Eye lens dosimetry: does the direction of rotation (vertical or horizontal) play a role in type testing?

With the International Commission on Radiological Protection (ICRP) lowering the annual dose limit for the eye lens to 20 mSv, precise monitoring of eye lens exposure has become essential. The personal dose equivalent at a depth of 3 mm, Hp(3), is the measurement method for monitoring the dose to the lens of the eye. Usual dosemeter type-test irradiations at non-normal angles of radiation incidence (alpha not equal 0 degrees) primarily use lateral radiation exposure scenarios, where radiation approaches from the left or right, necessitating rotation of the dosemeter-phantom setup around a vertical axis. However, this method does not adequately account for bottom-to-top radiation exposures which are common in real-world situations (such as radiation scattered by a patient reaching medical staff). This study examines oblique radiation exposure conditions using a typical eye lens thermoluminescent dosemeter (TLD), Eye-D, placed on a cylindrical phantom to assess dose response at different angles and exposure energies. The study employs both low-energy (N-30 radiation quality with a mean photon energy of 25 keV) and medium-energy (N-100 radiation quality with a mean photon energy of 83 keV) x-rays at irradiation angles of -60 degrees, 0 degrees, and +60 degrees, measured along the vertical and horizontal rotation axes of the dosemeter-phantom setup. The results show no significant difference between horizontal and vertical (polar and radial) rotation orientations of the dosemeter-phantom setup: recorded relative doses stayed well within +/- 1 %, i.e. by far within the attributed combined uncertainty of +/- 2 %.