Dear Editor,

We have read with great interest the article entitled ‘Modified Seoul National University Renal Stone Complexity score for retrograde intrarenal surgery’ by Jung et al. [1]. The authors analyzed 88 patients who underwent flexible ureterorenoscopy (f-URS) and developed a new nomogram called the modified Seoul National University Renal Stone Complexity (S-ReSC) score to predict stone-free (SF) after fURS. The Modified S-ReSC score is only based on the number of sites of renal stones involved, regardless of the size and number of the stones.

In the study, the authors claimed that ‘stone size and stone number were not hindrance to fURS except when the stone was large’. Also they stated that ‘too large stones were excluded and PNL was performed to these patients’ in the article. But the upper limit of ‘large’ or ‘too large’ stones had not been clearly stated. For example, can we reliably apply this nomogram for the stone of 40 mm? If yes, the total score will be same with 10 mm stone which is located at the same calyx.

Success rate after fURS is inversely proportional to stone size [2]. In addition, staged procedures are required for big stones. For this reasons, fURS can not be recommended as first-line treatment for stones >20 mm [3].

In conclusion, stone size remains one of the primary limiting factors of fURS and is a predictive factor on success after fURS.