Influence of dosimeter depth on estimating patient's eye lens dose in head computed tomography: A measurement and Monte Carlo simulation study

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The distance between the posterior cornea and anterior surface of the lens is approximately 3 mm, and it is unclear whether the dose measured near the ocular surface of the phantom accurately reflects the lens dose without any specific correction factors. This study aimed to investigate the influence of the depth for the estimation of the eye lens doses of the patient during head computed tomography (CT) examination using measurement and Monte Carlo simulation. During the measurement, the absorbed dose on the surface and at a depth of 3 mm from the surface of the cylinder phantom, corresponding to the positions of the right and left eyes, was obtained using optically stimulated luminescence dosimeters at tube voltages of 80 and 120 kV. In the Monte Carlo simulations, an anthropomorphic phantom and custom-made cylinder phantoms, which were made of polymethyl methacrylate (PMMA) and polyurethane, respectively, were used, and absorbed doses were obtained on the surface and at a depth of 3 mm from the surface of the phantoms. In the measurement, the absorbed dose at the 3-mm depth from the surface of the phantom was significantly higher than that on the surface of the phantom for both tube voltages (p < 0.01). In the simulation, the absorbed doses at a depth of 3 mm from the surface of the phantom model were significantly higher than those on the surface of the phantom model regardless of the material and structure of the phantom (p < 0.01). The ratio of the absorbed dose on the surface to that at a depth of 3 mm ranged from 0.93 to 0.95 regardless of the material and structure of the phantom. These results highlight the importance of lens dosimetry considering the depth of the lens.