Variation in thyroid volumes due to differences in the measured length or area of the cross‐sectional plane: A validation study of the ellipsoid approximation method using CT images

PURPOSE: This study examined the variation in the thyroid volume determined by the ellipsoid approximation method due to differences in the measured length or area of the cross‐sectional plane of CT images. METHODS: Forty‐five patients with Graves' disease were included in this retrospective study. We designated the three‐dimensional thyroid volumes extracted manually (V (CT)) as the reference data and calculated five approximate volumes for comparison: (a) the mean volume of 8100 different thyroid volumes depending on the diameter of the cross‐sectional plane at the midpoint of the major axis, (V (ellipsoid,mean)); (b) the volume using the maximum diameter and its orthogonal diameter, (V (ellipsoid,maxlength)); (c) the maximum (V (ellipsoid,maxvolume)); (d) minimum (V (ellipsoid,minvolume)) of the 8100 thyroid volumes; and (e) the volume determined with an equivalent circle diameter, (V (ellipsoid,Heywood)). RESULTS: Thyroid volumes obtained via the ellipsoid approximation method varied depending on the diameter of the cross‐sectional plane and included a mean error of approximately 20%, while the concordance correlation coefficient (CCC) differed for each approximate volume. Among these volumes, V (ellipsoid,mean) and V (ellipsoid,Heywood) were in good agreement with V (CT), according to single regression analyses and the resultant CCC values, with mean errors of 0.1% and 10.4%, respectively. CONCLUSION: While V (ellipsoid,Heywood) approximated thyroid volumes with vastly reduced errors, we recommend utilizing three‐dimensional thyroid volumetry if measurement accuracy is required.