Transcript Radiation Protection Paperwork

Elaine Holt
Catherine Ashmore
Lorna Sweetman
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Background
A draft report of the ICRP was issued in early 2011
 It reviewed the data relating to non-cancer effects and
tissue reactions
 It drew particular attention to threshold doses for tissue
injury of relevance to radiation protection.
 Special attention was paid to eye cataracts, due to emergent
evidence of higher occurrences than expected, after low
doses.
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Background
Sources of evidence:
 Japanese A-bomb survivors
 Therapeutic radiotherapy
 Repeated CT scans
 Astronauts
 Residents of contaminated buildings
 Victims of the Chernobyl accident
 Radiation technologists, interventional radiologists and
interventional cardiologists
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Cataract Types
Nuclear
Cortical
Posterior subcapsular
www.ndrs.scot.nhs.uk/Train/Handbook/drh-27.htm
Ionising radiation is associated predominantly
(though not exclusively) with opacities in the
cortical and posterior subcapsular regions
Previous guidelines
 In accordance with present ocular guidelines, cataract
formation is deterministic.
 ICRP threshold values for detectable opacities were as
follows: 5 Sv for chronic and 0.5-2.0 Sv for acute
exposures.
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Cataracts as a deterministic
effect
Classification based on several longitudinal studies
 NRPB (1996) produced a guidance document based on
the work of Merriam et al in the 1950s. Threshold ~ 1.3
Gy
 SSK (2007) – Tolerance dose ~ 2 Gy, but could be an
overestimate.
 ICRP (1990 and 2007) – Thresholds for radiation induced
cataracts are 2 Gy for acute exposure, 4 Gy for
fractionated exposure, higher for protracted exposures.
Shortcomings and caveats
 Phelps Brown (1997) – insufficient data, dose
estimation “necessarily crude”
 Smilenov et al (2008) – Timescales too short
 ICRP (2007) – The lens of the eye is maybe more
radiosensitive than previously thought
Conclusions drawn from new evidence
 Cataracts may form at significantly lower doses than
previously believed
 Some findings are consistent with no dose threshold
 Not all studies support this observation, approaches
are very different
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Threshold for lens changes
 Low doses result in lens changes which are slow to
appear
 Longer follow up and an abundance of data in the low
dose region have yielded evidence of a lower dose
threshold
 The report concluded that detectable lens changes
occur at 0.2-0.5 Sv.
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Resulting change to the eye dose limit
Proposed change to the annual eye dose limit from 150
mSv to 20 mSv.
 Cataracts are eminently curable unlike cancer.
 This is clearly a big change, which is likely to have
significant implications.
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Dose limits continued
 New dose limits likely to be included in the IAEA Basic
Safety Standards as well as the Euratom Basic Safety
Standards Directive.
 It is understood that the member of the public dose limit
and the classification level will each be 15 mSv per year.
 However, it is possible that staff who are likely to exceed
three tenths the dose limit (6 mSv) will require
classification.
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Cardiologist eye doses
Cardiologist Eye Doses 2010
90
80
70
Classification level at
3/10 the dose limit
Frequency
60
50
40
30
20
10
0
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
>10
Hp(3) Eye Dose (mSv)
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Implications
 Based upon 2010 data, very few cardiologists would need to
be classified, if a level of 15 mSv were adopted.
 However, if a classification level of three tenths the dose
limit is chosen 10-20 cardiologists would need to be
classified.
 This is likely to represent an underestimate as eye doses
fluctuate from year to year.
 With a 6 mSv level it may be necessary to classify all
cardiologists
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Requirements for classified staff
Under IRR99, the following requirements must be met for
classified staff:
 Doses to the eye and body must be assessed, as well as to
the hand where these are likely to be significant.
 The dose records must be kept by an approved dosimetry
service.
 Currently no approved dosimetry service exists for the
measurement of lens dose.
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Requirements for classified staff
 Annual health review
 Transfer of dose and health record when employee
commences work elsewhere
 Both records must be kept for a period of at least fifty years
from the time an individual ceases to be a classified worker.
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Avoiding classification – reducing eye doses
 It is obviously preferable to take measures to avoid
classification.
 Ceiling suspended shields and lead glass spectacles
can reduce eye doses
 Inverse Square Law
 ALARA principle
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ANGLE Vs DOSE
Greater angle of ‘C’ arm = 6 x increase in dose to the eye
P.A. ( Postero-anterior )
L.A.O. 40, CRANIAL 25
Shielding Devices
 The efficacy of lead glasses has been investigated in
a number of studies, both in the direct beam and in
more clinically realistic conditions. The best
protection is of the order of 80-90% but this is
variable.
 Face masks (0.1 mm Pb) found to reduce eye dose
by 85% at 80 kVp, can be worn over spectacles
 Ceiling suspended shields can afford a dose
reduction of approximately 98% at 80 kVp
Eye dose monitoring
 Individuals likely to incur > 1/10 the dose limit are
monitored routinely
 Large increase in the number of staff members required to
undergo eye dosimetry.
 Positioning of badges presents a problem; the dose can vary
by a factor of 3-5 depending on position of the badge with
respect to the tube1.
1. Jankowski, J. Methods of radiation exposure estimation of patient and medical staff during some
cardiology procedures. 5th Int. Workshop on Individual Monitoring of Ionising Radiation. Orai
Japan 2009.
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In conclusion
 The ICRP review suggests that the threshold for
cataract formation is lower than previously thought,
maybe zero
 Note that several studies question these conclusions
 However, on their basis, a change in the annual eye
dose limit from 150 mSv to 20 mSv is proposed.
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Furthermore
 Implications are likely to be significant, especially for
interventional and cardiology procedures
 Data must be gathered in order to determine the level
of monitoring required and whether an individual
requires classification.
 Care must be taken to utilise protective devices, and to
comply with monitoring requirements.
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