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Is monitoring of the radiation dose to the eyes necessary?


Background: Recently, the International Commission on Radiation Protection (ICRP) has drastically reduced its recommended maximum permissible value for the annual radiation dose to the eye lens: from 150 millisievert (mSv) to 20 mSv per year. This step was necessary, because it became increasing apparent that the previous maximum value does not suffice to prevent an eye cataract due to ionizing radiation. It could be assumed that the previous maximum permissible value of 150 mSv per year is not exceeded if the maximum permissible values for the person as a whole and the skin are not exceeded; for decades, these values have been routinely monitored by means of measurements for personnel (by means of so-called "whole-body and skin dosemeters") which is occupationally exposed to radiation. With the new maximum permissible value of 20 mSv per year for the eye lens, this is no longer valid. Therefore, it is now up to the German Radiation Protection Commission to make recommendations on the question of whether the radiation protection pursued so far is sufficient, e.g. for medical staff, or if more accurate dose measurements and additional protective measures are necessary. Suitability tests for different PTB dosemeter types are to help with these decisions.

If the German legislation should follow the recommendation of the ICRP and accordingly reduce the maximum permissible value, this could imply that – in addition to the previously worn whole-body and skin dosemeters - suitable dosemeters must be worn in the direct vicinity of the eye behind an appropriate eye protection. In order to protect their eyes, staff members who have attained their maximum annual dose, should not work in this field for the rest of the year.

The results of PTB in detail:

Dosemeters can be subdivided into three categories - according to the measuring purpose for which they have been designed. The different dosemeter types differ in their suitability for the determination of the radiation dose in the eye:

  1. Skin dosemeters provide reliable values for the eye lens dose only if they are used in the case of mere X-radiation (e.g. in radiology) in direct proximity to the eye, and if - additionally - its reverse side is permeable to X-radiation. In nuclear medicine, beta radiation is partly used. Here, skin dosemeters might overestimate the actual dose by a factor of one hundred, and more, and are therefore not suited. An example of a skin dosemeter is shown in Fig. 1.
  2. Eye dosemeters estimate the dose in a part of the eye lens which is particularly sensitive to radiation. However, only very few types of this kind of dosemeters exist, as the measurand has so far not been used. For the existing types it still has to be demonstrated that they are also suited for beta radiation. An example of an eye dosemeter is shown in Figure 2.
  3. According to their structure, whole-body dosemeters estimate the whole- body dose of the viscera. They are not suited for the determination of the eye lens dose, as they underestimate the radiation dose to the eye lens especially in the case of low-energy X-radiation and in the case of beta radiation. An example of a whole-body dosemeter is shown in Figure 3.

The further work of PTB to integrate the results in the national and international standardization is funded by the Federal Ministry of Economics and Technology by means of the project "Innovation with Standards (INS)" "Improved monitoring of the eye lens dose".

Source : https://www.mirion.com/gfx/landings/dsd_rings_lg.png

Figure 1 : skin dosemeter


Source : http://www.radcard.pl/obrazki/eye_d_brochure.jpg

Figure 2 : eye dosemeter


Source : http://www.ptb.de

Figure 3 : whole-body dosemeter