Biological Effects of Ionising Radiation Flashcards
1
Q
ionising radiation can be divided into (2)
A
by products of radioactive decay
artificially produced electromagnetic radiation
2
Q
by products of radioactive decay
divides into (3)
A
alpha particles
betal particles
gamma rays
3
Q
alpha particles
A
2 protons/2 neutrons
large particle
20um in water
4
Q
beta particles
A
electron
very small particle
less than 1cm in water
5
Q
gamma rays
A
high energy
travels long distances
10s of cm in water
also electromagnetic radiation
6
Q
artificially produced electromagnetic radiation is
A
X-rays for radiographic imaging
high or low energy
travels 10s of cm in water
7
Q
X rays and gamma rays
A
are identical
only differ in source
8
Q
atoms Vs Ions
A
atoms have equal numbers of proton and electrons
ions do not
9
Q
ionising radiation energy
A
is enough to turn atoms into ions
Does this by ‘knocking away’ electrons orbiting the nucleus of an atom
single photon of radiation can carry enough energy to ionise an atom
10
Q
ion pair =
A
negative electron and positive atom
11
Q
interaction of radiation
A
When radiation passes through matter it will ionise atoms along it’s path
Following each ionisation process, each ion pair, will deposit a certain amount of energy locally, approximately 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 4eV
12
Q
how does density of ionisation differ
A
Density of ionisation occurs differs for radiation
Gamma and electrons are sparsely ionising
Alpha particles, protons and neutrons and heavy ions are densely ionising
13
Q
ionising potential for gamma and electrons
A
sparsely ionising
14
Q
ionising potential for alpha particles, protons and neutrons
A
heavy ions are densely ionising
15
Q
effect of ionising process on structure of organic molecules
A
Cells of basic building elements
DNA in cell nucleus
16
Q
most significant effect of ionising radiation
A
damage to DNA
17
Q
evidence of DNA damage by radiation
A
can be seen in the faulty repair of chromosome breaks, leading to development of abnormal cell populations and the development of cancer
The majority of damage is easily repaired, depending on the category of damage
18
Q
evidence of faulty DNA repair
A
Faulty repair of breaks is seen in individuals who are exposed to large radiation doses
19
Q
is DNA damage repairable?
A
The majority of damage is easily repaired, depending on the category of damage
20
Q
2 types of DNA damage
A
direct effect
indirect effect
21
Q
direct effect of DNA damage
A
radiation interacts with the atoms of a DNA molecule or another important part of cell
22
Q
indirect effect of DNA damage
A
Radiation interacts with water in the cell (75% water)
when water molecule becomes ionised a highly reactive free radical ion is formed
- 2 of these can combine to form a hydroxyl radical which can diffuse short distances and cause DNA damage
Free radicals are unstable, highly reactive molecules
- these damage DNA
23
Q
free radicals
A
unstable, highly reactive molecules
24
Q
DNA damage when no radiation
A
can occur
frequency of more than 50 thousands per cell per day
25
DNA damage and repair - advantage of structure
An advantage of DNA being a double helix is that if only one strand of the helix breaks, the DNA is still held in place by the second and so it can be easily fixed
However, 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
26
what if both strands of DNA double helix break
However, 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
27
single strand DNA break
can usually be repaired
28
double stranded breaks (DSB)
more difficult to repair
Usually occur as a result of alpha radiation
- The increase of DNA damage complexity with ioniation density
If the DSB is misrejoinined, then this can lead to mutations which can affect cell function
29
increase of DNA damage is with
ionisation density
- alpha
30
biological effect depends on (4)
Type of radiation
Amount of radiation (dose/ energy absorbed)
Time over which the dose is received (dose rate)
The tissue or cell type irradiated
31
low doses of radiation produces
less damage compared to higher density doses
e.g. X rays less damage compared to alpha particles
32
alpha particles Vs X-rays in damage
there is a linear relationship for alpha particles which in turn kills more cells than a similar dose of X-rays would
33
dose rate of radiation on damage effect
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
34
organ cancer risks of radiation
Following large radiation exposures, there has only been higher incidence 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
35
tissue radiosensitivity
dependent on (2)
the function of the cells that make up the tissue
if the cells are actively dividing
36
stem cells radiosensitivity
Stem cells exist to produce cells for another cell population
-Divide frequently
Very radiosensitive
37
differentiated cells radiosensitivity
Do not exhibit mitotic (dividing) behaviour
Less sensitive to radiation damage
38
the more rapidly the cells is dividing the...
greater the sensitivity to radiation
39
highly radiosensitive tissues (5)
```
bone marrow
lymphoid
gastrointestinal
gonads
embryonic
```
40
moderately radiosensitive tissues
skin
vascular endothelium
lung
lens of the eye
41
least radiosensitive tissues
CNS
bone and cartilage
connective tissue
42
tissue weighting factors related to
Tissue radiosensitivity leading to tissue weighting factors
| - The more rapidly a cell is dividing, the greater the sensitivity to radiation
43
tissue weighting factor for:
| bone marrow, colon, lung, stomach, breast
0.12
44
tissue weighting factor for:
| gonads
0.08
45
tissue weighting factor for:
| bladder, oesophagus, liver, thyroid
0.04
46
tissue weighting factor for:
| bone surface, skin, brain, salivary glands
0.01
47
tissue weighting factor for:
| remaining tissues
0.12
48
sum of all tissue weighting factors
1.00
49
what are the possible outcomes after radiation hits a cell
radiation hits cell nucleus
- no change
- DNA mutation (3 outcomes)
50
3 outcomes of DNA mutation
mutation repaired -> viable cell
cell death -> unviable cells
cell survives but is mutated -> cancer?
51
experiments of ionising radiation on cell cultures
Doses from heavily ionising particle radiations are more damaging than similar doses of X-rays
Affected cells may be able to repair damage
Dividing cells are more susceptible to damage
Heavily damaged cells may be programmed to die
52
ionising radiation transfer of energy
Ionising radiation has the ability to transfer energy from one medium to another
e.g. X-ray tube to a pt
53
dose is
measure of the amount of energy that has been transferred and deposited in a medium
has units
- defined in order to quantify the level of biological damage and the overall effect of the dose
54
dose units defined to
defined in order to quantify the level of biological damage and the overall effect of the dose
55
what does dose not take into account
dose takes no account of the variations in the potential damage that different types of radiation produce, or the different sensitivities of tissues
56
absorbed dose (Gy)
a quantity that can be measured
absorbed dose measures the energy deposited by radiation
for example, for an intra-oral X-ray the typical Entrance Skin Dose at the collimator tip is around 2mGy
57
equivalent dose (Sv)
a derived quantity
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
58
effective dose (Sv)
a derived quantity
Equivalent dose to each organ, multiplied by the tissue weighting factor and summed.
Represents the stochastic health risk to the whole body,
- which is probability of cancer induction
A typical intra-oral X-ray effective dose is 5uSv
59
linear no threshold model (LNT)
effect of radiation on the population
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
The LNT model estimates the long term biological damage for radiation
- it assumes that radiation is always harmful with no safety threshold
- several small exposures would have the same effect as one large exposure
60
LNT assumes
it assumes that radiation is always harmful with no safety threshold
The LNT model estimates the long term biological damage for radiation
- several small exposures would have the same effect as one large exposure
61
LNT
the effective dose is directly proportional to
the risk of cancer (the damage)
62
associated lifetime risk of 1mSv dose
associated lifetime risk of cancer of 1 in 20,000
63
associated lifetime risk of an intra oral X-ray
less that 1 in 10,000,000
64
error bars on LNT at high dose end
tight
compared to large at low dose end
means several curves can be drawn through data points
65
error bars on LNT at low dose end
large
means several curves can be drawn through data points
66
why has the ICRP opted for linear no threshold (LNT) model
cautious approach to radiation reduction
67
aim for public radiation exposure
maintained the doses to public to As Low As Reasonable Practicable
accepted that exposures to natural low levels of radiation are inevitable
68
2 types of radiation effects
deterministic effects
stochastic effects (statistical approach)
69
deterministic radiation effects
tissue reactions
only occur above a certain (threshold) dose
the severity of the effect s related to the dose received
70
stochastic radiation effects (statistical approach)
the probability of occurence 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
71
deterministic effects
seen when
unusual to see in radiology but possible in high dose areas (e.g. interventional radiology)
often the effects will not show immediately, but rather several days after exposure
72
threshold dose for deterministic effects on
bone marrow blood cell depletion
>0.5Sv
73
threshold dose for deterministic effects on
cataracts
>0.5Sv
74
threshold dose for deterministic effects on
sterility
>3Sv
75
threshold dose for deterministic effects on
hair loss
>3Sv
76
threshold dose for deterministic effects on
skin damage/erythema
>5Sv
77
threshold dose for deterministic effects on
lethal dose
6 Sv to whole body
For comparison, a typical intra-oral X-ray effect dose is 5uSv (0.000005 Sv)
78
stochastic effects threshold
no known threshold for stochastic effects
| - there is no dose below which the effects will not occur
79
able to predict stochastic effects?
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
80
stochastic effects devlop
years after exposure
81
stochastic effects categories (2)
somatic - results in disease or disorder e.g. cancer
genetics - abnormalities in descedents
82
somatic stochastic effect
results in disease or disorder e.g. cancer
83
genetic stochastic effect
abnormalities in descendants
84
effect of radiation during preganancy
doses for any abnormalities to occur are moe 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 0.01 uSv and 8 uSv
- this is usually less than the estimated daily natural background dose received by the foetus
in early pregnancy, radiation exposure above 100 mGy 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 100 mGy before or immediately after implantation of the embryo into the uterin wall
during organogenesis (2 – 8 weeks post conception) when the organs are not fully formed, doses >250 mGy could lead to growth retardation
85
effect of radiation on pregnancy
- dental X-rays
doses for any abnormalities to occur are moe 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 0.01 uSv and 8 uSv
- this is usually less than the estimated daily natural background dose received by the foetus
86
childhood cancer and radiation
a number of studies have examined the risk of childhood cancer before the age of 15 years, following exposure in utero
the natural incidence of childhood cancer is low – 1 in 650 up to age of 15
the risk of cancer induction is 1 in 13,00 per 1mGy exposure in utero
the risk of fatal cancer from a 1mGy exposure is 1 in 40,000 up to age 15
- risk from a dental X-ray would be a million times less if beam is not directed towards abdomen
87
sources of natural background radiation (4)
cosmic rays, 320 uSv/yr at sea level 9 mSv/yr at 6000m
- round trip to Mars 0.6 Sv
internal radionuclides from diet 300 uSv annually
radionuclides in the air e.g. radon
external gamma radiation
- e.g. soil, rocks, the decay of uranium, building materials
88
estimated annual UK natural background radiation dose is
2.2mSv
can increase to 10mSv in some regions due to radon
89
sources of radiation exposure UK from highest to lowest
NATURAL (84%)
- radon gas from ground (50%)
- gamma rays from ground/buildings (13%)
- cosmic rays (12%)
- food and drink (9.5%)
ARTIFICIAL (16%)
- fallout (0.2%)
- occupational (0.2%)
- discharges (<0.1%)
- products (<0.1%)
- medical (15%)
90
effective doses for examination
intra oral X-ray
0.005mSv
lifetime risk of cancer, 1 in 10 million – 1 in 100 million
- negligible risk
91
effective doses for examination
lumbar spine X-ray
1 mSv
Lifetime risk of cancer, 1 in 10,000 – 1 in 100,000
- Very low risk
92
effective doses for examination
abdominal CT
10 mSv
Lifetime risk of cancer, 1 in 1,000 – 1 in 10,000
- Low risk
93
cumulative exposure from several X-rays
can be significant
94
radiation protection
guidance notes for dental practitioners on safe use of X-ray equipment 2nd edition
IRMER (pts)
- 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 programme for X-ray equipment, image processing, viewing, image quality
95
dose limits IRR17
employee
body: 20mSv
skin, extremities: 500mSv
eye: 20mSv
96
dose limits IRR17
trainee <18y
body: 6mSv
Skin, extremities: 150mSv
eye: 15mSv
97
dose limits IRR17
other
body: 1mSv
skin, extremities: 50mSv
eye: 15mSv
98
controlled areas IRR17
Should extend at least 1.5m from the X-ray tube and pt
The X-ray beam should always be directed away from staff
99
radiation protection philosophy from IRMER17 protecting the pt
justification
- Practices must have sufficient benefit to individuals or society in order to offset the detriment
optimisation
- Individual does and the number of people exposed should be kept as low as reasonably practicable (ALARP)
100
dose optimisation
is legal requirement
We need to make sure the dose to the patients is ALARP
- Still maintain adequate image quality
Rectangular collimators should be used
- Circular collimators have been shown to increase dose by 40%
101
3 ways patient doses can be reduced
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
102
diagnostic reference levels DRLs
Not appropriate to apply dose limits to medial exposure as there is a direct benefit to the pt
Quantitive guidance available – clinical audit tools to identify poor practice
Legislation requires employers to have established dose levels for typical examinations for standard sized patients
They are comparative standard that is used in optimisation
- They are compared to national reference levels
Individual X-ray units are compared to DRLs and national reference levels
- Enable identification of units giving higher doses
103
current DRLs for adult intra-oral examinations
0. 9mGy (digital sensor)
| 1. 2mGY (phosphor plates and film)
104
current DRLs for child intra-oral examinations
0. 6mGy (digital sensor)
| 0. 7mGy (phosphor plates and film)
105
image quality needs to be
Sufficient for clinical purpose
Not justified exposure if there is no adequate image quality attained
106
CR plates damage
CR plates are prone to damage by teeth marks
- Reduce damage by inserting the plates between 2 plastic sheets
Damaged detectors should be cleaned or replaced if necessary
107
images with minor artefacts and non-uniformities
should be saved
Refer to these images if there is a suspected artefact in a clinical image
Can also be used for training
108
CR plates artefacts due to:
fingerprints
scratches
109
radiation risk assessment
by the employer
identify all hazards with a potential to cause a radiation accident
Evaluate the risks of arising from the hazards
