Radiation Protection Flashcards
What is Absorbed Dose and what are it’s units?
Measure of the amount of energy deposited in a medium per unit mass of the medium.
Units = Gray (Gy)
What is absorbed dose measured in?
Measure in Joules/kg
What is the subunit of absorbed dose?
milligrays (mGy) (x10^-3)
What is the equivalent dose?
Takes into account the different radio-biological effectiveness of different types of radiation.
How is equivalent dose calculated?
Equivalent dose = absorbed dose x radiation weighting factor
What is the weighting factor?
Weighting factor = Wr
X-rays, gamma rays and beta particles Wr = 1
Alpha particles Wr = 20
What is the unit of equivalent dose?
Sievert (Sv)
Sub-unit = millisieverts (mSv) (x10^-3)
microsieverts (x10^-6)
What is the equivalent dose, measured in Sieverts, equal to?
It is equal to the absorbed dose, measured in Grays, as the radiation weighting factor = 1.
What is the effective dose?
The effective dose allows doses from different investigations of different parts of the body to be compared.
It converts the dose to the equivalent whole body dose, as some body parts are more sensitive to radiation than others.
Use a tissue weighting factor (Wt)
How is effective dose calculated and what is it measured in?
Effective dose (E) = equivalent dose x tissue weighting factor Unit = Sievert Sub-unit = millisievert
What does the term dose usually mean?
Effective dose
What is the UK’s background radiation?
Average 2.5mSv per year
What are the two types of biological affects of radiation?
Somatic = affecting those irradiated, either deterministic or non-deterministic (stochastic) Genetic = affecting offspring of those irradiated, non-deterministic (stochastic)
What is meant by Deterministic affects of radiation?
The effects will occur, there is a threshold dose at which the effect begins to occur. Above the threshold, the severity of the effect increases with dose.
What are some deterministic effects?
Cataracts, gastrointestinal upset, suppression of erythropoiesis .
Do deterministic effects of radiation occur in dentistry?
No, they shouldn’t
What are the effects of radiation on the unborn child?
Related to dose so deterministic
Susceptible when the organs are developing (2-9 weeks)
Results in malformations (reduced growth) or death
Name 6 non-deterministic effects (Stochastic) of radiation?
It may occur, development is random
No threshold at which stochastic events begin to occur
Any dose of radiation may cause stochastic effects
Probability of event occurring increases with dose
Severity of event not effected by dose, all or nothing
Leukaemia’s and solid tumours
Do non-deterministic (stochastic) effects of radiation occur in dentistry?
Yes
Which cells are the most radiosensitive?
Those with high mitotic rate
those that are going to undergo many future mitoses
Those most primitive in differentiation
Lymphocytes and oocytes which are highly differentiated and non-dividing.
Effects of radiation can be direct or indirect, explain each.
Direct - Interacts with biological macromolecules (DNA, RNA, proteins and enzymes) causing ionisation and damage
Indirect - water is abundant in the body - most common damage.
Hydrogen and hydroxyl fee radicals produced by action of radiation on water
Free radicals can form toxic substances such as hydrogen peroxide.
What happens when DNA is changed/damaged?
DNA is the primary mechanism for cell death, mutation and carcinogenesis
Strands break, loss/change of base, disrupt bonds between strands
Can DNA be repaired?
Yes, but if both strands are broken close to each other, there can be misrepair as the intact strand is usually used as a template.
What is the risk of cancer in Scotland?
Currently the risk of getting cancer in their lifetime in Scotland is 1 in 3.
What are the genetic effects of radiation?
Radiation to reproductive organs may damage the DNA in sperm/egg.
This may result in congenital abnormalities in offspring of those irradiated but no certainty as there is no threshold.
As with all stochastic effects, it is difficult to prove
What is the additional risk of cancer from each of the following Chest X-ray Head CT Intraoral Panoramic
Risk of fatal cancer Chest X-ray - 1 in 1,000,000 Head CT - 1 in 10,000 Intraoral - 0.06-0.7 in 1,000,000 Panoramic - 0.29-1.9 in 1,000,000
Why are younger patients more at risk?
These patients have more dividing cells so are more radiosensitive and have a longer lifespan over which to develop an affect
What are the three principles of radiation protection?
Justification
Optimisation
Dose Limitation
Explain justification of x-rays.
Exposure cannot be carried out unless it is justified
The benefit from examination must outweigh the detriment from exposure
Explain optimisation.
For each exposure, the operators must ensure that the doses arising from the exposure are kept as low as reasonably practicable and consistent with the intended diagnostic purpose.
What is ALARP?
As Low As Reasonably Practicable
What is the inverse square law?
The intesity is inversely proportional to distance^2, therefore ensure staff stand a sufficient distance from the patient and not in the direction of the primary beam.
What is the controlled area?
Designated area to protect staff and other persons
How to ensure it is a controlled area?
Generally within the primary beam until it has been sufficiently attenuated by distance or shielding
Within 1.5m of the tube and patient in any other direction
Usually exists only when x-rays are being generated
Who should be in the controlled area?
Only the patient
What should be the size of the controlled area?
The size depends on the kV of the machine, 1.5m for under 70kV but size depends on local rules.
What is the distance for the controlled area in DDH?
2m for all machines or behind an appropriate shield
What warnings should there be in place?
A visible light and an audible noise when x-rays are being produced
The operator should be able to prevent access to the controlled area
Control panels, switches, isolators etc. outside the controlled area
How should x-ray exposure be monitored?
Not normally required if the staff don’t have to enter the controlled area, unless the individual doses are likely to exceed 1mSv per year
Or if there is a high workload ie >100 intra-orals or >50 panoramics per week.
Is there any justification for routine use of lead aprons for patients during dental radiography?
No, however thyroid collars should be used in those few cases where the thyroid may be in the primary beam.
When is the foetus most susceptible?
During organogenesis (2-9 weeks)
Would there be any need to enquire about pregnancy for a dental radiograph?
No, as the primary beam will not be irradiating the pelvic area.
