Outcomes of Cystic Versus Solid Renal Masses After Surgery: A Prospective Single-centre Comparative Cohort Study

Background and objective: Cystic renal masses pose unique diagnostic and management challenges. The aim of this study is to compare the clinical characteristics, perioperative outcomes, histopathological features, and midterm oncological outcomes of patients with cystic versus solid renal masses treated with surgery at a referral academic centre. Methods: We prospectively collected data from patients with cT1-T2 renal masses who underwent partial or radical nephrectomy from January 2017 to May 2024. Cystic masses were classified according to the Bosniak 2019 classification. Kaplan-Meier curves were used to compare disease-free survival (DFS) among patients with malignant cystic versus solid tumours. Key findings and limitations: We included 993 patients (13% with cystic masses). Preoperative characteristics were similar between patients with solid and cystic masses, except for a larger median tumour diameter in the cystic group. Intra- and postoperative outcomes were also comparable between the study groups. The proportion of malignant tumours was 86% in cystic and 74% in solid masses (p < 0.001). While the histological spectrum of cystic renal masses was heterogeneous, Bosniak IV renal masses harboured a higher proportion of malignant tumours than Bosniak IIF-III renal masses (96% vs 74%, p < 0.001). At a multivariable analysis, independent predictors of malignancy were tumour size, smoking status, and tumour nature (Bosniak IV vs solid). At a median follow-up of 27 mo (interquartile range 12-48), the estimated DFS rates in patients with malignant tumours were 97%, 92%, and 89% at 12, 36, and 60 mo, respectively, with no difference between the groups. In a sensitivity analysis, DFS was not significantly different between patients with Bosniak IV and IIF-III malignant cystic masses. Conclusions and clinical implications: A non-negligible proportion of patients with surgically treated localised renal masses, especially if solid or Bosniak III cystic, harboured a benign tumour, warranting better preoperative tumour characterisation to provide value-based care. Midterm oncological outcomes were similar between malignant solid and cystic renal masses, confirming the need for risk-based postoperative follow-up regardless of tumour nature. Patient summary: We studied a large group of patients who underwent surgery for kidney masses, comparing those that were mainly filled with fluid (cystic) to those that were solid. Although cystic kidney masses were more often cancerous, the chances of remaining cancer-free after surgery were similar for both types. The cystic or solid nature of the mass alone did not determine whether it was malignant. Disease-free survival was similar between cystic and solid tumours, and DFS was not significantly different between patients with Bosniak IV and IIF-III malignant cystic masses. Our findings support an individualized approach to the management of cystic renal masses, with the goal of avoiding unnecessary surgery while still effectively addressing cancer risk.