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BDS3 Dental 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 2 main forms?

A
  • By product of radioactive decay

- Artificially produced electromagnetic radiation

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2
Q

Give 3 examples of by-products of radioactive decay?

A
  • Alpha particles (2 protons/2 neutrons)
  • Large particles, 20um in water
  • Beta particles (electron)
  • Very small particle, less than 1cm in water
  • Gamma ray (electromagnetic radiation)
  • High energy, travels long distances, 10s of cm in water
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3
Q

What is a type of artificially produces electromagnetic radiation?

A
  • X-rays - for radiographic imaging

- High or low energy, travels 10s of cm in water

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4
Q

Atoms have an equal number of protons and electrons. Do ions also have this?

A

No
- Ionising radiation has enough energy to turn atoms into ions. IT does this by ‘knocking out’ electrons orbiting the nucleus of an atom

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5
Q

When radiation passes through matter what will happen?

A
  • It will ionise atoms along its path
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6
Q

What is the result of ionisation?

A
  • The result is a free electron and a positively charged ion. This negative electron and positive ionised atom are called an ion pair
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7
Q

Explain the interaction of radiation? (3)

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, approx 35eV for air and tissue
  • This energy is greater than the energy involved in atomic bonds e.g. ionic and covalent bonds in molecules involve approximately only 4eV
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8
Q

What is the most significant effect of ionising radiation?

A
  • Damage to DNA
  • Evidence of DNA damage can be seen in the faulty repair of chromosome breaks, leading to the development of abnormal cell populations and the development of cancer
  • Faulty 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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9
Q

What are the 2 different categories that radiation damage to DNA can be split into?

A
  • Radiation can damage DNA directly or indirectly
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10
Q

What is the direct effect of radiation damage on DNA?

A
  • Radiation interacts with the atoms of a DNA molecule or another important part of the cell
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11
Q

What is the indirect effect of radiation damage on DNA?

A
  • Radiation interacts with water in the cell (75% water). When a water molecule becomes ionised a highly reactive free radical ion is formed, two of these can combine to form a hydroxyl radical which can diffuse short distances and cause DNA damage
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12
Q

What are free radicals?

A
  • Unstable, highly reactive molecules
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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 so it can be easily fixed
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14
Q

What is the problem if both strands of a double helix of DNA are broken?

A
  • If both strands break it becomes much more difficult to piece the DNA back together. The two remaining ends will seek to re-join with other free ends, not necessarily the correct matching end
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15
Q

How do double stranded breaks of DNA usually occur?

A
  • Usually occur as a result of alpha radiation - the increase of DNA damage with complexity with ionisation density
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16
Q

If a double strand break of DNA is repaired but misrejoined what will result?

A
  • This can lead to mutations which can affect cell function
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17
Q

The biological effect of DNA damage will depend on a number of factors. Give 4 examples of these factors?

A
  • Type of radiation
  • Amount of radiation (dose)
  • Time over which the dose is received (dose rate)
  • The tissue or cell type irradiated
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18
Q

A weighting factor is used depending on the type of radiation. What is the weighting factor for beta, gamma and x-rays?

A

1

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19
Q

A weighting factor is used depending on the type of radiation. What is the weighting factor for alpha particles?

A

20

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20
Q

IS radiation delivered at a low dose rate more or less damaging than radiation delivered at a high dose rate?

A
  • Radiation delivered at a low dose rate is less damaging
  • Cells can repair less serious DNA damage before further damage occurs
  • At high dose rates, the DNA repair capacity of the cell is likely to be overwhelmed
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21
Q

Are organs at risk of cancer when exposed to radiation?

A
  • Following large radiation exposures, there has only been higher incidences of cancer in certain 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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22
Q

The radio-sensitivity of tissues is dependent on 2 factors. What are these?

A
  • The function of the cells that make up the tissues

- If the cells are actively dividing

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23
Q

Stem cells exist to produce cells for another cell population. What are 2 properties of stem cells?

A
  • Divide frequently

- Very radiosensitive

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24
Q

Do differentiated cells exhibit mitotic (dividing) behaviour?

A

No

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25
Q

Are differentiated cells more or less sensitive to radiation damage compared to stem cells?

A
  • Less sensitive to radiation damage
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26
Q

What property of a cell makes it more sensitive to radiation?

A
  • The more rapidly it divides the more sensitive to radiation it is
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27
Q

Give examples of some highly radiosensitive tissues. (5)

A
  • Bone marrow
  • Lymphoid
  • GI
  • Gonads
  • Embryonic
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28
Q

Give examples of moderately radiosensitive tissues? (4)

A
  • Skin
  • Vascular endothelium
  • Lung
  • Lens of the eye
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29
Q

Give examples of least radiosensitive tissues? (3)

A
  • Central nervous system
  • Bone and cartilage
  • Connective tissue
30
Q

What are the 4 possible outcomes after radiation hits a cell nucleus?

A
  • No change
  • DNA mutation -> Mutation repaired -> Viable cell
  • DNA mutation -> cell death -> unviable cell
  • DNA mutation -> cell survives but is mutated -> Cancer?
31
Q

What is more damaging: doses from heavily ionising particle radiations or similar doses of x-rays?

A
  • Heavily ionising particle radiations
  • Affected cells may be able to repair damage
  • Dividing cells are more susceptible to damage
  • Heavily damaged cells may be programmed to die
32
Q

Ionising radiation has the ability to transfer energy from one medium to another. What is dose a measure of?

A
  • Dose is a measure of the amount of energy that has been transferred and deposited in a medium
  • However, dose takes no account of the variations in the potential damage that different types of radiation produce, or the different sensitivities of tissues
  • Additional dose units have been defined in order to quantify the level of biological damage and the overall effect of the dose
33
Q

What is Absorbed dose (Gy)?

A
  • A quantity that can be measured. Absorbed dose measures the energy deposited by radiation. For example, for an intra-oral x-ray they typical entrance skin dose at the collimator tip is around 2 mGy
34
Q

What is Equivalent dose (Sv)?

A
  • We have seen that different types of radiation can cause different levels of damage to tissue
  • Equivalent dose is the absorbed dose multiplied by 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
35
Q

What is effective dose (Sv)?

A
  • Equivalent dose to each organ, multiplied by the tissue weighting factor and summed
  • Represents the stochastic health risk to the whole body, which id the probability of cancer induction. A typical intra oral x-ray effective dose is 5uSv
36
Q

Current practical radiation theory assumes that the coefficient for risk against dose remains constant no matter how small the dose. It assumes that the linear no threshold (LNT) model is valid. What is the LNT?

A
  • 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 affect as one large exposure
37
Q

What is the effective dose directly proportional to?

A
  • The risk of cancer (the damage)
38
Q

What is the associated lifetime risk of cancer in a dose of 1mSv?

A
  • 1 in 20,000 risk of cancer
39
Q

What is the associated risk of cancer form an intra-oral x-ray?

A
  • Less than 1 in 10,000,000
40
Q

What are the 2 types of radiation effect?

A
  • Deterministic effects

- Stochastic effects (statistical approach)

41
Q

Explain deterministic effects of radiation?

A
  • Tissue reactions
  • Only occur above a certain (threshold) dose
  • The severity of the effect is related to the dose received
42
Q

Explain the stochastic effects of radiation?

A
  • 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
43
Q

Deterministic effects of radiation are unusual to see in radiology. However, when is this possible?

A
  • Possible in high dose areas such as interventional radiology
  • Often the effects will not show immediately but rather several days after exposure
44
Q

There is no known threshold for stochastic effect. What does this mean?

A
  • There is no dose below which the effect will not occur
45
Q

We cannot predict if stochastic effects will occur in an exposed individual or how severe they will be. What do we know though?

A
  • Know that the likelihood of the effect occurring increases as the dose increases
46
Q

When can stochastic effects develop?

A
  • Can develop years after exposure
47
Q

Stochastic effects can be subdivided into 2 categories. What are these?

A
  • Somatic - results in disease or disorder e.g. cancer

- Genetics - abnormalities in descendants

48
Q

Are doses of intra-oral radiographs damaging to pregnant women?

A
  • Doses for any abnormalities to occur are more than 1000 times greater than that of an intra-oral x-ray
49
Q

Do we need to be wary about dental x-rays and pregnancy?

A
  • Pregnancy does not need to be taken into account for dental x-rays because the dose to the foetus is so low; between 0.01uSv and 8.00uSv. This is usually less than the estimated daily natural background dose received by the foetus
50
Q

What do we need to be aware of when taking radiographs of pregnant women? (4)

A
  • The foetus must not be irradiated inadvertently nor should the x-ray beam be directed towards the patients 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 doses of the order of 100mGy before or immediately after implantation of the embryo into the uterine wall
  • During organogenesis (2-8 weeks post conception) when the organs are not fully formed, doses > 250mGy could lead to growth retardation
51
Q

What is the natural incidence of childhood cancer?

A
  • 1 in 650 up to the age of 15 years
52
Q

What is the risk of cancer induction per 1mGy exposure in utero?

A

1 in 13,000

53
Q

What is the risk of fatal cancer from a 1mGy exposure, up to the age of 15 years?

A

1 in 40,000

- Risk from a dental x-ray would be a million times less if beam is not directed towards abdomen

54
Q

Give examples of natural background radiation? (4)

A
  • Cosmic rays
  • Internal radionuclides from diet
  • Radionuclides in the air e.g. radon
  • External gamma radiation e.g. soil , rocks
55
Q

What is the estimated annual UK natural background radiation dose?

A
  • 2.2mSv which could increase to 10mSv in some regions due to radon
56
Q

Intra-oral x-rays have a dose of 0.005mSv. What is the lifetime risk of cancer?

A
  • 1 in 10 million - 1in 100 million

- Negligible risk

57
Q

Lumbar spine x-rays have a dose of 1mSv. What is the lifetime risk of cancer?

A
  • 1 in 10,000 - 1 in 100,000

- Very low risk

58
Q

Abdominal CT’s have a dose of 10mSv. What is the lifetime risk of cancer?

A

1 in 1,000 - 1 in 10,000

- Low risk

59
Q

Can cumulative exposures from a number of x-ray examinations be significant?

A
  • Yes
60
Q

In order to protect staff, what should the controlled area be when taking an x-ray?

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 the staff member
61
Q

What is justification of radiographs?

A
  • Practices must have sufficient benefit to individuals or society in order to offset the detriment
62
Q

What is optimisation of radiographs?

A
  • Individual doses and the number of people exposed should be kept as low as reasonably practicable (ALARP)
63
Q

Is dose opsonisation a legal requirement?

A
  • Yes
64
Q

We need to make sure that the dose is ALARP. How can we do this?

A
  • Still maintain adequate image quality
  • Circular collimators have been shown to increase the dose by 40%
  • Rectangular collimators should be used
65
Q

Patient doses can be reduced using a variety of methods. Give examples of theses? (3)

A
  • 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
66
Q

What are diagnostic reference levels?

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 opsonisation
  • They are compared to national reference levels
  • Individual x-ray units are compared to the DRL’s and national reference levels (enables identification of nits giving higher doses)
67
Q

What are the current DRL’s for intra-oral examinations? (2)

A

Adult: 0.9mGy (digital sensors) and 1.2mGy (phosphor plates and film)
Child: 0.6mGy (digital sensors) and 0.7mGy (phosphor plates and films)

68
Q

CR plates are prone to damage by teeth marks. How can we try to prevent this? (2)

A
  • Reduce damage by inserting the plates between 2 plastic sheets
  • Damaged detectors should be cleaned or replaced if necessary
69
Q

Can we still use images with minor artefacts or non-uniformities?

A
  • Yes these should be saved
  • Refer to these images if there is suspected artefact in a clinical image
  • Can also be used for training purposes
70
Q

What is a radiation risk assessment?

A
  • So before starting any work with ionising radiation the employer must undertake prior Risk Assessment. The purpose is to identify control measures that may need to be put in place to restrict exposures to employees and others.
  • This risk assessment should identify all hazards with a potential to cause a radiation accident and should evaluate the risks arising from them.

BDS3 Dental Radiology (13 decks)

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