Catherine Ashmore Lorna Sweetman

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Elaine Holt

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 survivorsTherapeutic radiotherapyRepeated CT scansAstronautsResidents of contaminated buildingsVictims of the Chernobyl accidentRadiation technologists, interventional

radiologists and interventional cardiologists

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Cataract Types

www.ndrs.scot.nhs.uk/Train/Handbook/drh-27.htm

Nuclear

Cortical

Posterior subcapsular

Ionising radiation is associated predominantly (though not exclusively) with opacities in the cortical and posterior subcapsular regions

Previous guidelinesIn 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 effectClassification 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 changesLow 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 continuedNew 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 dosesCardiologist Eye Doses 2010

0

10

20

30

40

50

60

70

80

90

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)

Fre

qu

en

cy

Classification level at 3/10 the dose limit

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ImplicationsBased 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 staffAnnual 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 DOSEGreater angle of ‘C’ arm = 6 x increase in dose to the eye

P.A. ( Postero-anterior ) L.A.O. 40, CRANIAL 25

Shielding DevicesThe 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 monitoringIndividuals 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 conclusionThe 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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FurthermoreImplications 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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