Accurate Assessment of Interval Change in Stone Burden Among Patients with Medullary Sponge Kidney: A Volumetric Approach

<b><i>Introduction:</i></b> Medullary sponge kidney (MSK) and nephrocalcinosis (NC) are radiographically characterized by the formation of innumerable intrarenal concretions. Accurate determination of disease progression is challenging given the large and typically sporadic calcific burden, limiting the utility of traditional manual linear measurements on non-contrast computerized tomography (NCCT). Progression of stone size or quantity is an important predictor of morbidity in recurrent stone formers. As such, among these patients, we sought to test the accuracy of serial NCCT reports indicating a "stable/unchanged" stone burden against actual stone volume measurements using 3-dimensional Slicer®. <b><i>Materials and Methods:</i></b> 20 MSK/NC patients with a "stable/unchanged" stone burden report between January 2018 and September 2023 were retrospectively enrolled in this study. A second group of 50 patients with a single stone was also evaluated during the same time period. Volumetric stone burden measurements were completed by a physician reviewer with over 200 hours of experience with 3D Slicer®. The reviewer was blinded to the dates of the NCCT. Based on volume measurements, we determined the failure to detect a 15%, 25%, or 50% increase in stone volume. A two-tailed chi-square test was used to compare the two patient groups. <b><i>Results:</i></b> The mean time between paired NCCTs was 15.23 months ±8.58. In 49% of MSK patients, the "stable/unchanged" stone burden report was incorrect. Indeed, among these patients, there was a median stone growth of 297 mm³ (IQR: 128.88-745.55 mm³). An increase in stone burden of 15%, 25%, or 50% was missed in 49%, 39%, and 20% of cases, respectively. In contrast, for the single-stone patients the rate of "stable/unchanged" stone burden misclassification was significantly lower, with an increase in stone burden of 15%, 25%, or 50% being missed in 24%, 9%, and 7% of cases, respectively. <b><i>Conclusion:</i></b> NCCT volumetric progression of nephrolithiasis at a mean time of 15 months among MSK patients was misread as "stable/unchanged" stone disease in almost half of our patients.