Comparison of Measurement Methods for Stone Volume Estimation: An In Vitro Study.

Abstract

Introduction: Urolithiasis guidelines still rely on the maximum stone diameter to propose treatment strategy, although this measure is known to have many pitfalls. Stone volume (SV) could represent a more accurate measurement, helping to plan the treatment or follow-up. Various methods to measure SV have been proposed. We aimed to compare different methods to estimate SV. Methods: Fifteen stones (human and artificial) were assessed. Real SV was measured using the water displacement method. Volume estimation included three diameter-based formulas (Ackerman, 4/3 Pi r³ and r³/2) and two 3D segmentation methods (Horos and Kidney Stone Calculator [KSC]). All measurements were done by a single operator. Spearman correlation test and comparative analyses were conducted between the real and the estimated SV. Results: Compared with real SVs, Ackerman and r³/2 formulas estimated volume accurately in 2/15 (13%) of stones each. No accurate measurement was reported using the sphere formula. KSC did estimate volume accurately in 4/15 (27%) stones compared with the reference SV; Horos did it in 7/15 (47%) stones. Both segmentation methods presented strong correlation coefficients (r = 0.9642 and 0.9659, p < 0.0001), while formula correlation was moderate (r = 0.7531, p < 0.0001). Conclusion: Formulas and segmentation methods for SV estimation resulted in divergent outcomes. Segmentation methods (Horos and KSC) presented higher accuracies in SV estimation, compared with real SV. Formulas were the least accurate.