When stones grow: a practical guide from EAU endourology to changes in volume with increasing size.

BACKGROUND: Accurate assessment of kidney stone volume is essential for treatment planning, yet significant heterogeneity exists in measurement methodologies. The choice of volume estimation formula can substantially impact calculated stone burden, potentially influencing clinical decisions. OBJECTIVE: To quantify inter-method variability in kidney stone volume estimation across clinically relevant stone morphologies. METHODS: Twenty-five stone configurations spanning clinically relevant dimensions (3–30 mm) were analyzed using four validated volume estimation formulas: scalene ellipsoid, oblate ellipsoid, prolate ellipsoid, and spherical approximation. Inter-method variability was quantified using coefficient of variation (CV). Error propagation analysis assessed the impact of CT measurement uncertainty (± 1.5 mm). Clinical reclassification potential was evaluated at guideline-based treatment thresholds (10 mm, 15 mm, 20 mm). RESULTS: Inter-method CV ranged from 0% for spherical stones to 88.4% for highly elongated configurations. A strong linear relationship was observed between shape index and CV (r²=0.94). Measurement uncertainty (± 1.5 mm) produced volume variations of -66% to + 120% for small stones (5 mm) versus − 21% to + 24% for larger stones (20 mm). At clinical thresholds, elongated stones demonstrated up to 6.0-fold volume differences between formulas. CONCLUSIONS: Formula selection significantly impacts calculated stone volume, particularly for non-spherical morphologies.