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Eye Dose Threshold Limits. Elaine Holt. Catherine Ashmore Lorna Sweetman. Background. A draft report of the ICRP was issued in early 2011 It reviewed the data relating to non-cancer effects and tissue reactions

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Eye dose threshold limits

Eye Dose Threshold Limits

Elaine Holt

Catherine Ashmore

Lorna Sweetman

North Western Medical Physics


Background
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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Background1
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
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
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
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
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
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
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 0.2-0.5Sv.

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Resulting change to the eye dose limit
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
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

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Implications
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
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 staff1
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
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
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 devices
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
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
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
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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