Transcript Xray RP Principles 2011-comp

Principles of Radiation Protection –
Managing Radiation Protection
The Management Principles
• JUSTIFICATION
• OPTIMISATION
• LIMITATION
Justification
• No practice involving exposures to
radiation should be adopted unless it
produces sufficient benefit to the exposed
individuals or to society to offset the
radiation detriment it causes.
• Justification of exposures is primarily the
responsibility of the medical professional
i.e. the Radiologist.
• The expected clinical benefit associated
with each type of procedure should have
been demonstrated to be sufficient to
offset the radiation detriment.
Justification
Benefit of the radiation exposure must outweigh
the risk of exposure
vs
Optimisation
• All exposures and radiation doses must be
kept
– As
– Low
– As
– Reasonably
– Practicable
ALARP
• With economic and social factors taken
into account
OPTIMISATION
• For every exposure, operators must
ensure that doses arising from the
exposure are kept as low as reasonably
practicable and consistent with the
intended diagnostic purpose.
• THIS IS OPTIMISATION
OPTIMISATION
• You are defined as an IRMER Operator
– Your ‘optimisation’ is ensuring you leave the
X-ray unit/LINAC in a safe condition fit for
clinical use
– Handover procedure
Optimisation – Staff Dose
Investigation Level (DIL)
Dose Investigation Level
• Once you start work with ionising radiation, you
are subject to legal dose limits – 6 mSv per year
for non-classified workers
• However, we have to define a Dose Investigation
Level
– 1.2 mSv per year
– Or 0.1 mSv per month
• This is a level of dose that should trigger an
investigation in conjunction with your RPA, and
ensures that you do not receive anywhere close
to the legal limit.
Limitation
• In the UK, legislation stipulates annual
limits for the amount of radiation that may
be received by staff and members of the
public
• Limits are set such that deterministic
effects never happen
• Limits are set such that chances of
stochastic effects are minimised
Legal Dose Limits - Patients
• For examinations
directly associated
with illness – there
are no dose limits
Legal Dose Limits – Radiation
Workers
• Radiation workers are those exposed to
radiation as part of their occupation
• No benefit – only risk
• Two subgroups depending on level of
exposure:
– Classified radiation worker
– Non-classified radiation worker
Legal Dose Limits – Classified
Workers
•
•
•
•
•
Receive high levels of radiation exposure
Very unlikely for dental
Require annual health check
Compulsory dose monitoring
For classified worker
– Whole body 20 mSv per year effective dose (18 years
old and above)
– Lens of eye 150 mSv per year equivalent dose
– Skin 500 mSv per year equivalent dose
– Extremities (hands and feet etc) 500 mSv per year
equivalent dose
Legal Dose limits for non-classified
workers (all radiation workers in
this Trust)
• Very unlikely you will need to be classified
– You only need to be classified if you are considered to
approach 3/10ths of any dose limit
• Relevant dose limit for you is 6 mSv whole body
effective dose
• Very unlikely to exceed this per year
• We use a dose constraint of 0.1 mSv per month
for RT and X-ray engineers
• Risk assessment usually show it is very unlikely
this will be exceeded
• We monitor routinely with dose badges
Legal Dose Limitation - Public
• The annual dose limit
for a member of the
public (e.g. office
worker in room next
door to x-ray)
– 1 mSv/yr
– But we use a dose
constraint of
0.3mSv/yr
Radiation Dose
• Absorbed Dose (Jkg-1)
– Amount of energy deposited per kilogram
– Dose to an organ or tissue
– Unit is the Gray (Gy)
• DOSE TO A CERTAIN PLACE IN THE BODY
RADIATION
TISSUE
• Effective Dose (Jkg-1)
– This is the average dose to whole body
– Unit is the Sievert (Sv)
– This gives us the risk of contracting cancer of the x ray
exposure
• THIS IS THE OVERALL DOSE TO THE WHOLE
BODY
Tissue Weighting Factors
Tissue
Weighting Factor
Breast
0.12
Red Bone Marrow
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Gonads
0.08
Bladder
0.04
Liver
0.04
Oesophagus
0.04
Thyroid
0.04
Skin
0.01
Bone surface
0.01
Brain
0.01
Kidneys
0.01
Salivary glands
0.01
Remainder
0.12
Risks Associated with X rays
• Adult Exposure (per 1 mSv)
–
–
–
–
Fatal cancer (all types)
Fatal leukaemia
Non fatal cancer
Heritable effects
1 in 20,000
1 in 200,000
1 in 100,000
1 in 80,000
• Childhood exposure
– Fatal cancer
1 in 10,000
• Foetal exposure
– Fatal cancer to 15 years
– All cancers to 15 years
– Heritable effects
1 in 10,000
1 in 17,000
1 in 42,000
Small Risks, So why worry?...
• Average effective dose for radiography ~0.5 mSv
• Risk of fatal cancer only 1 in 40,000
• But, large number of patients
– 40 000 000+ procedures.
– Therefore, 700 patients ‘killed’ each year due to x-rays.
• So:
– All exposures must be JUSTIFIED.
– Doses to patients, and staff, must be As Low As
Reasonably Achievable (ALARA principle).
Typical doses in Diagnostic
Radiology
Other Risks
Doses in Perspective
•
•
•
•
Effective dose from natural background radiation in the UK is approximately
2.7 mSv
This is 2000 times greater than a dental exposure
This natural radiation comes from
– cosmic rays,
– rocks and soil,
– food,
– radon.
Artificial radiation comes from:
– Fallout from nuclear explosions
– Radioactive waste discharged from nuclear power plants
– Medical and dental exposures
– Occupational exposures
Practical methods to restrict YOUR
radiation exposure
• Time
• Distance
• Shielding
The real risk to staff
X-ray Tube
Primary Beam
Scattered Radiation
Patient
Staff
•Double distance = 1/4 dose
•Triple distance = 1/9th dose.
In air, x-rays obey
the Inverse Square
Law.
I∞1/d2
Distance
• Operator B receives only a quarter of the
radiation received by Operator A if he is
standing twice the distance from the
source
• Operator B receives only one ninth of the
radiation received by Operator A is he is
standing 3 times the distance from the
source
Shielding
Shielding
Dose monitoring
• Film badges
• Thermoluminescent dosemeters (TLD)
– Badge
– Extremities
• Ionisation chambers
Film badges
• Plastic frame
• Worn outside of clothes
for 1 to 3 months
– Advantages:
– Provide a permanent
record of dose
– Measure type and energy
of radiation
– Simple and robust
– Disadvantages
– No immediate indication of
exposure
– Processing can lead to
errors
– Prone to filter loss
TLD
• Similar use as for film
badges
• They absorb radiation
and release this as light
when heated
–
–
–
–
–
–
Advantages:
Re-usable
Easy to read out
Disadvantages:
Read out is destructive
Limited info on type of
radiation
Ionization chambers
• Used by scientific personnel to measure beam
dose
• Radiation ionises air inside chamber which
produces a current of electricity
• Advantages
– Very accurate
– Immediate read out
• Disadvantages
– No permanent record
– No indication of type of radiation
– Fragile and easliy damaged
Patient Doses in Radiography Patient Dose Limitation &
Practical Principles of
Radiation Protection
What are patient doses?
• Absorbed dose in mGy
– Dose to a certain place in the body (eg skin
dose)
• Effective dose in mSv
– Takes into account the tissues that have been
exposed
How do we measure these in
practice?
• Dose Area Product meters (DAP)
• Skin dose:
– kV, mAs, focus to skin distance
• Screening time
• Dose Length Product (CT Scanning)
Dose Area Product
•Stochastic risks approx. proportional to DAP
•Skin dose is DAP / area irradiated
•1 Gy.cm2  3 mGy skin dose
•1 Gy.cm2  0.2 mSv effective dose .
Largest Exposure from man-made radiation is Medical
46 million medical & dental x-rays in UK annually
Angiography (non-CT)
1%
CT
7%
Interventional (non-CT)
1%
Dental
26%
Conventional (less dental)
Dental
CT
Angiography (non-CT)
Interventional (non-CT)
Conventional (less dental)
65%
Major Contributors to UK collective dose from medical x-rays
2008 data – HPA-CRCE-012 published Dec 2010
RADIATION EXPOSURE OF THE UK POPULATION FROM MEDICAL AND DENTAL X-RAY EXAMINATIONS
From NRPB/HPA data
2008 data – HPA-CRCE-012 published Dec 2010
RADIATION EXPOSURE OF THE UK POPULATION FROM MEDICAL AND DENTAL X-RAY EXAMINATIONS
From NRPB/HPA data
UK Annual Collective Dose (man Sv)
Hundreds of cancer cases blamed
on dentist x-rays
Independent.co.uk By Jeremy Laurence, Health Editor Friday, 30 January 2004
Radiation from X-rays in dentist surgeries and
hospitals causes 700 people in Britain to
develop cancer each year, researchers say
today.
30 January 2004
700 CANCER CASES
CAUSED BY X-RAYS
X-RAYS used in everyday detection of diseases and broken bones are responsible for
about 700 cases of cancer a year, according to the most detailed study to date.
The research showed that 0.6 per cent of the 124,000 patients found to have cancer
each year can attribute the disease to X-ray exposure. Diagnostic X-rays, which are
used in conventional radiography and imaging techniques such as CT scans, are the
largest man-made source of radiation exposure to the general population. Although
such X-rays provide great benefits, it is generally accepted that their use is associated
with very small increases in cancer risk.
Researchers from Oxford University and
Cancer Research UK estimated the size of the
risk based on the number of X-rays carried out
in Britain and in 14 other countries.
According to their findings, published in the
medical journal The Lancet, the results showed
that X-rays accounted for 6 out of every 1,000
cases of cancer up to the age of 75, equivalent
to 700 out of the 124,000 cases of cancer
diagnosed each year.
Un-necessary exposures
Those exposures that are:
• unlikely to be helpful to patient
management, or
• are not As Low As is Reasonably
Practicable in order to meet a clinical
objective.
Practical Optimisation for Patient
Protection - ALARA
Factors affecting Patient Dose
•
•
•
•
•
Field Size (Collimation)
Tube voltage (kV)
Beam filtration
Tube to patient distance
Film/Sensor speed – (Direct Digital or CR)
Collimation, Collimation,
Collimation
• Cover only the area needed
• Small fields
give lower dose
(and less scatter,
therefore better
image)
• Avoid more
radiosensitive
areas - e.g.
gonads, female
breast
• Position
carefully.
• Optimal collimation will result in :
– Lower patient dose
– Lower occupational dose
– Improved image quality
Tube Voltage (kV) – Intra-oral
•Higher kV (Quality) = lower skin dose
–trade off = less contrast
Filtration
• Low energy radiation = patient skin dose
for no diagnostic value.
• Added filtration = lower patient skin dose
– Increases beam quality
– trade off = less contrast
Minimum Filtration
•
•
•
•
 70kVp  1.5 mm Al
> 70kVp  2.5 mm Al
<1.5 mm Al – Filtration must be increased
Also possible to have too much filtration.
Tube to Patient Distance or
Focus to Skin Distance (FSD)
•Greater FSD = lower patient dose
•Greater FSD = less magnification (so fewer
distortions).
Digital Sensors
• Higher doses = clearer images
• Lower doses = noisier images
• Easy to not optimise doses as it is not so
obvious when overexposure occurs.
• But underexposure results in grainy
images.
• In our experience CR is slightly higher
dose than film
• DR is lower dose than film and CR
Patient Doses – Diagnostic Reference
Levels
There are no patient dose limits!!!
• Whilst there are no dose limits set for patient
exposures, various surveys conducted over the
past 25 years indicate a wide variation in doses
for the same examination.
• It is therefore considered that there is significant
scope for improvement in the optimisation of
patient protection.
National
Diagnostic Reference Levels (DRLs)
Conclusion
•
•
•
•
•
•
Justify – Optimise – Limit
Time – Distance - Shielding
Collimate
Choose Correct Voltage & Dose
Use the handover procedure
Remember to Consult your RPA, they will
give you Relevant Protection Advice – if in
doubt ASK.
Management of Radiation
Protection in this Trust
In This Trust
• Radiation Safety Policy is embedded into the
general Health & Safety Policy CP137
• Radiation Physics website www.hullrad.org.uk
contains additional guidance:
–
–
–
–
–
–
Staff & Patient Pregnancy
Diagnostic Reference Levels
Dose Investigation Levels
Duties of the RPS
Personal Dose Monitoring
Local Rules
Help …
• Radiation Protection Advisers
– John Saunderson – x76-1329
– Craig Moore – x76-1385
• Rad’n Prot’n Team
– Andrew Davis, Dave Strain, Tim Wood – ext. 76-1330
• X-Ray engineers
– Andy Patchett & team – Medical Physics - HRI ext. 5756
• Oncology Physics Team
– Sean McManus and Co. x 76-1367
• Radiation Protection website
– www.hullrad.org.uk
• Trust Policy CP137
END