Eye lens dose in CT examinations
Description
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 study was designed as two-part investigation where first the current state of patient exposure levels was assessed. Thereafter identified issues were addressed and a follow-up assessment was made to verify the improvements. The dataset included 329 pre-training and 51 post-training adult brain CT scans. The collected data from patients included: age, sex, CTDIvol, DLP, 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. Non-zero gantry tilt had been used in a large proportion of the CT-examinations in the retrospective setting, 84.8%. However, the lenses were successfully excluded from the scan area in only 1.8 % of the examinations. In the prospective part, the gantry tilt was used in 98% of the studies and the proportion of successful examinations rose from 1.8% to 11.8%. The lens dose decreased significantly when the eyes were excluded from the imaging area. The modelled lens dose was 25.9 mGy (17.8 - 49.2 mGy) when the eyes were included and 1.7 mGy (0.4 - 2.4 mGy) when the eyes were excluded. The data in the file includes: patient age and sex, gantry tilt in degrees, classification (A-C), CTDIvol, DLP, lens dose (mGy) and notes about possible implants.
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