4. How Much Radiation Do You Use? Flashcards
LOs
Key terms to do with radiation dose What are they and what do they mean?
- The word, ‘dose’ appears in many different areas with several different meanings
- 4 main terms that include the word ‘dose’
- Radiation dose
→ describes the amount of radiation that is received by patients from different x-ray examinations - reference dose
→ dose level which is set for every radiographic examination which should not be exceeded - radiation dose limits
→ describe maximum levels of radiation to individuals which again should not be exceeded - natural background radiation dose
→ describes naturally occurring radiation levels
What are the 3 main dose units in radiation dosemetry?
- radiation absorbed dose (D)
- equivalent dose (H)
- effective dose (E) I
RADIATION ABSORBED DOSE
1.
Defined as?
2.
Current Unit?
- Subunit?
- Why’re subunits used?
- Original unit?
- the measure of the amount of energy absorbed from the radiation beam per unit mass of tissue
- This is what the physicists are actually able to measure, using sophisticated dosemeters.
2.
unit = Gray (Gy) → measured in joules per kilogram. That is energy per mass of tissue.
- Milligrays (mGy) (10^-3)
4.
Gray = quite a large unit, so frequently absorbed doses are quoted in milligrays
5.
- RAD (radiation absorbed dose)
- measured in ergs per gram
- 1 Gy v 100 RADS
body that is being irradiated. To achieve this various mathematical calculations are performed and different dose units are used.
1.
Why do we need to be able to compared radiation gives to patients?
2.
Why can we not use radiation absorbed dose as a comparible figure?
1.
- For the amount of radiation delivered to patients from different investigations to be meaningful, we really need to be able to compare one investigation with another
2.
- because it is only a measure of the amount of energy absorbed from the radiation beam
- does not consider how dangerous the type of radiation might be, or the sensitivity of the particular part of the body being irradiated
EQUIVALENT DOSE
1.
Defined as?
- Current unit?
3.
Subunit
- Original unit?
1.
- measure that indicates the radio biological effectiveness of different types of radiation
2.
Sievert (Sv)
3.
- Millisievert (mSv) (10^-3)
- Microsievert (10^-6)
- rem
- 1 sievert = 100 rems
EQUIVALENT DOSE (H)
1.
How is it calculated?
2.
What is radiation quality-weighting factor (WR -(little R))?
- EG of WR values for x-rays, gamma rays, neutrons, alpha particles and what they mean?
1.
H = D x WR (little R)
Equivalent dose = radiation dose absorbed x radiation quality-weighting factor
2.
describes the damaging nature of different types of radiation
3.
X-rays/ gamma rays = 1
Neutrons = 10
Alpha particles = 20
- this means that alpha particles are 20 x more damaging to the tissues than x-rays or gamma rays for the same absorbed dose
- The extra damaging effect of the type of radiation being used is acknowledged and taken into account.
Why is calculating H useful?
- By calculating the equivalent dose we now have a common measurement of dose irrespective of the type of radiation that is being used
- A second mathematical calculation can now be performed to take into account the part of the body that was being irradiated = effective dose
EFFECTIVE DOSE (E)
1.
What is it?
2.
Units?
3.
Why is it useful?
- How to calculate?
5.
What is WT?
- takes into account different parts of body
- converts all doses to an equivalent whole body dose
- this is what patients may ask about
2.
SI = sieverts
3.
- allows doses from different investigations of different part of the body to be compared
- done by converting all doses to an equivalent whole body dose
4.
E = H x WT
Effective does = equivalent dose x weighting factor
5.
tissue weighting factor
1.
How is tissue weighting factor determined?
- Bone marrow, salivary gland, thyroid gland WT?
1.
- International Commission of Radiological Protection (ICRP) determines and publishes them
2.
Bone marrow= 0.12
Thyroid = 0.05
Salivary grand = 0.01
Salivary glands previously did not have a WT but now do, Why?
- because recent research has highlighted the increased incidence of salivary gland tumours as a result of increased exposure during dental panoramic radiography
What are the effective doses from dental radiography?
- intra oral periapical dental radiography
→ 0.008- 0.001 mSv
-
Why is there a range for the effective dose and not a single figure?
Examples of equipment that lower the effective dose? Why is this useful?
The discrepancy in range is due to a variety of equipment
Old traditional equipment :
0.008mSv
50-60 low kV
100 mm fsd
Round collimation
D-speed
Effective doses from dental radiography vs medical radiography?
What is a reference dose?
- the published maximum absorbed dose for every investigation – both medical and dental
Who sets the reference dose?
What happens if equipment delivers a dose in excess of reference?
1.
- set by the Health Protection Agency (the HPA)
- formerly known as the National Radiological Protection Board (the NRPB) who have carried out large surveys of different types of equipment to measure what the absorbed dose to patients is from various medical and dental investigations
2.
- If equipment delivers a dose in excess of the reference dose, then steps need to be taken to reduce patient exposure
- Reference doses are then set so that 75% of the equipment that has been surveyed delivers a dose less than the reference dose
- Adult mandibular molar periapical reference dose?
2.
panoramic radiograph of an adult reference dose?
1.
- Health Protection Agency recommends a reference dose = 4 mGy
- Remember, the reference dose is an absorbed dose, therefore the unit that is used is the mGy
2.
65 mGy per millimetre
- studies shows that there is no guarantee that different X- ray machines producing the same radiograph will in fact deliver the same dose to the patients
-The reference dose is a mechanism designed to try to protect patients by identifying very high dose equipment which can then be adjusted or taken out of service
How are Radiation dose limits recommended ?
by the International Commission on radiological protection (ICRP)
-information has been gathered for many years by multiple sources
ICRP general principles?
- 3 general principles
- JUSTIFICATION
→ No practice should be adopted unless it has a net positive benefit
2.
OPTIMISATION
→ All exposures shall be kept as low as reasonably possible (ALARP), taking social and economic factors into account
3.
LIMITATION
→ equivalent dose to individuals shall not exceed the limits recommended by the commission
the International Commission divides the population into three groups for the purpose of recommendations and setting specific dose limits
What are they?
- Patients
2.
Occupationally exposed (workers)
- General public
Patients are divided into subgroups depending on why they’re being x-rayed → What are they?
- Examination directly related to an illness
- Periodic health checks ( systemic examinations )
- Examinations for occupational , medico-legal or insurance purposes
- Medical research
EXAMINATIONS DIRECTLY ASSOCIATED WITH ILLNESS
- Dose limit?
- Number of radiographs that can be taken?
- Decision to carry out investigation should be based on?
No set dose limits
Clinical decides number + type of radiographs taken
Decision based on :
- correct assessment indications
- how much info they will gain
- how the result will influence diagnosis + treatment
- clinician should have knowledge of physical properties + biological effects of ionising radiation
Radiation dose limits are different for different categories of people - what are the categories?
What are the ionising radiation annual dose limits?
Why have lower ionising radiation annual dose limits been put into place?
- Classified workers (Eg nuclear power plant)
→ 20 mSv - non classified workers (EG dental team)
→ 6mSv - general public (patients)
→ 1 mSv - all jobs carry a risk
- recently new lower limits have been put into place
- as recently been linked to health issues and thought to be 3x more damaging than originally thought
What is a thermoluminescent dosimeter? And electric personal dosemter?
- help measure your radiation exposure
- most common ones used = yellow thermo luminescent dosimeter (tld) or the little blue film badges
→ wear them, usually for a month
→ They are sent to a medical physics department and are read, and that gives you an indication of how much radiation you have received during that time period - OR electronic personal dosimeter
→ gives an actual digital readout as to how much radiation you have received.
Why is useful to monitor your radiation exposure
- To compare it to the annual dose limits
- If you follow routine practical radiation safety measures, then these personal monitoring devices should give a reading as close to zero as possible
Natural background radiation dose? Where does it come from?
Gamma rays + other radioactive emissions
Approx 2.6 mSv annually
- rock
- soil
- outer space
- certain floods
1.
Average periapical and modern panoramic film equivalent in terms of background radiation?
2.
Why is it useful to know this?
Risks of ionisation radiation classifications?
RISKS OF IONISING RADIATION
For intra oral films
RISKS OF IONISING RADIATION
For panoramic films
How are these ionising radiation risks estimated?
Does size of ionising radiation exposure effect severity?
Is risk of ionising radiation effected by age?
how do the risks from radiation compare with the risks from other potentially dangerous activities?
Genetic stochastic effects
