Radiation dose limit for eye lens slashed
The Hindu A Phakonit Cataract operation in progress at a hospital in Guntur. File photo
The lens of the eye is one of the most radiation sensitive tissues in the body. If the eye lens which is normally crystal clear receives a high enough radiation dose it may become partly cloudy or totally opaque depending on the dose. Radiation protection agencies have prescribed dose limits to the lens to prevent induction of lens opacity or cataract.
On April 21, this year, the International Commission on Radiological Protection (ICRP) which issues recommendations on radiation protection, slashed the dose limit for the lens of the eye to 20mSv in a year, averaged over defined period of five years, with no single year exceeding 50 mSv.
Earlier dose limit
The earlier dose limit was 150mSv in a year. (Sv is a unit of biologically effective dose. The radiation energy absorbed in a sievert (Sv) is one Joule per kilogramme of material; since the unit is large, a sub-multiple such as one thousandth of a Sv or milliSv —mSv — is normally used).
Several studies over the past few years led the Commission to reduce the dose limit steeply.
There are three main forms of cataract depending on its anatomic location in the eye lens: nuclear, cortical and posterior sub capsular (PSC). Among these, PSC is the least common and is commonly associated with exposure to ionizing radiation. Radiation Effects Research foundation (RERF) describes the formation of radiation cataract thus: “There is a transparent layer of cells covering the interior frontal side of the capsule that covers the eye lens.
This layer maintains the function of the lens by slowly growing toward the centre, achieved through cell division at the periphery. Because irradiation is especially harmful to dividing cells, exposed cells at the equator are most prone to damage.
Unknown reasons
For unknown reasons, damaged cells move toward the rear of the lens before converging on the centre. Such cells prevent light from travelling straight forward resulting in opacity.”
So far, scientists believed that cataract will be formed only after the lens receives a typical radiation dose called the threshold. ICRP assumed that threshold was 2Gy for a single dose and 5 Gy when the exposure occurs in a protracted way.
Not any more. Recent studies appear to show the formation of radiation induced cataracts at much lower doses than the current standards. (Gy is the unit of absorbed dose; the dose is said to be one gray — Gy — when the ionizing radiation energy absorbed per kilogramme of material is one joule).
ICRP now considers that the threshold dose for cataract is 0.5Gy. ICRP also stated that although uncertainty remains, medical practitioners must be made aware that the absorbed dose threshold for circulatory disease may be as low as 0.5Gy to the heart or brain.
“Doses to patients of this magnitude could be reached during some complex interventional procedures, and therefore particular emphasis should be placed on optimization in these circumstances,” ICRP cautioned the specialists. The procedures include angioplasty.
The June 2010 on-line version of Catheterization and Cardiovascular Interventions and October 210 issue of Radiation Research have published studies on increased risk of cataracts among interventional cardiology professionals. Though the numbers of professionals monitored in the studies was limited, the results demand urgent action.
Chernobyl effect
Cataract analysis of 8607 Chernobyl clean up workers,12 and 14 years after exposure, indicated that posterior sub-capsular or cortical cataracts appeared in 25 per cent of the participants (Radiation Research, February 2007). Researchers found evidence of a dose threshold of less than 0.7Gy.
The researchers noted that the workloads tend to increase in catheterization suites. This, together with lack of training in radiation protection and unavailability or non-use of radiation protection accessories may result in doses to the eyes of cardiology professionals sufficient to cause cataracts.
Studies show that leaded glass alone reduced the dose to the lens by 5 to 10 times; scatter-shielding drapes alone reduced the dose rate by 5 to 25 times; using both reduced the dose rate by 25 times or more
In BioMed Central Public Health (2010), Dr Sophie Jacob from the French Institute of Radiological Protection and Nuclear Safety (IRSN) and other specialists listed 14 peer reviewed studies showing evidence for low dose radiation-induced cataracts.
The results of their study on occupational cataracts and lens opacities in interventional cardiology involving 1700 interventional cardiologists in France is expected to be available this year.
The jury is no more out on radiation induction of cataract. The present ICRP recommendations must serve as a wake up call for interventional cardiology and radiology professionals.
Raja Ramanna Fellow, Department of Atomic Energy (ksparth@yahoo.co.uk)
