RADIOISOTOPES

The recommended annual limit of equivalent dose to the lens of the eye has decreased to 20 mSv y⁻¹ from the current limit of 150 mSv y⁻¹. In terms of occupational exposure to radiation workers, exposure inhomogeneity plays an important role when estimating equivalent dose to the eye lens from readings of dosimeters worn by workers on their trunk. The authors focused on homogeneous exposure situations that radiation workers may encounter in nuclear and accelerator facilities. Moreover, the authors investigated how radiation workers are exposed non-homogenously in homogeneous exposure scenarios, where radiation workers may usually be encountered, regardless of their radiation works. In our previous study, we proposed a methodology to quantitatively evaluate the inhomogeneity of exposure. The homogeneity of exposure was evaluated by performing Monte Carlo calculations with a mathematical phantom under the isotropic and rotational irradiation geometries due to gamma and beta rays. H_p(3)_eye/H_p(10)_trunk exceeded 1.0 even in the case of homogenous exposure to gamma rays. Even in the homogeneous exposure scenarios, the H_p(3)_eye might exceed the revised annual dose limit for radiation workers who are exposed close to annual dose limit. For exposure due to beta rays from ⁹⁰Y, the equivalent dose of the lens of the eye might overestimate around four times when estimated from the H_p(0.07) measured on worker’s trunk.