Biological Effects of Ionising Radiation

Question 1

ionising radiation can be divided into 2 main forms ~ what are these

Answer 1

1) By-products of radioactive decay
• Alpha particle (2 protons / 2 neutrons)
§ Large particle (20 µm in water)
• Beta particle (electron)
§ Very small particle (less than 1cm in water)
• Gamma ray (electromagnetic radiation)
§ High energy
§ Travels large distances
§ 10s of cm in water
§ Identical to x-rays - only difference is their source

2) Artificially produced electromagnetic radiation
• X-rays for radiographic imaging
§ High or low energy
§ Travels 10s of cm in water

Question 2

do atoms have equal numbers of protons and electrons

Answer 2

yes

Question 3

do ions have equal numbers of protons and electrons

Answer 3

no

Question 4

how does ionising radiation work on atoms and ions

Answer 4

Ionising radiation has enough energy to turn atoms into ions

It does this by “knocking away” electrons orbiting the nucleus of an atom

Question 5

what property sets the ionising radiation hitting the atom apart from other types of lower energy radiation

Answer 5

is that a single photon of radiation can carry enough energy to ionise an atom

Question 6

what is the result of ionisation

Answer 6

The result of ionisation is a free electron and a positively charged ion

Question 7

what is an ion pair

Answer 7

This negative electron and positively charged ion are called an ion pair

Question 8

what happens during the interaction of radiation

Answer 8

• When radiation passes through matter it will ionise atoms along its path
• Following each ionisation process, each ion pair, will deposit a certain amount of energy locally, approximately 35 eV for air and tissue
• This energy is greater than the energy involved in atomic bonds eg ionic and covalent bonds in molecules approximately only 4 eV

• Ionising radiation deposits energy along a track
○ the density at which ionisation occurs differs with different types of radiation
○ Gamma rays and electrons are sparsely ionising
○ Alpha particles, protons and neutrons and heavy ions are densely ionising

Question 9

what is the most significant effect of ionising radiation

Answer 9

Damage to DNA

Question 10

where can evidence of DNA damage be seen and what occurs

Answer 10

• Evidence of DNA damage can be seen in the faulty repair of chromosome breaks, leading to development of abnormal cell populations and the development of cancer

○ Fault repair of breaks is seen in individuals who are exposed to large radiation doses

○ The majority of damage is easily repaired, depending on the category of damage

Question 11

how does radiation damage DNA directly

Answer 11

Radiation interacts with the atoms of a DNA molecule or another important part of the cell

Radiation interacts directly with the atoms of a DNA molecule or another important part of the cell

Question 12

how does radiation damage DNA indirectly

Answer 12

○ Radiation interacts with water in the cells (which is 75% water)

○ When a water molecule becomes ionised a highly reactive free radical ion is formed

○ Two of these ions can combine to form a hydroxyl radical which can diffuse short distances and cause DNA damage

○ Free radicals are unstable, highly reactive molecules

○ Radiation interacts with water in the cell, producing free radicals which can cause damage

Question 13

what is an advantage of DNA being a double helix

Answer 13

if only one strand of the helix breaks, the DNA is still held in place by the second strand and so it can be easily fixed

Question 14

what happens if both strands of DNA breaks

Answer 14

if both strands break it becomes far more difficult to piece the DNA back together
The 2 remaining ends will seek to re-join with other free ends, not necessarily the correct matching end

Question 15

what do double strand breaks usually occur as a result of

Answer 15

Usually occur as a result of alpha radiation

The increase of DNA damage complexity with ionisation density

Question 16

what happens in the double strand break repair is mis-joined

Answer 16

then this can lead to mutations which can affect cell function

Question 17

what does the biological effect depend on

Answer 17

○ Type of radiation

○ Amount of radiation (dose)
§ The energy absorbed

○ Time over which the dose is received (dose rate)

○ The tissue or cell type irradiated

Question 18

what is the weighting factor for beta gamma and x-rays

Answer 18

1

Question 19

what is the weighting factor for alpha particles

Answer 19

20

Question 20

what does low doses of radiation produce

Answer 20

less damage

Question 21

at low dose rates, what is the effect of the radiation

Answer 21

Radiation delivered at a low dose rate is less damaging

Cells can repair less serious DNA damage before further damage occurs

Question 22

what is the effect on DNA at high dose rates

Answer 22

At high dose rates, the DNA repair capacity of the cell is likely to be overwhelmed

Question 23

what are the organ cancer risks of radiation

Answer 23

• Following large radiation exposures, there has only been higher incidences of cancer in certain tissues, not all tissues
• Most medical exposures do not irradiate the body uniformly
• Risk will vary depending on the organ that receives the highest dose

Question 24

list organs at risk of radiation

Answer 24

• oesophagus
• thyroid
• lungs
• skin
• breast
• stomach
• liver
• colon
• gonads

(actually pretty sure this is most of the organs so maybe ignore this q? up to you lol)

Question 25

what is the radiosensitivity of tissues dependent on

Answer 25

• The radiosensitivity of tissues is dependent on two factors ○ The function of the cells that makes up the tissues ○ If the cells are actively dividing

Question 26

what are stem cells

Answer 26

• Stem cells exist to produce cells for another cell population ○ Divide frequently ○ Very radiosensitive

Question 27

what are differentiated cells

Answer 27

○ Do no exhibit mitotic (dividing) behaviour ○ Less sensitive to radiation damage

Question 28

what happens the more rapidly a cell is dividing

Answer 28

the greater the sensitivity to radiation

Question 29

what are highly radiosensitive tissues

Answer 29

``` ○ Bone marrow ○ Lymphoid ○ Gastrointestinal ○ Gonads ○ Embryonic ```

Question 30

what are moderately radiosensitive tissues

Answer 30

○ Skin ○ Vascular endothelium ○ Lung ○ Lens of the eye

Question 31

what are the least radiosensitive tissues

Answer 31

○ Central nervous system ○ Bone and cartilage ○ Connective tissue

Question 32

what is the tissue weighting factor for bone marrow, colon, lung, stomach and breasts

Answer 32

0.12

Question 33

what is the tissue weighting factor for gonads

Answer 33

0.08

Question 34

what is the tissue weighting factor for bladder, oesophagus, liver and thyroid

Answer 34

0.04

Question 35

what is the tissue weighting factor for bone surface, skin, brain and salivary glands

Answer 35

0.01

Question 36

what is the tissue weighting factor for remaining tissues

Answer 36

0.012

Question 37

what is the sum of all the weighting factors

Answer 37

1

Question 38

what are the 2 outcomes when radiation hits the cell nucleus

Answer 38

- no change | - DNA mutation

Question 39

what can happen if DNA mutation occurs

Answer 39

1. mutation repaired = viable cell 2. cell death = unviable cell 3. cell survives but is mutated = cancer?

Question 40

what cells are more susceptible to damage

Answer 40

dividing cells

Question 41

are doses from heavily ionising particle radiations more damaging than similar doses of x-rays

Answer 41

yes

Question 42

are affected cells able to repair

Answer 42

yes

Question 43

what might happen to heavily damaged cells

Answer 43

may be programmed to die

Question 44

what has ionising radiation the ability to do

Answer 44

Ionising radiation has the ability to transfer energy from one medium to another eg an x-ray tube to a patient

Question 45

what is dose

Answer 45

dose is a measure of the amount of energy that has been transferred and deposited in a medium

Question 46

what does dose not take account of

Answer 46

dose takes no account of the variations in the potential damage the different types of radiation produce, or the different sensitivities of tissues

Question 47

what is absorbed dose (Gy)

Answer 47

○ A quantity that can be measured ○ Absorbed dose measures the energy deposited by radiation ○ For example: an intra-oral x-ray ~ the typical entrance skin dose at the collimator tip is around 2 mGy

Question 48

what is equivalent dose

Answer 48

• Equivalent Dose, a derived quantity (Sv) ○ Equivalent dose is the absorbed dose multiplied by a radiation weighting factor depending on the type of radiation § For beta, gamma and x-rays the weighting factor is 1 § For alpha particles it is 20 ○ We have seen that different types of radiation can cause different levels of damage to tissue

Question 49

what is effective dose

Answer 49

• Effective dose, a derived quantity (Sv) ○ Equivalent dose to each organ, multiplied by the tissue weighting factor and summed ○ Represents the stochastic health risk to the whole body, which is the probability of cancer induction ○ A typical intra-oral x-ray effect dose is 5 µSv

Question 50

what is the LNT model

Answer 50

Linear No Threshold (LNT) Model • Current practical radiation theory assumes that the coefficient for risk against dose remains constant no matter how small the dose. ○ It assumes that the LNT model is valid • The LNT model estimates the long term biological damage from radiation • It assumes that radiation is always harmful with no safety threshold ○ Several small exposures would have the same effect as one large exposure * The effective dose is directly proportional to the risk of cancer (the damage) * A dose of 1mSv has an associated lifetime risk of cancer of 1 in 20,000 * The risk from an intra-oral x-ray is less than 1 in 10,000,000 • It is expected that exposure to radiation at low levels is going to happen (both from nature and human technology) The aim is maintain the dose to the public to as low as reasonably practiticable

Question 51

what are the 2 types of radiation effects

Answer 51

1) Deterministic Effects • Tissue reactions • Only occur above a certain (threshold) dose • The severity of the effect is related to the dose received 2) Stochastic Effects (statistical approach) • The probability of occurrence is related to the dose received § Basis of LNT model • No threshold to the effect and severity of that effect is not dependent on dose

Question 52

what are deterministic effects

Answer 52

* Unusual to see in radiology although possible in high dose areas such as interventional radiology * Often the effects will not show immediately, but rather several days after exposure

Question 53

what can occur in deterministic effects

Answer 53

- bone marrow blood cell depletion (>0.5Sv) - cataracts (>0.5Sv) - sterility (>3Sv) - hair loss (>3Sv) - skin damage / erythema (>5Sc) - lethal dose (6Sv to whole body)

Question 54

what is a typical intra-oral x-ray effective dose

Answer 54

5µSv (0.000005 Sv)

Question 55

what are stochastic effects

Answer 55

• No known threshold for stochastic effects ○ There is no dose below which the effects will not occur • Cannot predict if these effects will occur in an exposed individual or how severe they will be ○ The likelihood of the effect occurring increases as the dose increases • Effects can develop years after exposure

Question 56

what can stochastic effects be subdivided into

Answer 56

1) Somatic - Results in disease or disorder eg cancer 2) Genetics - Abnormalities in descendants of those exposed

Question 57

what are the effects of radiation during pregnancy

Answer 57

• doses for any abnormalities to occur are more than 1000 times greater than that of an intra-oral x-ray • Pregnancy does not need to be taken into account for dental x-rays because the dose to the foetus is so low (between 0.01µSv and 8µSv) ○ This is usually less than the estimated daily natural background dose received by the foetus * The foetus should not be irradiated inadvertently nor should the x-ray beam be directed towards the patient's abdomen * In early pregnancy, radiation exposure above 100mGy could damage or kill enough of the cells for the embryo to undergo resorption * Lethal effects can be induced by the doses of the order of 100 mGy before or immediately after implantation of the embryo into the uterine wall * During organogenesis (2-8weeks post conception) when the organs are not fully formed, doses > 250mGy could lead to growth retardation

Question 58

what are the risks of childhood cancer following exposure in utero

Answer 58

• A number of studied have examined the risk of childhood cancer before the age of 15 years, following an exposure in utero • The natural incidence rate of childhood cancer is low ○ 1 in 650 up to age of 15 years • The risk of cancer induction is 1 in 13,000 per 1 mGy exposure in utero • The risk of fatal cancer from a 1 mGy exposure is 1 in 40,000 up to age 15 years ○ Risk from a dental x-ray would be a million times less if beam is not directed towards abdomen

Question 59

what are the sources of natural background radiation

Answer 59

``` • There are many sources of natural background radiation ○ Cosmic rays - 320 µSv / year at sea level - 9 mSv / year at 6000m - Round trip to mars 0.6 Sv ``` ○ Internal radionuclide from diet 300µSv annually ○ Radionuclides in the air - Eg Radon ``` ○ External gamma radiation - Eg soil - Eg rocks □ The decay of uranium □ Building materials ``` • The estimated annual UK natural background radiation dose is 2.2mSv which could increase to 10 mSv in some regions due to radon

Question 60

what does a radon atlas show

Answer 60

A radon atlas shows bands of radon risk in each 1km grid square

Question 61

what are the effective doses from examinations

Answer 61

• Intra-oral x-ray = 0.005 mSv ○ Life time risk of cancer: 1 in 10milllion - 1 in 100 million ○ Negligible risk

Question 62

what do the guidance notes for dental practitioners on the safe use of x-ray equipment include

Answer 62

• IRMER17 (patients) ○ Duty holders, employer's procedures, accidental and unintended exposures, MPE, training • IRR17 (staff and public) ○ Safety and warning systems, risk assessment, controlled areas, personal dose monitoring, local rules, RPS, RPA • QA ○ QA programme for x-ray equipment, image processing, viewing, image quality • Appendices with examples of forms and procedures

Question 63

how are staff protected in a controlled area

Answer 63

• The controlled area should extend at least 1.5m from the x-ray tube and patient The x-ray beam should always be directed away from staff members

Question 64

explain justification

Answer 64

Practices must have sufficient benefit to individuals or society in order to offset the detriment

Question 65

explain optimisation

Answer 65

Individual doses and the number of people exposed should be kept as low as reasonably practicable (ALARP)

Question 66

what is dose optimisation

Answer 66

• Dose optimisation is a legal requirement • We need to make sure the dose to the patients is ALARP ○ Still maintain adequate image quality ○ Circular collimators have been shown to increase the dose by 40% ○ Rectangular collimators should be used • Patient doses can be reduced using a variety of methods ○ Use E speed film or faster (fewer x-ray photons required) ○ Use a kV range of 60kV to 70kV ○ The focus to skin distance should be >200mm

Question 67

what are DRLs

Answer 67

Diagnostic Reference Levels (DRLs)

Question 68

explain DRLs

Answer 68

* Legislation requires employers to have established dose levels for typical examinations for standard sized patients * They are a comparative standard that is used in optimisation * They are compared to national reference levels • Individual x-ray units are compared to the DRLs and national reference levels ○ Enables identification of units giving higher doses ``` • Current DRLs for intra-oral examinations ○ Adult § 0.9mGy (digital sensors) § 1.2mGy (phosphor plates and film) ○ Child: § 0.6mGy (digital sensors) § 0.7mGy (phosphor plates and film) ```

Question 69

what is included in a radiation risk assessment

Answer 69

Identify all hazards with a potential to cause a radiation accident Evaluate the risks of arising from the hazards