Impact of Active Surveillance on Nephrometric Scores of Bosniak III and IV Renal Cysts.

Purpose: Active surveillance (AS) has been widely explored for small renal masses, but its role in complex renal cysts (CRCs) categorized as Bosniak (Bk) III and IV remains unclear. This study aims to evaluate changes in nephrometry scores (s-RENAL and s-PADUA) during AS and their potential impact on surgical complexity. Methods: A retrospective, multicentric, observational study was conducted, analyzing 101 CRCs from 95 patients across three institutions. Nephrometry scores were evaluated at baseline and after a minimum follow-up of two years using computed tomography (CT) and magnetic resonance imaging (MRI). Statistical analysis included chi-square and marginal homogeneity tests, with data managed using REDCap. Results: Among the 101 CRCs, 39 were Bk IV and 62 Bk III, with a mean follow-up of 4.2 years. The initial mean lesion size was 2.77 cm (0.50-11.8 cm), increasing to 3.0 cm (0.40-13.10 cm) at follow-up. Complexity scores increased in 4% of cases according to s-RENAL and 5% according to s-PADUA. Tumor growth was observed in 45 cases (44.5%), while 37 (36.6%) remained stable, and 19 (18.8%) decreased in size. Bk IV cysts tended to increase in size more than Bk III and had a smaller chance of decreasing. Median growth and shrinkage velocities were 0.19 cm/year and 0.21 cm/year, respectively. Conclusion: Active surveillance is a viable and safe strategy for managing select Bk III and IV CRCs. A small but significant proportion of CRCs under AS exhibit increased nephrometry complexity, which may impact surgical planning. These findings suggest that nephrometry score evolution should be considered when selecting patients for AS versus early surgical intervention.