase in fatal cancer, probability reduces with age.

Question 10

Give three exams and typical doses for these? (3 marks)

Answer 10

Chest PA x-ray = 0.02 mSv
Ba Swallow = 1.5 mSv
CT abdo = 10 mSv

Question 11

What effects can radiation have on an embryo? (4 marks)

Answer 11

• Reduction in IQ
• Death
• Gross malformations
• Cancer induction

Question 12

Give two ways in which surface dose can be measured or estimated? (3 marks)

Answer 12

• Directly using TLDs
• Can estimate it using DAP or outputs from QA measurements. Use output at 1 m, mAs, distance correct and account for backscatter.

Question 13

How can organ dose be measured? (2 marks)

Answer 13

Use TLD or estimates to get ESD or DAP and use look-up tables from HPA to convert to organ dose values.

Question 14

What are the pros and cons of direct and indirect measurements? (6 marks)

Answer 14

Direct measurements such as TLDs are more accurate, however they are most costly and require processing. Only surface dose possible.

Indirect methods such as using outputs and DAP values are cheaper and quicker, but potentially less accurate methods. Can be done retrospectively.

Question 15

Give four CT dose related quantities? (8 marks)

Answer 15

CTDI = CT dose index, dose profile area measured using pencil chamber

CTDI_w = Weighted = 1/3 central + 2/3 peripheral

CTDI_vol = pitch and mA corrected

DLP = dose length product, vol x length