Three-Dimensional Volumetric Segmentation of Pituitary Tumors: Assessment of Inter-rater Agreement and Comparison with Conventional Geometric Equations

Abstract

Background The assessment of pituitary tumor (PT) volume is important in the treatment and follow-up of patients with PT. Previously, PT volume estimation has been performed by conventional geometric equations (CGE) such as abc/2 (simplified ellipsoid volume equation) and 4πr ³ /3 (sphere), both presuming a symmetric tumor shape, which occurs uncommonly in patients with PT. In contrast, three-dimensional (3D) voxel-based software segmentation takes the irregular and asymmetric shapes that PTs often possess into account and might be a more accurate method for PT volume segmentation. The purpose of this study is twofold. (1) To compare 3D segmentation with CGE for PT volume estimation. (2) To assess inter-rater reliability in 3D segmentation of PTs. Methods Nineteen high-resolution (1mm slice thickness) T1-weighted MRI examinations of patients with PT were independently analyzed and manually segmented, using the software ITK-SNAP, by two certified neuroradiologists. Concurrently, the volumes of the PTs were estimated with abc/2 and 4πr ³ /3 by a clinician, and the results were compared with the corresponding segmented volumes. Results There was a significant decrease in PT volume attained from the segmentations compared with the calculations made with abc/2 ( p < 0.001, mean volume 18% higher than segmentation) and 4πr ³ /3 ( p < 0.001, mean volume 28% higher than segmentation). The intraclass correlation coefficient (ICC) for the two sets of segmented PTs was 0.99. Conclusion CGE ( abc/2 and 4πr ³ /3 ) significantly overestimates PT volume compared with 3D volumetric segmentation. The inter-rater agreement on manual 3D volumetric software segmentation is excellent.

Keywords: abc/2; pituitary tumor volume; segmentation; volumetric analysis.

Fig. 1

Illustration of an ellipsoid. The abc/2 equation for volume estimation is derived from the equation for an ellipsoid's volume, (length)*(width)*(height)*π/6, by approximating π to 3. Picture published under the Creative Commons Attribution Share Alike 3.0 License, attained from Wikimedia Commons, author Peter Mercator.

Fig. 2

View of three-dimensional segmentation of a pituitary tumor. The tumor is marked in red on axial (upper left), sagittal (upper right), and coronal views (lower right) on magnetic resonance images.

Fig. 3

The diagram illustrates the difference (in %) between the volumes attained by segmentations and the equations abc/2 and 4πr ³ /3. The x-axis shows the segmented volume (the mean value of two independent segmentations per PT, performed by two certified neuroradiologists) of the pituitary tumors in increasing order. PT, pituitary tumor.

Fig. 4

The Bland–Altman plot showing the difference in measured pituitary tumor volume in 19 patients. The volumetric segmentations were performed independently by two certified neuroradiologists.