Slice thickness matters: CT-based evaluation of stone-free status with grade classification after RIRS.

PURPOSE: This study aimed to evaluate the influence of computed tomography (CT) slice thickness on stone-free status (SFS) assessment following retrograde intrarenal surgery (RIRS). METHODS: This retrospective single-center cohort study included 145 patients who underwent RIRS with MOSES technology between October 2022 and March 2023. Postoperative SFS was assessed using low-dose CT images reconstructed at 1-mm and 5-mm axial slice thicknesses. SFS was defined as follows: Grade A, no stones visible; Grade B, residual stone ≤ 2 mm; Grade C, residual stone 2.1-4 mm; and Grade D, residual stone > 4 mm. SFS grade distributions were compared using the McNemar-Bowker test. Residual stones were evaluated according to maximum stone diameter (MSD) and maximum attenuation value. RESULTS: SFS grade distributions differed significantly between the two slice thicknesses (p < 0.001), with 5-mm images shifting patients toward more favorable SFS categories. Grade A increased from 42.8% (62/145) to 57.2% (83/145). Favorable SFS (Grades A-B) increased from 83.4% (121/145) to 93.8% (136/145), an absolute increase of 10.3% points (95% CI 5.4-15.3). Overall, 35 patients (24.1%; 95% CI 17.9-31.7%) were reclassified, with no reclassification to a worse grade. MSD and attenuation values were significantly lower on 5-mm images, indicating undermeasurement of residual stone size and attenuation values with thicker-slice reconstructions. CONCLUSION: CT reconstruction slice thickness significantly affected SFS assessment after RIRS. Thicker reconstructions may artifactually increase the apparent stone-free rate by underdetecting or undermeasuring small residual stones. When evaluating SFS with CT, the imaging protocol, including slice thickness, should be clearly reported.