EXPRESS: Positional Impact on Ocular Lens and Thyroid Radiation Doses During Feline Cranial CT: A TLD Study of 70 Cases.

VD positioning during feline cranial CT reduces thyroid radiation dose by 24.4% without additional equipment.

This prospective study investigated the effect of patient positioning on radiation doses absorbed by the ocular lenses and thyroid gland during feline cranial CT examinations. Seventy cats were equally divided into dorsoventral (DV, n=35) and ventrodorsal (VD, n=35) positioning groups, with thermoluminescent dosimeters (TLD-100) placed at anatomical projections of the target organs to measure absorbed radiation doses. Results demonstrated no statistically significant difference in ocular lens doses between the two positioning groups (p>0.05). However, VD positioning produced a clinically and statistically significant 24.4% reduction in mean thyroid gland radiation dose compared to DV positioning (1.33 ± 0.17 mGy vs. 1.76 ± 0.35 mGy; p=4.97×10⁻⁹). The authors attribute this thyroid dose reduction to a 'natural shielding' effect provided by the dorsal axial skeleton and epaxial musculature, which interpose between the X-ray source and the thyroid gland in the VD position. Importantly, this dosimetric advantage was achieved without requiring additional shielding equipment or compromising diagnostic image quality. Given that feline patients may undergo repeated CT examinations and that the thyroid gland is a radiosensitive organ of particular concern in cats, the findings have meaningful implications for radiation safety. The study supports adopting VD positioning as a routine standard for feline cranial CT to minimize thyroid radiation exposure in accordance with ALARA (As Low As Reasonably Achievable) principles.

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