What is the significance of the Modified Seoul Renal stone complexity score in evaluating renal stones and predicting surgical outcomes for Retrograde Intrarenal Surgery (RIRS)?

Modified Seoul Renal Stone Complexity Score (S-ReSC)

Overview and Purpose

The Modified Seoul National University Renal Stone Complexity (S-ReSC) score is a validated nephrolithometric scoring system specifically designed to predict stone-free rates after Retrograde Intrarenal Surgery (RIRS), with scores ranging from 1-12 based on stone location and number of sites involved. 1, 2

Scoring System Components

The Modified S-ReSC assigns points according to the anatomical distribution of stones within the kidney 2:

• Renal pelvis (#1): 1 point per site involved 2
• Superior and inferior major calyceal groups (#2-3): 1 point per site 2
• Anterior and posterior minor calyceal groups: Divided into superior (#4-5), middle (#6-7), and inferior calyx (#8-9) locations, each receiving 1 point per site 2
• Additional weighting for inferior locations: If stones occupy inferior sites (#3, #8-9), one additional point per site is added to the original score 2

The total score ranges from 1 to 12, with higher scores indicating greater stone complexity 1, 2.

Risk Stratification Categories

The Modified S-ReSC stratifies patients into three distinct prognostic groups 1:

• Low-score group (1-2 points): Stone-free rate of 86.7-94.2% 1, 2
• Intermediate-score group (3-4 points): Stone-free rate of 70.2-84.0% 1, 2
• High-score group (5-12 points): Stone-free rate of 45.5-48.6% 1, 2

These categories demonstrate statistically significant differences in outcomes (p<0.001) 1, 2.

Predictive Performance and Validation

The Modified S-ReSC demonstrates superior predictive accuracy compared to other nephrolithometric scoring systems for RIRS outcomes. 3, 2, 4

Comparative Performance Metrics

• Area under the curve (AUC) for S-ReSC: 0.755-0.869, indicating excellent discriminative ability 3, 5
• Comparison to RUSS (Resorlu-Unsal Stone Score): S-ReSC shows higher AUC (0.806 vs 0.692, p=0.012) 2
• Comparison to S.T.O.N.E. score: S-ReSC demonstrates comparable or superior performance (AUC 0.755 vs 0.725) 3
• Comparison to R.I.R.S. score: Both show similar predictive accuracy (AUC 0.755 vs 0.752) 3

Reliability and Validity

The Modified S-ReSC has been externally validated with robust reliability metrics 1:

• Interobserver reliability: Almost perfect level of agreement between different evaluators 1
• Test-retest reliability: Consistent scoring when repeated by the same evaluator 1
• Calibration: Predicted probability of stone-free rate shows concordance with observed frequency 1

Clinical Application Algorithm

When to Use S-ReSC vs Other Scoring Systems

For single renal stones, S-ReSC should be the preferred scoring system; for multiple stones, RUSS may be more appropriate. 3

• S-ReSC is optimal for: Single stone assessment, lower caliceal stone evaluation, and when precise anatomical localization is available 3, 2
• S-ReSC performs well for: Stones in any location, including lower pole stones where the additional weighting factor improves prediction 2
• Alternative considerations: RUSS may be preferred when assessing multiple stones across different locations, as it was identified as an independent predictive factor in multivariate analysis 3

Preoperative Counseling Based on Score

Surgeons should use the Modified S-ReSC to set realistic expectations 1, 2:

• Scores 1-2: Inform patients of >85% likelihood of complete stone clearance in single procedure 1, 2
• Scores 3-4: Counsel patients about 70-84% success rate; may require staged procedures 1, 2
• Scores ≥5: Discuss <50% stone-free rate with RIRS; consider alternative approaches including PCNL for stones >20mm per AUA guidelines 6, 7, 1, 2

Integration with Treatment Guidelines

Alignment with AUA/Endourological Society Guidelines

The Modified S-ReSC complements guideline-based treatment selection 6, 7:

• For stones >20mm with high S-ReSC scores: PCNL should be offered as first-line therapy rather than RIRS, as PCNL achieves 94% stone-free rates vs 75% for URS 6, 7
• For lower pole stones >10mm with high S-ReSC scores: Avoid SWL (58% success rate); consider URS (81%) or PCNL (87%) based on complexity 6, 8
• For stones ≤10mm with low S-ReSC scores: Either SWL or URS are acceptable first-line options 6, 8

Special Considerations for Lower Pole Stones

The Modified S-ReSC's additional weighting for inferior locations (#3, #8-9) specifically addresses the challenge of lower pole stones 2:

• Lower pole anatomy matters: Infundibulopelvic angle (IPA) <44.5° and infundibulopelvic length (RIL) >24.5mm predict higher residual stone rates 4
• S-ReSC accounts for this: The extra point assigned to inferior locations reflects the reduced stone clearance from dependent calyces 2
• Clinical implication: High S-ReSC scores in lower pole locations should prompt consideration of PCNL over RIRS, particularly for stones >10mm 6, 8, 4

Limitations and Caveats

What S-ReSC Does Not Predict

The Modified S-ReSC accurately predicts stone-free rates but does not reliably predict postoperative complications. 5

• Complication prediction: None of the nephrolithometric scoring systems (including S-ReSC) effectively predict complications after stone surgery 5
• Patient factors not included: Body habitus, skin-to-stone distance, stone density (Hounsfield units), and stone composition are not incorporated into S-ReSC 3, 2
• Operator experience: The score does not account for surgeon expertise, which significantly impacts RIRS outcomes 3, 1

When to Consider Alternative Scoring Systems

• For PCNL planning: Original S-ReSC (not modified version) was designed and validated for percutaneous nephrolithotomy 2, 5
• For stones >20mm: R.I.R.S. scoring system showed superiority in multivariate analysis for large stones undergoing RIRS 4
• For comprehensive assessment: S.T.O.N.E. score includes stone density and obstruction, which may be relevant for treatment planning 3

Practical Implementation

Required Preoperative Imaging

To calculate Modified S-ReSC accurately 1, 2:

• Non-contrast CT scan: Provides precise anatomical localization of stones across all nine renal sites 1, 2
• Identify each involved site: Count renal pelvis, major calyces (superior/inferior), and minor calyces (anterior/posterior in superior/middle/inferior groups) 2
• Apply additional points: Add one point for each inferior location (#3, #8-9) beyond the base score 2

Documentation and Communication

• Record the numerical score: Document the total Modified S-ReSC score (1-12) in the operative plan 1, 2
• Specify risk category: Note whether patient falls into low (1-2), intermediate (3-4), or high (≥5) complexity group 1
• Discuss implications: Use the predicted stone-free rate to inform consent discussions and set expectations 1, 2