Simple renal cysts are a common benign renal condition. While typically asymptomatic, their progressive growth can lead to compression of adjacent renal parenchyma, resulting in localized atrophy and functional impairment. The formation of a "crater" at the base of the cyst, where parenchymal loss is most pronounced, is a potential contributor to decreased glomerular filtration. This study aimed to evaluate the impact of simple renal cysts on renal function, explore the relationship between cyst size, volume of atrophied parenchyma, and glomerular filtration rate (GFR), and propose objective criteria for surgical intervention.

We conducted a prospective cohort study involving 109 patients with solitary simple renal cysts (Bosniak category I–II) located in either kidney. All patients underwent contrast-enhanced computed tomography (CT) of the urinary system to assess total and lost renal parenchyma volume. Renal scintigraphy was performed to obtain split renal function. The estimated GFR (eGFR) was calculated using multiple standard formulas. The relationship between cyst characteristics and renal function was assessed via correlation and multivariate logistic regression. Ethical approval was obtained prior to patient enrollment.

The median maximum cyst diameter was 80 mm. The median total volume of the affected kidney parenchyma was 174 ml, with a median atrophied volume of 49 ml, accounting for a 28% parenchymal loss. The median total GFR was 77.07 ml/min, with the affected kidney contributing a median of 34.46 ml/min, compared to 45.49 ml/min from the contralateral kidney. The median inter-kidney GFR difference was 11 ml/min (statistically significant). Among eGFR formulas tested, the Cockcroft–Gault equation (adjusted for body surface area) showed the closest correlation with scintigraphy results. A significant positive correlation was found between cyst size and proportion of lost parenchyma (ρ = 0.37, 95% CI [0.20; 0.52], p < 0.001). Multivariate regression identified the percentage of parenchyma loss as a significant predictor of reduced GFR (OR = 1.13; p < 0.001).

This study demonstrates that increasing size of simple renal cysts is associated with progressive parenchymal atrophy and a corresponding decline in renal function. The volume of lost parenchyma is significantly correlated with decreased GFR in the affected kidney. Based on our findings, the presence of ≥20% parenchymal loss may be a reasonable threshold for performing renal scintigraphy and considering surgical treatment in otherwise asymptomatic patients.