Eye lens dose in CT examinations
In 2007-2010, approximately 660 million X-ray examinations were performed in Europe. Of these studies, 8,7 % were CT scans. However, they accounted for 52 % of diagnostic and interventional healthcare radiation. The lens of the eye is particularly sensitive to ionizing radiation and excessive radiation can cause cataracts. Cataracts are the leading cause of blindness and visual disability worldwide. Radiation-induced cataracts are currently classified as deterministic with threshold-dose of 0.5 Sv, but there is evidence that the cataract risk is also increases by radiation doses lower than that. The purpose of our study was to investigate how well the ALARA principle was followed in the Tampere University Hospital regarding the radiation protection of the lens in the brain CT scans by the use of optimal gantry tilt and scan length. The dataset included 329 adult brain CT scans. The collected data from patients included: age, sex, CTDIvol and gantry tilt angle. The eye lens absorbed doses in Class A and Class C were modeled by using CT-Expo v. 2.7. Three different categories were used to classify if the gantry tilt and scan length were optimal. In Class A the scan area is fitted to supraorbitomeatal line. In Class C the scan area is fitted to infraortibomeatal line or below. The Class B is between Class A and C where the scan area is fitted around orbitomeatal line. The lowest image in the CT image stack was used to classify the study. The tilt of the gantry had been used in a large proportion of the CT studies, 84.8%. The optimization to avoid lens irradiation was successful by using gantry tilt and image stack placement in 1.8 % of examinations (Class A), and partially successful in 68.5 % (Class B). The whole lens was exposed in 29.7 % of examinations (Class C). The modeled median (min-max) lens aborbed doses for the cases where the lenses were completely excluded (Class A) and included (Class C) in the exposed area were 4.4 mGy (0.7 mGy - 4.9 mGy) and 25.9 mGy (17.8 mGy - 49.2 mGy) respectively. The data in the file includes: patient age and sex, gantry tilt in degrees, classification (A-C), CTDIvol, lens dose (mGy) and notes about possible implants or imaging protocols.
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