|Year : 2012 | Volume
| Issue : 1 | Page : 1-3
The new dose limit for the lens of the eye and its implications
C/o. RP&AD, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Web Publication||6-May-2013|
B C Bhatt
C/o. RP&AD, Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: DST., Conflict of Interest: None
|How to cite this article:|
Bhatt B C. The new dose limit for the lens of the eye and its implications. Radiat Prot Environ 2012;35:1-3
For many years in the International Commission on Radiological Protection (ICRP) publications, eye lens equivalent dose (H lens ) has been reported as limiting quantity and a value of 150 mSv/y has been recommended in ICRP publication 60 (1991). The ICRP publication 103 (2007) provided updated information on dose thresholds for deterministic effects. The criterion used by the ICRP to define a threshold for any tissue reaction is the amount of radiation to cause the effect to be observable clinically in 1% of the exposed individuals. On this basis, the Commission judged that the occupational dose limit for the lens of the eye given in the ICRP publication 60 (1991) remained applicable. However, it was commented that new data on the radio-sensitivity of the eye were expected and stated that the Commission would consider those data when they became available. Subsequently, the ICRP reviewed the recent epidemiological evidence suggesting that there are some tissue reaction effects, particularly those with very late manifestation, where threshold doses are or might be lower than previously considered. A statement was issued by the ICRP in this regard in April, 2011.
Subsequently, the Commission has brought out an ICRP Publication 118 (2012) on "ICRP Statement on Tissue Reactions and Early and Late Effects of Radiation in Normal Tissues and Organs - Threshold Doses for Tissue Reactions in a Radiation Protection Context." For the lens of the eye, the threshold in absorbed dose is now considered to be 0.5 Gy. For occupational exposure in planned exposure situations, the Commission recommends an equivalent dose limit for the lens of the eye of 20 mSv in a year, averaged over the defined periods of 5 years, with no single year exceeding 50 mSv. This new recommendation has been included in the revised International Basic Safety Standards of International Atomic Energy Agency (IAEA-BSS-2011).
This has led to scientific debates in various forums, and national and regional bodies responsible for radiation protection and regulations find it necessary to assess its impact in the existing programs on radiation protection and safety, particularly in the practice of interventional radiology and cardiology (IR/IC). Due to lowering of the annual dose limit for the eye lens from 150 mSv to 20 mSv (i.e., by a factor of 7.5) for occupational exposures, the radiation protection requirements for protecting the eye and monitoring the dose to the lens of the eyes to comply with the new dose limit are now more demanding than formerly assumed.
The International Commission on Radiation Units (ICRU) recommendations (ICRU 47, 1993) introduced the personal dose equivalent H p (d) for individual monitoring. H p (d) is defined as the dose equivalent in soft tissue, at an appropriate depth, d (mm), below a specified point on the body. H p (10) is indicated for the whole body exposure evaluation, H p (0.07) for the extremities and H p (3) for eye lens dosimetry. H p (3) dosimeters are designed to monitor the eye lens dose, H lens , as the radiation-sensitive part of the lens lies about 3 mm within the eye. In order to comply its theoretical definition, ICRU-57 (1998) and ICRP-74 (1996) introduced a simplified 30 cm × 30 cm × 15 cm 4-element ICRU slab phantom in which sets of conversion coefficients between measurable quantities (e.g., fluence or air kerma) and H p (d) were calculated. Data for H p (3) are not available in the reports but were evaluated, on the same phantom by Till et al.
One of the major issues is regarding development of methods for dosimetry of the eye lens using suitable phantom(s) and calibration procedures. The first systematic efforts in this direction have been made under the Optimization of Radiation Protection of Medical Staff (ORAMED) project, where coordinated measurements on occupationally exposed medical staff were performed in different hospitals in Europe (www.oramed-fp7.eu). This collaborative project (2008-2011) was supported by the European Commission within its seventh framework programme. The main objective was to obtain a set of standardized data on extremity and eye lens doses for staff involved in IR/IC and to optimize radiation protection aspects. Special attention was given to the measurement of the dose to the eye lens. Under the project, a new eye lens dosimeter responding in terms of H P (3) was designed, optimized and tested. ,
The dosimeter consists of an LiF: Mg,Cu,P (MCP-N) thermoluminescent (TL) detector (in the form of pellet Ø4.5 mm × 0.9 mm) inside a polyamide capsule, with a wall thickness of 3 mm. The dosimeter holder enables comfortable wearing it on a head, at position fixed close to an eye. The test measurements and Monte Carlo calculations of the photon energy response and angular response produced very satisfactory results; all obtained values were within about 20% around unity (with respect to Cs-137). The dosimeter fulfills all requirements for its application in dosimetry in IR. The dosimeter was named EYE-D and is the first dosimeter available commercially from the RADCARD company. It is stated that the construction and energy response of the dosimeter tend to suggest that it can be applied without any modifications for typical beta-ray radiation fields encountered in nuclear medicine. Appropriate recommendations for type tests of passive eye lens dosimeters measuring the quantity H p (3) are contained in a recently published standard of the International Electrotechnical Commission (IEC): IEC 62387 (2012).
A new 20 cm high × 20 cm diameter cylinder recently suggested by the ORAMED project (www.oramed-fp7.eu) for eye lens dosimetry, as the use of a new cylindrical phantom, more representative of the head, allows the calculation of a useful set of conversion coefficients H p (3)/Ka for the reference X-ray beams for use in type test and calibration purposes.  In parallel with the ORAMED project activities on the operational quantity, detailed computational studies on the H T (eye lens) both for electrons and photons were carried out by Behrens and Dietze  and Behrens et al.,  with an improved model of the eye.
Behrens  has commented that the slab phantom is widely available and has been in use in many calibration laboratories for many years, whereas cylinder phantoms would have to be produced and made commercially available. It has been suggested  that in pure photon radiation fields, e.g., in IR, H p (0.07) dosimeters calibrated on a slab phantom are appropriate to monitor the lens dose when worn near the eye and if they detect radiation scattered back from the head (which is usually the case when their back consists of thin plastic). There are many commercially available extremity dosimeters (e.g., TL and optically stimulated luminescence (OSL) ring dosimeters) which may be useful for monitoring eye dose in pure photon fields. In beta radiation fields, e.g., in nuclear medicine, H p (0.07) dosimeters may overestimate the lens dose by a factor of 100 or more. Thus, they are unsuitable for this application. In radiation fields dominated by beta radiation dosimeters calibrated in terms of H p (3) on a slab phantom should be used.
As per Landauer's product catalogue nanoDot Al 2 O 3 : C OSL dosimeters can be used for eye dose monitoring.
Using a single collar dosimeter outside the lead apron is a common practice in many countries, including UK, and can be used to give a direct measure of eye dose (using H p (3) or H p (0.07)) plus an indication of the whole body dose. A below apron dosimeter may additionally be required if the collar badge reading is high, but formulae to determine effective dose would be needed. 
Active personal dosimeters (APDs) have been found to be very efficient tools to reduce occupational doses in many applications of ionizing radiation, but exhibit limitations in pulsed radiation fields. A study concerning the optimization of the use of APDs in IR/IC was performed in the framework of the ORAMED project.  Results indicate that most of APDs provide a response in pulsed mode more or less affected by the personal dose equivalent rate, which means they could be used in routine monitoring provided that correction factors are introduced. These results emphasized the importance of adding tests in pulsed mode in type-test procedures for APDs.
There is a general opinion that if the dose limit for the lens of the eye is reduced to 20 mSv, with higher workload, many physicians could exceed this limit. Therefore, as a measure of the optimization of radiation protection, in addition to the routine monitoring of eye dose, the proper use of radiation protective devices, such as ceiling suspended shields, lead glass spectacles will even be more important. Training in radiation protection is crucial for Medical Physicists, Interventional Radiologists and Cardiologists, Paramedical Staff, Radiation Protection Officers, University Lecturers.
There is a need to collate all these international efforts to produce a document which should provide specific guidelines in respect of suitable dosimeters for monitoring photon and electron dose to the eye lens, H lens , use of suitable phantom(s), establishing calibration procedures in terms of H p (3), recommendations for type tests of passive eye lens dosimeters for measuring the operational quantity H p (3). It is therefore suggested that IAEA may develop a Safety Guide similar to the one published earlier under IAEA Safety Standards Series on "Assessment of Occupational Exposure Due to External Sources of Radiation," Safety Guide No. RS-G-1.3 (1999). This would serve as a useful guidance document in the area of personnel dosimetry where the eye lens doses are hardly ever measured in practice in most of the countries. Such a document would also be important for preparedness to ensure compliance by the member states with the new ICRP recommendation on the dose limits for the lens of the eye.
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