How much radiation do you use? Flashcards
What is meant by ‘radiation dose’?
It is the amount of radiation that is received by patients from different xray examinations
What is meant by the term ‘reference dose’?
Dose level that is set for every radiographic examination that should not be exceeded
What is meant by the term ‘radiation dose limit’?
Maximum level of radiation to individuals which must not be exceeded
What is meant by the term ‘natural background radiation dose’?
Naturally occurring radiation levels
What are the 3 main dose units?
- Radiation absorbed dose (D)
- Equivalent dose (H)
- Effective dose (E)
What is the Radiation Absorbed Dose (D)?
It is a measure of the amount of energy absorbed from the radiation beam per unit mass of tissue
What is the unit for the Radiation Absorbed Dose (D)?
Gray (Gy) measured in joules/kg (energy/mass of tissue).
Gray is quite a large unit of measurement, so it’s subunit is milligray (mGy)
What was the original unit of measurement for Radiation Absorbed Dose (D)? And what is the conversion rate between the new unit and old unit?
Original unit: rad (measured in ergs/gm) > stood for Radiation Absorbed Dose
Conversion: 1 Gray = 100 rads
Why can’t Radiation Absorbed Dose be used as a comparable figure? (3 points)
Because it is only a measure of the amount of energy absorbed from the radiation beam.
It does not allow for:
- how dangerous the type of radiation might be
- the sensitivity of the particular part of the body being irradiated
What is the equivalent dose? (H)
Measure that indicates the radiobiological effectiveness of different types of radiation - i.e. allows for the differing biological effects of different types of radiation and thus provides a common unit
What is the ‘radiation quality-weighting factor’?
It is a figure which describes the damaging nature of different types of radiation
How is equivalent dose (H) calculated?
By multiplying the radiation absorbed dose by the radiation quality-weighting factor
H = (radiation absorbed dose) x (radiation quality-weighting factor)
What is the radiation quality weighting factor for x-rays and gamma rays?
1
What is the radiation quality weighting factor for Neutrons?
10
What is the radiation quality weighting factor for alpha particles?
20
What is the significance of the radiation quality weighting factor - if this is a higher value, is the radiation more or less damaging?
If the value is higher - the radiation is more damaging.
Why may the equivalent dose be a better way of comparing radiation?
Because it accounts for the damaging effect of the type of radiation that is being used
What is the modern unit for the equivalent dose?
Sievert
BUT because the sievert is a large unit, the subunits millisievert or microsievert are more frequently used.
What was the original unit for the equivalent dose?
What is the conversion from the original unit to the modern unit?
Rem.
1 Sievert = 100 rems
What dose type would be best to use if trying to use a common measurement of dose irrespective of the type of radiation that is being used?
The equivalent dose
What is the effective dose?
This allows doses from different investigations of different parts of the body to be compared - by converting all doses to an equivalent whole body dose.
How is the effective dose calculated?
(Equivalent dose) x (tissue weighting factor)
What is the ‘tissue weighting factor’?
What is this value used to calculate?
- It is a relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue. It accounts for the variable radiosensitivities of organs and tissues in the body to ionising radiation.
- It is used to calculate the effective dose
What organisation determines and publishes the weighting factors for various parts of the body?
The International Commission of radiological Protection (ICRP).
Give an example of where weighting factors have been edited by the International Commission
Breast tissue: from 0.05 to 0.12 (increase)
Gonads: 0.2 to 0.05 (Decreased)
What is the radiation dose from 1 intraoral dental film?
0.008 - 0.001 millisieverts (effective dose)
Why is there a range for the effective dose of an intraoral dental radiograph?
Because the dose varies between surgeries - due to differences in x-ray equipment and image receptors used.
e.g. in older machines a higher dose would usually be delivered
How could the dose be reduced in older, more traditional x-ray equipment?
Change speed of film to faster E speed film - this will half dose.
If 70kV equipment and 200mm focus to skin distance - dose halved again.
Use rectangular collimator - half dose again.
What are the recommendations of x-ray equipment for dental radiographs? (4)
- Rectangular collimator
- 70 kV equipment
- Long focus to skin distance of 200mm
- E speed film (or faster F speed film)
Reduces the dose so therefore reduces the risk
What gland did the International Commission suggest required their own weighting factor in 2005, that is especially relevant to dentists?
Salivary glands
Is it more appropriate and realistic to include salivary glands when calculating effective dose of a panoramic radiograph?
Yes - as the International Commission believes the salivary glands deserve their own weighting factor
What is a reference dose?
It is the published maximum absorbed dose for every investigation - both medical and dental
What must happen if equipment delivers a dose in excess of the reference dose?
Steps will need to be taken to reduce patient exposure
Who sets the reference dose values?
The Health Protection Agency (HPA)
What reference dose is recommended by the HPA for an adult mandibular molar PA radiograph?
4 mGy
What reference dose is recommended by the HPA for an adult panoramic radiograph?
65 mGy per millimetre
What is the purpose of measuring and recommending reference doses?
It is to protect patients by identifying very high dose equipment which can then be adjusted or taken out of service
Who sets the recommendations on specific dose limits?
The International Commission on Radiation Protection (ICRP)
What are the International Commission’s 3 general principles regarding radiation safety and dose limitation?
- JUSTIFICATION > No practice shall be adopted unless its introduction produces a net positive benefit
- OPTIMISATION > ALARP (as low as reasonably practicable), taking social and economical factors into account
- LIMITATION > the equivalent dose to individuals shall not exceed the limits recommended by the commission
How does the International Commission divide the population for the purpose of recommendations and setting specific dose limits?
Patients
Workers
General public
What are the 4 subgroups patients are divided into, which is dependent on why they are being x-rayed?
- Examinations directly associated with illness (e.g. toothache)
- Systemic examinations (periodic health checks)
- Examinations for occupational, medico-legal or insurance purposes
- Medical research
What are the dose limits for patients whose examinations are directly associated with illness?
The are NO set limits
Who is responsible for the number and type of radiographs taken for a patient?
The clinician
The decision to carry out a radiographic investigation should be based on what 4 factors?
- Correct assessment of indications
- The expected yield
- The way in which the results are likely to influence the diagnosis and subsequent treatment
- The clinician having adequate knowledge of the physical properties and biological effects of ionising radiation
How are workers divided into 2 subgroups?
Classified workers e.g. people who work in nuclear power industry
Non-Classified workers e.g. members of dental team
(depends on levels of occupation exposure and risks involved)
What UK legislation governs the use of ionising radiation and lays down the new annual dose limits?
Ionising Radiation Regulations (1999)
IRR
What is the annual dose limit for classified workers? (set by IRR)
20 mSv
used to be 50
What is the annual dose limit for non-classified workers?
6 mSv
used to be 15
What is the annual dose limit for the general public?
1 mSv
used to be 5
Why were the annual dose limits for classified, non-classified and the general public lowered by the ICRP?
Because the IC reassessed all evidence available linking ionising radiation to health issues/problems, and concluded that ionising radiation is approx 3 times more damaging to human tissue than originally thought.
How could you measure how much radiation you receive when you are at work?
You could use personal monitoring devices:
> Yellow thermo luminescent dosimeter or blue film badges
Wear them for a period (usually month), then sent to medical
physics dept and are read.
> Electronic personal dosimeter - gives digital readout. Can use throughout the year and then compare to annual dose limit.
These should give reading as close to zero as possible.
Are dentists required to measure how much radiation they are exposed to?
No - as they are unclassified workers, it is your choice to measure it or not.
It is probably sensible to be monitored for a few months to keep safe when practising
What is the natural background radiation dose throughout the UK (annually)?
2.6 mSv
Where does the natural background radiation originate from?
Rocks and soil in the Earth’s crust, from outer space and certain foods
These emissions are responsible for the radioactive environment in which we live
How many hours of background radiation is an average PA radiograph equivalent to?
16 hours
How many hours of background radiation is an average modern panoramic radiograph equivalent to?
2 days (48 hours)
Name the 3 effects of ionising radiation
Which type are dentists most concerned with?
- Somatic stochastic effects
- Somatic deterministic effects
- Genetic stochastic effects
Most concerned with somatic stochastic effects (chance effects)
What are the chances of one fatal malignancy from one intra-oral film for:
a. ) traditional x-ray equipment
b. ) modern equipment
a. ) 1 in every 2 million exposures
b. ) 1 in every 20 millions exposures
What is the risk estimate for fatal cancers from panoramic radiograph using modern equipment?
1 fatal malignancy per 1 million exposures
How are risks of radiograph exposure estimated?
Using dose effect graphs constructed using available research data (dose on x axis, effect on y axis).
Any amount of radiation will produce an effect -as demonstrated by calculating and extrapolating the average and plotting it, as it passes through zero
You can measure actual dose from an examination and use the yellow line and go across to effects from which you can estimate the risk
What effect on the chance of stochastic effects does increasing the dose have?
It is more likely that a malignant change will occur with a higher dose
How does risk vary with patient’s age?
The risk factors are all calculated assuming an 30 year old adult.
If a child is less than 10 years old, the risk factors are 3 times greater
In the elderly, it gets less and less
How does the risk of death from a panoramic radiograph common to the risk of death from smoking 10 cigarettes a day?
Panoramic radiograph: 1 in 1 million
10 cigarettes: 1 in 200
Approximately how many intraoral films are taken annually in the UK?
25 million
Approximately how many panoramic films are taken annually in the UK?
3 million
How many of annual UK fatal malignancies are estimated to be from dental radiography?
10-12
assuming that not all films are taken using modern equipment and techniques
What have the genetic stochastic effects been estimated to be for dental radiography in the UK?
1 congenital abnormality every 3 years
