Risk and dose Flashcards
What is the unit of absorbed dose?
Gy (gray)
What is 1 gray?
1 joule of energy removed from the beam by interaction with a 1kg block
= not a lot of energy but effect is dependent on how the energy is packaged (I.e. Individual particles have less of an impact than a mass of particles)
What is equivalent dose (H)?
A measure which allows the potential harm (radio biological effectiveness) from different types of radiation to be considered
How do you calculate equivalent dose?
Equivalent dose = absorbed dose (D) X radiation weighting factor (WR)
What are the units of equivalent dose?
Sv (Sievert)
What is the radiation weighting factor?
The quality factor (Q) / ionising capacity Varies between radiation types: - X-rays and gamma rays = 1 - beta particles = 1 - protons and neutrons = 5-10 - alpha particles = 20
Protons, neutrons and alpha particles = greater mass deposited in a smaller area = greater ionising capacity
N.b where radiation weighting factor = 1 then the number of Gy=Sv
What is effective dose (E)?
Allows doses from different investigations of different parts of the body to be compared (because it will vary between tissues)
How do you calculate effective dose?
Effective dose = Equivalent dose X tissue weighting factor
What are the units of effective dose?
Sv (sievert)
Why is the effective dose important?
Different parts of the body (e.g. Go ads and lens etc.) are more radio sensitive than others
If more than 1 organ is exposed what is the total effective dose?
The sum of the effective doses to all exposed organs
How much dose is a patient allowed to recite every in a year?
As much as it takes to diagnose and treat them effectively BUT NEEDS JUSTIFICATION SO USE ALARP
Order these dental and medical investigations from low effective dose (mSv) to high:
Barium meal, panoramic film, dental intraoral film, CT chest, CT head, Chest X-ray and mammography…
Dental intraoral film Panoramic film Chest X-ray Barium meal CT head CT chest Mammography
How many hours of natural background radiation is equivalent to the effective dose for dental intraoral films?
16
How many hours of natural background radiation is equivalent to the effective dose for panoramic film?
3 days
What is the risk from 1 intraoral film?
1 malignancy in every 2 million exposures (traditional X-ray set and D-speed film)
1 malignancy in every 20 million exposures (70kV set and E-speed film)
What is the risk from 1 panoramic film?
Using modern equipment
Risk for fatal cancers = 0.21 - 1.9 malignancies over million exposures s
Risk factors for dying in any 1 year…
Smoking 10 cigarettes a day All natural causes, age 40 All kinds of violence or poisoning Influenza Accident on the road Leukaemia Accident at home Accident at work Radiation (working within radiation industry) Homicide FROM 2 DENTAL BITEWING RADIOGRAPHS Lightening strike Release of radiation from nearby power station
Why may dose vary for a single intraoral film?
- different X-ray tube (round/ collimating)
- different detector systems (film speeds/ digital)
Due to the huge variation in doses what do we need to do regularly?
Monitor and use diagnostic reference levels
What does film speed determine?
How sensitive a film is to radiation
(Higher letter = more expensive but more sensitive)
-> decreases exposure for same quality
What is absorbed dose (D)?
A measure of the amount of energy absorbed from the radiation be a per unit mass tissue/loss of energy of the beam
Why do children have a higher multiplication factor for risk of cancer than adults?
Children’s cells are undergoing more replication so any damage is amplified
What is the multiplication factor for risk at 30 years?
1
Risk in relation to age…. Multiplication factors:
What is collective dose?
A measure used when considering the total effective dose to the population
How do you calculate collective dose?
Collective dose = effective dose X population
What are the units for collective dose?
Man-sievert (man-Sv)
Who determines dose limitation?
The international commission on radiological protection (ICRP)
Which 3 categories is the population divided into for dose limitation?
Patients
Occupationally exposed
General public
What is the dose limit for patients?
There isn’t one
What should the decision to carry out a radio graphic investigation be based on (4)?
- the correct assessment of the indications
- the expected diagnostic yield I.e. Usefulness
- the way the results are likely to influence the diagnosis and subsequent treatment
- the clinicians having an adequate knowledge of the physical properties and biological effects of ionising radiation
What is the dose limit for the occupationally exposed?
Depends on level of occupational exposure and the risk involved:
Classified workers
Non-classified workers
“The risk to any worker who receives full dose limit will be such that the worker will be at no greater risk than a worker in another hazardous but non radioactive environment”
What exposure is required for an occupationally exposed individual to be a classified worker?
Dose >6mSv per annum
What is the dose limitations for comforters and carers?
They don’t have a dose limit but they have dose constraints
What classes an individual as a comforter and carer?
An individual that willingly incurs an exposure in the support and comfort of another undergoing/ who has undergone a medical exposure
What are the 3 golden rules of dose reduction?
- Distance
- Shielding
- Time
What law does distance follow?
The inverse square law
What controlled area (IRR99) is needed for sets operating up to 70kV?
1.0m
What controlled area (IRR99) is needed for sets operating above 70kV?
1.5m
(Preferably the operator should be able to stand 2m or more away from the patient and X-ray beam but not in the line of the primary beam)
What should happen when a controlled area extends to a door?
A warning notice should be provided at the entrance to indicate when radiography is in progress
What needs to be visible throughout the exposure?
X-ray tube and warning light should be visible throughout the exposure
Which materials can be used for shielding?
Lead (Pb)
Lead rubber/lead glass (Pb equivalent in mm)
Concrete
Barium plaster
How do we reduce the dose to operators/comforters/carers?
Stand behind lead screen
If necessary hold/ wear lead rubber apron (n.b. There is no justification for the routine use of lead aprons for patients in dental radiography as no real benefit and aprons may block radiograph which may need retaking)
When should thyroid collars be used?
If the thyroid is in the primary beam
How do we reduce the dose to innocent bystanders?
Close X-ray room doors (local rules)
Perform X-rays in a room only unless
Only allow those required in the room
How do we change time to reduce exposure?
Limit the amount of time exposed to radiation
Which other ways can we reduce dose to the patient?
Collimating
Careful technique (adequate training, accurate positioning, exposure selection and good explanations)
Digital radiography
Perform appropriate projections (follow established protocols)
Equipment
Quality assurance
When dose a dental X-ray set give a lower dose?
Higher kV (60-70) Rectangular collimating Adequate aluminium filtration Long focus to skin distance (>20cm) Accurate timer Adequate warning signs Restriction of exposure to dentition (decreases effective dose by 50%)
How does long focus to skin distance
Angle of field smaller = doesn’t spread as much in tissue
How do we manage quality assurance?
Regular checks of equipment (at least every 3 yrs) = includes processing equipment
Image quality evaluation and audit
What are the advantages of digital imaging over film?
= quicker results (direct/indirect) = potentially less patient dose = less storage requirements = constantly evolving = environmentally better
What are the different types of dose measurements?
Entrance surface dose (esd)
Dose-area product (dap)
What is entrance surface dose?
Represent the max dose in tissues
Measured using thermoluminescent dosimeters (tlds) and ionisation chambers
What are the units for entrance surface dose?
mGy
What is dose area product?
Measures ionisation and areas of ionising
What is the units for dose area product?
mGy/cm^2
When should a person wear a personal dosimeter?
When risk assessment indicates that individual doses could exceed 1mSv / yr
In practice… Should be considered for those staff who’s weekly workload exceeds 100 intraoral/ 50 panoramic films
What is a film badge?
Photographic film under Windows of different thicknesses
What are the advantages of film badges?
Cheap
Provides a permanent record
Can be reassessed
Provides information on type of radiation received
What are the disadvantages of film badges?
Requires processing
What is thermoluminescence?
A form of luminescence when stored radiation energy is emitted As light on heating
Which material do thermoluminescent dosimeters contain?
Lithium fluoride crystals
What are the advantages of thermoluminescent dosimeters?
Sensitive
Reusable
Provide quick read out
No processing required
What are the disadvantages of thermoluminescent dosimeters?
Destructive read out
Expensive
Provides limited I for on energy of radiation received (don’t know type exposed to)
