Biological Effects of Ionising Radiation Flashcards
ionising radiation can be divided into (2)
by products of radioactive decay
artificially produced electromagnetic radiation
by products of radioactive decay
divides into (3)
alpha particles
betal particles
gamma rays
alpha particles
2 protons/2 neutrons
large particle
20um in water
beta particles
electron
very small particle
less than 1cm in water
gamma rays
high energy
travels long distances
10s of cm in water
also electromagnetic radiation
artificially produced electromagnetic radiation is
X-rays for radiographic imaging
high or low energy
travels 10s of cm in water
X rays and gamma rays
are identical
only differ in source
atoms Vs Ions
atoms have equal numbers of proton and electrons
ions do not
ionising radiation energy
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
ion pair =
negative electron and positive atom
interaction of radiation
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
how does density of ionisation differ
Density of ionisation occurs differs for radiation
Gamma and electrons are sparsely ionising
Alpha particles, protons and neutrons and heavy ions are densely ionising
ionising potential for gamma and electrons
sparsely ionising
ionising potential for alpha particles, protons and neutrons
heavy ions are densely ionising
effect of ionising process on structure of organic molecules
Cells of basic building elements
DNA in cell nucleus
most significant effect of ionising radiation
damage to DNA
evidence of DNA damage by radiation
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
evidence of faulty DNA repair
Faulty repair of breaks is seen in individuals who are exposed to large radiation doses
is DNA damage repairable?
The majority of damage is easily repaired, depending on the category of damage
2 types of DNA damage
direct effect
indirect effect
direct effect of DNA damage
radiation interacts with the atoms of a DNA molecule or another important part of cell
indirect effect of DNA damage
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
free radicals
unstable, highly reactive molecules
DNA damage when no radiation
can occur
frequency of more than 50 thousands per cell per day
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
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
single strand DNA break
can usually be repaired
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
increase of DNA damage is with
ionisation density
- alpha
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
low doses of radiation produces
less damage compared to higher density doses
e.g. X rays less damage compared to alpha particles
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
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
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
tissue radiosensitivity
dependent on (2)
the function of the cells that make up the tissue
if the cells are actively dividing
stem cells radiosensitivity
Stem cells exist to produce cells for another cell population
-Divide frequently
Very radiosensitive
differentiated cells radiosensitivity
Do not exhibit mitotic (dividing) behaviour
Less sensitive to radiation damage
the more rapidly the cells is dividing the…
greater the sensitivity to radiation
highly radiosensitive tissues (5)
bone marrow lymphoid gastrointestinal gonads embryonic
moderately radiosensitive tissues
skin
vascular endothelium
lung
lens of the eye
least radiosensitive tissues
CNS
bone and cartilage
connective tissue
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
tissue weighting factor for:
bone marrow, colon, lung, stomach, breast
0.12
tissue weighting factor for:
gonads
0.08
tissue weighting factor for:
bladder, oesophagus, liver, thyroid
0.04
tissue weighting factor for:
bone surface, skin, brain, salivary glands
0.01
tissue weighting factor for:
remaining tissues
0.12
sum of all tissue weighting factors
1.00
what are the possible outcomes after radiation hits a cell
radiation hits cell nucleus
- no change
- DNA mutation (3 outcomes)
3 outcomes of DNA mutation
mutation repaired -> viable cell
cell death -> unviable cells
cell survives but is mutated -> cancer?
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
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
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
dose units defined to
defined in order to quantify the level of biological damage and the overall effect of the dose
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
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
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
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
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
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
LNT
the effective dose is directly proportional to
the risk of cancer (the damage)
associated lifetime risk of 1mSv dose
associated lifetime risk of cancer of 1 in 20,000
associated lifetime risk of an intra oral X-ray
less that 1 in 10,000,000
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
error bars on LNT at low dose end
large
means several curves can be drawn through data points
why has the ICRP opted for linear no threshold (LNT) model
cautious approach to radiation reduction
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
2 types of radiation effects
deterministic effects
stochastic effects (statistical approach)
deterministic radiation effects
tissue reactions
only occur above a certain (threshold) dose
the severity of the effect s related to the dose received
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
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
threshold dose for deterministic effects on
bone marrow blood cell depletion
> 0.5Sv
threshold dose for deterministic effects on
cataracts
> 0.5Sv
threshold dose for deterministic effects on
sterility
> 3Sv
threshold dose for deterministic effects on
hair loss
> 3Sv
threshold dose for deterministic effects on
skin damage/erythema
> 5Sv
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)
stochastic effects threshold
no known threshold for stochastic effects
- there is no dose below which the effects will not occur
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
stochastic effects devlop
years after exposure
stochastic effects categories (2)
somatic - results in disease or disorder e.g. cancer
genetics - abnormalities in descedents
somatic stochastic effect
results in disease or disorder e.g. cancer
genetic stochastic effect
abnormalities in descendants
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
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
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
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
estimated annual UK natural background radiation dose is
2.2mSv
can increase to 10mSv in some regions due to radon
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%)
effective doses for examination
intra oral X-ray
0.005mSv
lifetime risk of cancer, 1 in 10 million – 1 in 100 million
- negligible risk
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
effective doses for examination
abdominal CT
10 mSv
Lifetime risk of cancer, 1 in 1,000 – 1 in 10,000
- Low risk
cumulative exposure from several X-rays
can be significant
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
dose limits IRR17
employee
body: 20mSv
skin, extremities: 500mSv
eye: 20mSv
dose limits IRR17
trainee <18y
body: 6mSv
Skin, extremities: 150mSv
eye: 15mSv
dose limits IRR17
other
body: 1mSv
skin, extremities: 50mSv
eye: 15mSv
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
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)
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%
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
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
current DRLs for adult intra-oral examinations
- 9mGy (digital sensor)
1. 2mGY (phosphor plates and film)
current DRLs for child intra-oral examinations
- 6mGy (digital sensor)
0. 7mGy (phosphor plates and film)
image quality needs to be
Sufficient for clinical purpose
Not justified exposure if there is no adequate image quality attained
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
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
CR plates artefacts due to:
fingerprints
scratches
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
