Radiation protection Flashcards
Means of expressing dose
Radiation energy deposited in unit mass of tissue
-absorbed dose (Gray, Gy)(1Gy = 1J/kg)
Absorbed dose weighted for harmfulness of different radiations
-dose equivalent (Sievert, Sv)
Dose equivalent weighted for radiation susceptibility of different tissues. Mainly a measure of risk, rather than a physical dose
-Effective Dose (Sievert, Sv)
Tissue weighting factors: bone marrow, breast, colon, lung, stomach
0.12
Tissue weighting factors: bladder, oesophagus, gonads, liver, thyroid
0.05
Tissue weighting factors: bone surface, brain, kidneys, salivary glands, skin
0.01
Tissue weighting factors: remainder tissues
0.10
Summed total tissue weighting factors
1 = Whole body effective dose
Where does radiation come from?
14.3% medical
13.5% gamma from ground and buildings
11.6% internal (food)
10% cosmic
50.1% radon
0.5% other sources
(85% natural sources, 15% man-made)
Average annual UK background dose
2.5mSv
How can radiation affect DNA?
Directly: secondary electron breaks DNA strand
Indirectly: e- leaves trail of water molecules behind, highly reactive OH* free-radicals created in this way very damaging
Why is radiation so dangerous
Instantaneous whole body dose of 4Sv could potentially be fatal
!!!
Low risk
1 strand damage
High risk
Both strand breaks (more from indirect)
Consequences of DNA mutation
Mutation repaired –> viable cell
Cell death –> unviable cell
Cell survives but mutated –> cancer?
Cancer
Development of cancer has several stages, can take year
Cells not helpless, can repair DNA to some extent
Even in absence of radiation, each cell in body sustains 5-10000 DNA damage events an hour from constant onslaught of free radicals produced as by-product of reactions within cell
Two types of effect:
Deterministic and stochastic
Deterministic effect
Direct consequence of dose above minimum threshold, > in severity with dose
Threshold specific to tissue
Threshold normally of order of few Gy
LOOK AT GRAPH
Stochastic effect
> in likelihood but not severity with > dose, with no minimum threshold
Severity is indipendent of dose
e.g. induction of cancer, where extremely low doses involve > risk of cancer. As dose > severity of individual malignancies does not >, but probability that larger proportion of exposed population may develop malignancy
LOOK AT GRAPH
Who is affected?
Somatic effects are those occurring in individual as result of exposure to dose of radiation
Genetic effects are those which arise in offspring of individual exposed to dose of radiation
Thresholds for deterministic effects: whole body exposure
Detectable chromosome damage >0.1Sv Detectable blood count change >1Sv Radiation sickness >1 Possible death >3Sv Certain death >10Sv
Dental radiography risks
Intraoral/ periapical 2-10 μSv, cancer risk 0.06-0.7 per million exposure
CT Mandible 480-3300 μSv, cancer risk 24-242 per million exposures
Levels of risk for common X-ray exams
Negligible risk (< 1 in 1,000,000): a few days of background radiation
-chest, teeth, arms and legs, hands and feet
Minimal (1 in 1,000,000 to 1 in 100,000): a few weeks
-skull, head, neck
Very low risk (1 in 100,000 to 1 in 10,000): few months to a year
-breast, hip, spine, abdomen, pelvis, CT scan of head
Low risk (1 in 10,000 to 1,000): a few years
-kidneys and bladder, stomach, colon, CT of chest, CT of abdomen
Frequency of dental radiographs in UK
~20M radiographs in GDP
~3< panoramic (OPT) radiographs
Three golden rules of radiation protection
Time
Distance
Shielfing
Time
The longer you spend near a source of radiation, the higher your dose
Minimise the exposure time
LOOK AT GRAPH
Distance
As you move away from source, the dose rate decreases rapidly according to inverse square law
Twice the distance = quarter the dose
LOOK AT GRAPH
Shielding
Where possible use shielding, it attenuates the radiation, reducing the dose
Attenuation proportional to density
Might not make your job easier but it is there for your safety
Includes lead screens, aprons
Room shielding
Normally room shielding specified by radiation protection advisor before build takes place
They will deicde how thick walls should be and what they should be made of and how much lead in foor
On existing builds where additional shielding needed, wall can be re-plastered in barium plaster or lead panels can be affixed
Staff protection
Dose limits (annual) -6mSv to whole body -50mSv to lens of eye -150mSv to skin/ extremities Scatter PPE Controlled areas
How to keep staff protected
Training
-don’t do what you haven’t been trained to
Risk assessment-do risk assessment before starting new work with radiation
Local rules (RPS)
-read and understand them
Dose badges
Patient protection
No legal dose limits for pxs but benefits must justify risk
Still expectation that doses will not be ‘much greater than intended’
DRLs (diagnostic reference levels) guidelines for px dose
Exposures must be optimised
How to optimise px protection: choose appropriate factors
Filtration
kV
-high energy xray photons more penetrating so needed for thicker pxs but will reduce image contrast
mA
-> current > number of photons, > dose and improving image contrast
FSD (focus-to-skin distance)
-closer the focus to px the less xrays are scattered so higher the dose but image less noisy
Collimation
-defines beam size, must be no more than 6cm diameter
Filtration
Gets rid of low energy x-rays that would get through px
1. spectrum with no filtration
2. spectrum through tube housing (inherent filtration)
3. Additional filtration
LOOK AT GRAPH
Public protection
IRR99 Dose Limits (annual) -1mSv whole body -15mSv to lens of eye -50mSv to skin/ extremities -HPA recommends 0.3mSv whole body Room shielding (materials) Warning lights/ notices
People who can help
Radiation protection adviser
-will need one if you set up your own practice
Radiation protection supervisor
-RPS local to where x-rays being used
-make sure local rules are followed
Medical physic expert
-assist with optimisation - set exposure parameters to give best images to lowest dose
-may be same person as RPA, but separate responsibilities
Thresholds for deterministic effects: localised exposures
Erythema >5Sv
Depilation temporary >4Sv
Permanent Depilation >7Sv
Desquamation >20Sv
