A cataract is a clouding of the crystalline lens of the eye. Radiation cataract has been known for over a century, but its underlying mechanisms remain incompletely understood. In 2011, the International Commission on Radiological Protection (ICRP) issued the Seoul Statement to lower the dose threshold for vision-impairing cataract. ICRP deduced its acute threshold from epidemiological evidence demonstrating zero threshold, but concluded that cataract is a tissue reaction with a threshold albeit small. This paper is the first to hypothesize that there will be early- and late-onset cataracts that may arise by different pathomechanisms, and discusses this hypothesis according to available epidemiological and biological evidence. On one hand, early-onset posterior subcapsular cataract (PSC) may arise with a threshold after acute exposure. On the other, late-onset PSC or cortical cataract may arise without threshold after exposure regardless of the rate of dose delivery. Early-onset PSC may be attributable to excessive proliferation of lens epithelial cells, and late-onset PSC may involve its impaired proliferation. Late-onset cortical cataract may be associated with acceleration of aging process. If this hypothesis holds true, cataract will be a tissue reaction as well as a stochastic effect, challenging the current framework to classify radiation effects into these two categories.
After the Fukushima Daiichi Nuclear Power Station accident, the Nuclear Emergency Response Guidelines developed by the Nuclear Regulatory Authority of Japan has introduced Operational Intervention Levels (OILs). Particularly, OIL4 that is the screening level for decontamination against surface contaminations on the skin, clothes and others was adopted, and the default OIL4 value is 13,000 cpm. The guideline mentions that conversion of count rate is necessary when using different models of GM tube survey meters. Thus, count rate measurements using a surface radiation source of 10 cm square were performed with four typical GM tube survey meters in this work, and the instrument efficiencies were compared. In addition, the dependences of the distance from the detector window of the GM tube survey meter to the source are also evaluated between 1 cm and 5 cm. The resulting count rates for OIL4 of the tested GM tubes survey meters varied between 7,000 and 9,000 cpm. Count rates were decreasing as distance from the source increased with the different ratios depending on the model of GM tube survey meter. The screening levels showed between 3,000 and 6,000 cpm when distances between the source and the detector windows were 5 cm. This study suggested the importance to evaluate the intrinsic characteristics of the model and to derive the appropriate screening level at a few centimeters from the screening object in order to detect contamination reliably.
In April 2011, the International Commission on Radiological Protection (ICRP) issued the statement on tissue reactions. This stimulated interest in many countries. The Expert Committee on Radiation Protection of the Lens of the Eye was established in the Japanese Health Physics Society, and in April 2013, started discussion about the international developments and recent studies related to the dosimetry of the lens of the eye. This committee now publishes the interim report consisting of parts I-VI. Of these, this Part I overviews the structure of the eye and lens, cataract types and the scientific evidence of its new dose threshold and equivalent dose limit newly recommended by the ICRP.
A brief review is given of the history and methodology of external dosimetry for the lens of the eye. Under the 1989 revision to domestic radiological protection regulations, the concept on the effective dose equivalent and the dose limit to the lens of the eye (150 mSv/y) both introduced in the ICRP 1977 recommendations has changed nationwide the external monitoring methodology in non-uniform exposure situations to the trunk of a radiological worker. In such situations, which are often created by the presence of a protective apron, the worker is required to use at least two personal dosemeters, one worn on the trunk under the apron and the other, typically, at the collar over the apron. The latter dosemeter serves the dual purpose of providing the dose profile across the trunk for improved effective dose equivalent assessment and of estimating the dose to lens of the eye. The greater or appropriate value between Hp(10) and Hp(0.07), given by the dosemeter, is generally used as a surrogate of Hp(3) for recording the dose to the lens of the eye. The above-mentioned methodology was continued in the latest 2001 revision to the relevant regulations.
The magnitude of dose due to the Fukushima Daiichi Nuclear Power Plant accident was revealed by the UNSCEAR 2013 report published in April 2014. In the report, effective doses and thyroid doses were estimated for 1-year-old infants and 10-year-old children as well as adults. The UNSCEAR Committee formally agreed to rely principally on data available and literature published before the end of September 2012. However, studies on dose estimation have been conducted after that. Then, recent papers and activities of “Fukushima Health Management Survey” are presented. In relation to this, the effective dose estimated by the UNSCEAR report was compared with the estimation by another method (extrapolation from the result of Fukushima Health Management Survey with data on individual dose measurements using personal dosimeters). As a result, they were generally consistent each other. The UNSCEAR report states, it is likely that some overestimation has been introduced generally by the methodology used by the Committee. In the case of internal dose, direct measurements of the radioactive content of the thyroid indicated doses being lower than those estimated by the Committee, by a factor of about 3 to 5. Thus, studies on reconstructing a more realistic dose for residents are still needed.