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BDS3 - radiology > Biological Effects of Ionising Radiation > Flashcards

Biological Effects of Ionising Radiation Flashcards

1
Q

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

A

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

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2
Q

do atoms have equal numbers of protons and electrons

A

yes

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3
Q

do ions have equal numbers of protons and electrons

A

no

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4
Q

how does ionising radiation work on atoms and ions

A

Ionising radiation has enough energy to turn atoms into ions

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

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5
Q

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

A

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

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6
Q

what is the result of ionisation

A

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

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7
Q

what is an ion pair

A

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

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8
Q

what happens during the interaction of radiation

A
  • 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

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9
Q

what is the most significant effect of ionising radiation

A

Damage to DNA

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10
Q

where can evidence of DNA damage be seen and what occurs

A

• 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

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11
Q

how does radiation damage DNA directly

A

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

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12
Q

how does radiation damage DNA indirectly

A

○ 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

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13
Q

what is an advantage of DNA being a double helix

A

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

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14
Q

what happens if both strands of DNA breaks

A

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

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15
Q

what do double strand breaks usually occur as a result of

A

Usually occur as a result of alpha radiation

The increase of DNA damage complexity with ionisation density

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16
Q

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

A

then this can lead to mutations which can affect cell function

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17
Q

what does the biological effect depend on

A

○ 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

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18
Q

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

A

1

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19
Q

what is the weighting factor for alpha particles

A

20

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20
Q

what does low doses of radiation produce

A

less damage

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21
Q

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

A

Radiation delivered at a low dose rate is less damaging

Cells can repair less serious DNA damage before further damage occurs

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22
Q

what is the effect on DNA at high dose rates

A

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

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23
Q

what are the organ cancer risks of radiation

A
  • 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
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24
Q

list organs at risk of radiation

A
  • 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)

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25
Q

what is the radiosensitivity of tissues dependent on

A

• 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

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26
Q

what are stem cells

A

• Stem cells exist to produce cells for another cell population

○ Divide frequently

○ Very radiosensitive

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27
Q

what are differentiated cells

A

○ Do no exhibit mitotic (dividing) behaviour

○ Less sensitive to radiation damage

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28
Q

what happens the more rapidly a cell is dividing

A

the greater the sensitivity to radiation

29
Q

what are highly radiosensitive tissues

A 
○ Bone marrow
○ Lymphoid 
○ Gastrointestinal
○ Gonads
○ Embryonic
30
Q

what are moderately radiosensitive tissues

A

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

31
Q

what are the least radiosensitive tissues

A

○ Central nervous system
○ Bone and cartilage
○ Connective tissue

32
Q

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

A

0.12

33
Q

what is the tissue weighting factor for gonads

A

0.08

34
Q

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

A

0.04

35
Q

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

A

0.01

36
Q

what is the tissue weighting factor for remaining tissues

A

0.012

37
Q

what is the sum of all the weighting factors

A

1

38
Q

what are the 2 outcomes when radiation hits the cell nucleus

A
  • no change

- DNA mutation

39
Q

what can happen if DNA mutation occurs

A
  1. mutation repaired = viable cell
  2. cell death = unviable cell
  3. cell survives but is mutated = cancer?
40
Q

what cells are more susceptible to damage

A

dividing cells

41
Q

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

A

yes

42
Q

are affected cells able to repair

A

yes

43
Q

what might happen to heavily damaged cells

A

may be programmed to die

44
Q

what has ionising radiation the ability to do

A

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

45
Q

what is dose

A

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

46
Q

what does dose not take account of

A

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

47
Q

what is absorbed dose (Gy)

A

○ 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

48
Q

what is equivalent dose

A

• 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

49
Q

what is effective dose

A

• 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

50
Q

what is the LNT model

A

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

51
Q

what are the 2 types of radiation effects

A

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

52
Q

what are deterministic effects

A
  • 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
53
Q

what can occur in deterministic effects

A
  • 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)
54
Q

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

A

5µSv (0.000005 Sv)

55
Q

what are stochastic effects

A

• 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

56
Q

what can stochastic effects be subdivided into

A

1) Somatic
- Results in disease or disorder eg cancer

2) Genetics
- Abnormalities in descendants of those exposed

57
Q

what are the effects of radiation during pregnancy

A

• 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
58
Q

what are the risks of childhood cancer following exposure in utero

A

• 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

59
Q

what are the sources of natural background radiation

A 
• 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

60
Q

what does a radon atlas show

A

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

61
Q

what are the effective doses from examinations

A

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

62
Q

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

A

• 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

63
Q

how are staff protected in a controlled area

A

• 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

64
Q

explain justification

A

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

65
Q

explain optimisation

A

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

66
Q

what is dose optimisation

A

• 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

67
Q

what are DRLs

A

Diagnostic Reference Levels (DRLs)

68
Q

explain DRLs

A
  • 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)
69
Q

what is included in a radiation risk assessment

A

Identify all hazards with a potential to cause a radiation accident

Evaluate the risks of arising from the hazards

BDS3 - radiology (19 decks)

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