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Published in the Bulletin of the CNS, Vol. 37, No. 3, Oct. 2016
1. Introduction
Protecting the human body from the effects of ionizing radiation is essential to forestall stochastic
1
effects and
require placing limits on the effective dose. Dose limits on specific organs are also necessary to reduce the
deterministic
2
effects and tissue reactions.
The standard for radiation protection was ISO 15382 (2002) which mainly dealt with beta radiation for nuclear power
plant workers. Clearly an update is required to allow for new technology and the proliferative use of radiation in
medical practices. There is a need for more explicit radiation monitoring to operators and staff. ICRP118
(International Commission on Radiological Protection), Ref. 1, evolved their recommendations to include eye lens
doses as a follow on to their publication 103 and to focus on radiation exposures. It provides updated estimates of
‘practical’ threshold doses for tissue injury at the level of 1% incidence.
This paper discusses the current status and the recommendation for a drastic reduction of the dose limit to the eye
lens.
2. Typical Workplace Exposures
In workplace situations, radiation fields can
be predictable when measured over long
periods, if its variability is minimal, and thence
worker doses can be estimated. In medical
practices the annual doses can be as shown in
Table 1 measured by a whole body dosimeter.
But If the dosimeter is worn under protective
shielding it will underestimate the dose to the
unshielded surfaces, such as the eyes. Of
concern, is the inducement of cataract
which is the loss of transparency of
the lens of the eye which lies behind
the iris and the pupil Fig. 1 and starts
with lens opacities, Fig. 2. Cataracts
may be induced by genetic
components, is age related, and is
exacerbated by additional factors viz.
too much sunlight (UV), imbibing,
smoking, diabetes, the use of
corticosteroids, and of course by
radiation.
1
Stochastic effects are the non-lethal transformation of cells that can still cell divide, but can lead to cancer after a latency period if it is a
somatic cell, or may lead to hereditary effects if it is a germ cell. It is proportional to the dose received with no threshold. The ‘detriment-
adjusted nominal risk coefficient of dose’, which includes the risks of all cancers and hereditary effects, is 5% per Sievert (Sv). Ref. 4.
2
Deterministic effects are clinically observable when the dose exceeds a certain threshold and where the severity
increases with increasing dose. Examples include radiation burns, hair loss, cataracts, nausea, and such radiation
caused symptoms.
Table 1 Medical Practice Exposures
Operator/Worker
Annual Dose Range in mSv
Nuclear Medicine Staff
5-40
Interventional Radiologists (hands)
10-200
Interventional radiologists (legs)
10-200
Interventional radiologists (legs with shield)
1-15
Cardiologists (hands)
5-100
Cardiologists (legs)
5-100
Cardiologists (legs with shield)
0.5-10
Source: IRPA14 Lecture Notes by Filip Vanhavere
Fig. 1 Eye Schematic with Cataract Lens
Fig. 2 Vision by Normal Eye and by
Cataract Eye
Clinical data have shown that radiation associated posterior lens opacities was 52% for interventional cardiologists,
45% for nurses, and 9% for controls, Ref. 2. Estimated cumulative ocular doses ranged from 0.01Gy to 43Gy. This
Indicated a strong dose-response relationship of increased risk of the posterior lens opacities for interventional
cardiologists and nurses when radiation protection tools are not used.
3. Regulating the Exposure from
Ionizing Radiation
The epidemiological evidence suggest
that the threshold absorbed dose for
the induction of deleterious effects on
the eye lens is about 0.5 Gy, Ref. 3, and summarized in Table 2. Based on this finding, the International Commission
on Radiological Protection (ICRP) recommends that the dose equivalent to the eye lens should be reduced from 150
mSv to 20 mSv in a year, averaged over a 5y period with exposure not exceeding 50 mSv in any single year. On 21 April 2011
the ICRP recommendations followed their statements on tissue reactions by stating it is more appropriate to treat
cataract induction as a stochastic rather than as a deterministic effect. The ICRP recommendations are summarized in Table 3.
Hence, within this context, it is illogical to apply the same dose limit for a uniformly irradiated whole body to the lens of the eye.
There are two possibilities to address this issue viz.
a- Assigning an appropriate tissue weighting factor
to the dose limit of the eye lens, and including it in
the computation of the effective dose, or
b- By having a composite approach involving the
use of a tissue weighting factor for effective dose
computations together with a special limit on the
equivalent dose to the lens of the eye.
This approach would ensure that no individual
would be subjected to an unacceptably high risk of
inducting clinically significant cataracts. The IAEA Ref. 4, has developed two safety guides viz. Radiation Safety in the Medical
Uses of Ionizing Radiation No. RS-G-1.5; and Occupational Radiation Protection IAEA Safety Standards Series No. RS-
G-1.1 to provide guidance on the control of occupational exposures in the medical and other fields where there is
harmful ionizing radiation.
The rationale for not including the equivalent annual dose limit of 500 mSv in the 20 mSv effective dose limit is based
on preventing deterministic effects to the skin. Ref. 5.
Recent occupational findings in chronically radiation exposed workers suggest there is a long term risk for cataracts
and the need for eye protection even at low doses. In 2012 IRPA formed a Task Group (TG) to assess the impact of a
reduced dose limit to the eye lens for occupational workers. The TG proposed the following:
Reduce the number of sessions the staff can do per year in order to keep within the new dose limit.
Continue using the available protective measures but ensuring their optimum usage.
Increase risk awareness to potentially exposed workers. This can be achieved by mentoring, training, and by
implementing a safety culture.
However, in the nuclear power industry there is the alternate methodology of placing the operator in a remote
location. Yet, issues prevail in the implementation of the new recommendations of dose reduction Ref. 6. Ontario
Power Generation (OPG) identified issues requiring higher dose limits and has brought it to the attention of
Regulatory authorities. There is the potential for extra costs in training programs and changes in work methods. On
the positive side, increasing the use of robotics have reduced the dose significantly.
Equipment to Reduce Dose
Table 2 ICRP118 Estimates for Threshold Doses
Effect
Time to
Develop
Effect
Acute
Exposure
Gy
Highly Fractionated
(2Gy fractions or
equivalent)
Annual Dose Rate
over many years
Cataract
> 20 y
~ 0.5
~ 0.5
~ 0.5
Table 3 ICRP Recommended Dose Limits
Type of Limit
Occupational Exposure
Exposure to Public
Annual Effective Dose
20 mSv/y averaged over 5y 1
1 mSv/y
Annual Equivalent
Dose: Eye Lens
20 mSv/y averaged over 5y1
1 mSv/y
Skin 2
Hands and Feet
500 mSv
500 mSv
50 mSv
50 mSv
1- Provided that the Effective Dose does not exceed 50 mSv in any single year.
Additional restrictions apply to the occupational exposure of pregnant women.
2- Averaged over 1 cm2 area of skin regardless of the area exposed.
Public dose limit stays at 15 mSv/y
NASA limit of 8 mSv in total for astronauts on a mission
Some manufacturers have developed special equipment, such as Cathpax CRT Fig. 3, and Lemer Pax Fig. 4 which are
more like radiation shielding cabins dedicated to procedures under fluoroscopy. The Cathpax® provides optimal
radiation protection and obviates the need, and the discomfort, of a lead apron.
4. Dosimetry
5.1 Eye Lens Exposure to Photon Radiation
This is more applicable in the medical field in interventional radiology
where exposure to x-ray fields of <150 keV occur. This exposure is
mainly from scattered radiation emanating from the patient who is
undergoing an examination. Data show that with exposure to high-
energy photons (> 200 keV), it can be assumed that the dose
equivalents Hp(10) provides a good estimate of the eye lens dose.
Ref. 7.
5.2 Eye Lens Exposure to
Beta Radiation
Beta particles (electrons)
with energies < 3 MeV
have low depth
penetration in tissue
where the dose
transmission is localised at
the tissue surface.
Electrons with energies <
0.7 MeV have a range of < 3 mm tissue depth and therefore do not contribute to the eye lens dose in practice.
Table 4 shows the types of dosimeters, their limits and their usage, derived from Ref. 7. With all the shown overlaps
it is recommended that efforts be made to obtain international clarification as to how the eye lens dose be calculated
with the advent of Hp(3). In applications of interventional cardiology and radiology, the staff work in close proximity
to the x-ray sources. The exposures received are non-uniform because the staff wear lead aprons to shield the body,
but the head/eye are not protected. Hence Hp(3) is the recommended dosimeter (IRPA14 Task Group) to provide the
Equivalent Dose at 3 mm depth. In nuclear power plants and non-medical centers a whole body dosimeter is
considered sufficient. Unfortunately Hp(3) dosimeter is not yet widely available hence both Hp(0.07) and Hp(10) are
used. A number of methods of estimating the eye dose based on ratios of Hp(3)/Hp(10) have been proposed for both
the nuclear and medical sectors; but are not yet definitive.
In item [iii] of Table 4, the skin dose Hp(0.07) is the
dose equivalent in 0.07 mm depth in the body at the
application point. The Dose Equivalent HT,R is the
product of the organ absorbed dose DT,R averaged
over tissue organ T generated by the radiation R and
the weighting factor WR
HT, R = WR * DT, R
If the radiation has multiple energies and with
different weighting factors then use the summing
equation below and pick the appropriate weighting
factor from Table 5.
Table 4. Types of Dosimeters
Depth (d) in mm
Hp(d) is for estimating
Annual Limit mSV
Typical usage Ref. 7
Hp(10)
Equivalent dose, E
20
Gamma and X-Ray [i]
Hp(3)
Equivalent Eye Dose, Hlens
150
Medical applications and for
close work, fluoroscopy [ii]
Hp(0.07)
Equivalent Skin Dose, Hskin
500
Photon radiation fields [iii]
[i] The Hp(10) adequately estimates the eye lens dose at energies of <100 keV.
[ii] The Hp(3) is sufficient to determine eye lens dose in interventional applications e.g. cardiology. Must
be calibrated on a water slab phantom.
[iii] The Hp(0.07) adequately estimates the eye lens dose at energies of <200 keV. Hence there is no need
to an additional dose equivalent quantity Hp(3). Must be calibrated on a water slab phantom.
Table 5 Radiation Weighting Factors for Dose Equivalent
Radiation Type and Energy Range
Radiation Weighting Factor WR
Photons, all energies
1
Electrons and muons, all energies
1
Neutrons:
< 10 keV
5
10 keV to 100 keV
10
>100 keV to 2 MeV
20
>2 MeV to 20 MeV
10
> 20 MeV
5
Protons, except for recoil protons,
> 2 MeV
5
Alpha particles, fission fragments,
heavy nuclei
20
WR * DT,
HT, R =
R
Fig. 4 Lemer Pax
Fig. 3 Cathpax CRT
6. Conclusions
Dose to the eye lens is to be considered as a stochastic and not a deterministic effect.
A unified international approach is required to clarify how the eye lens dose be calculated.
Can use the HP 0.07 dosimeter where the photon energy is <200 keV. This is commonly used in interventional
radiology.
Yet, there were calls for the threshold dose to be set at an even lower level, due to clinical evidence. Since the
recommendation for a new eye lens dose limit some 5 years ago, a complete resolution of all the practical issues
has not been achieved. Such a drastic reduction in dose limit needs time to be implemented. Its application will
deeply change previous operating procedures.
References
Ref. 1 ICRP 118. ICRP Statement on Tissue Reactions / Early and Late Effects of Radiation in Normal Tissues and
Organs Threshold Doses for Tissue Reactions in a Radiation Protection Context, 2012.
Ref. 2 Risk for Radiation-induced Cataract for Staff in Interventional Cardiology: Is There Reason for Concern?
Ciraj-Bjelac O1, Rehani MM, Sim KH, Liew HB, Vano E, Kleiman NJ. PubMed 2010 Nov 15; 76(6):826-34.
Ref. 3 Regulating Exposure of the Lens of the Eye to Ionising Radiations. M.C. Thorne, J. Radiol. Prot. Jun. 2012.
Ref. 4 Implications for Occupational Radiation Protection of the New Dose Limit for the Lens of the Eye. IAEA
Tecdoc 1731, in Objectives 1.4
Ref. 5 George E. Chabot, Physics. Professor Emeritus. Pinanski Energy Center 207. Email: george_chabot@uml.edu
Phone: 978-934-3288.
Ref. 6 John Chase, Sr. Health Physicist at OPG, at the CRPA Annual Conference presentation, May, 2016.
Ref. 7 Monitoring the Eye Lens Dose; Statement by the German Commission on Radiological Protection with
Scientific Reasoning. Adopted at their 240th meeting on 2 February 2010.
ResearchGate has not been able to resolve any citations for this publication.
Article
The International Commission on Radiological Protection (ICRP) has reviewed recent epidemiological evidence suggesting that, for the lens of the eye, the threshold in absorbed dose for the induction of deleterious health effects is about 0.5 Gy. On this basis, the Commission recommends that for occupational exposure in planned exposure situations, the equivalent dose limit for the lens of the eye should be 20 mSv in a year, averaged over defined periods of 5 yr, with exposure not exceeding 50 mSv in any single year. This paper summarises the data that have been taken into account by the ICRP and critically examines whether the proposed downward revision of the dose limit is justified. Overall, it is concluded that the accumulating radiobiological and epidemiological evidence makes it more appropriate to treat cataract induction as a stochastic rather than a deterministic effect. Within this framework, it is illogical to have the same dose limit for the lens of the eye as for the whole body irradiated uniformly. This could be addressed either by removing the special dose limit for the lens of the eye, assigning it an appropriate tissue weighting factor and including it in the computation of the effective dose, or through a composite approach involving the use of a tissue weighting factor for effective dose computations together with a special limit on the equivalent dose to the lens of the eye to ensure that no individual was subject to an unacceptably high risk of induction of clinically significant cataracts.
Risk for Radiation-induced Cataract for Staff in Interventional Cardiology: Is There Reason for Concern? Ciraj-Bjelac O1
  • M M Rehani
  • K H Sim
  • H B Liew
  • E Vano
  • N J Kleiman
  • Pubmed
Ref. 2 Risk for Radiation-induced Cataract for Staff in Interventional Cardiology: Is There Reason for Concern? Ciraj-Bjelac O1, Rehani MM, Sim KH, Liew HB, Vano E, Kleiman NJ. PubMed 2010 Nov 15; 76(6):826-34.
Health Physicist at OPG, at the CRPA Annual Conference presentation
  • John Chase
  • Sr
John Chase, Sr. Health Physicist at OPG, at the CRPA Annual Conference presentation, May, 2016.