Introduction: Oncocytomas account for 3–7% of all solid renal masses. Renal mass biopsy (RMB) can accurately diagnose oncocytomas and facilitate active surveillance (AS) of these lesions. We sought to characterize the natural history and intermediate-term outcomes for oncocytomas undergoing AS at our institution.

Methods: This was a single-center, retrospective study of a prospectively maintained database of AS patients with biopsy-proven oncocytomas undergoing serial imaging and followup between 2004 and 2021. Time on AS was calculated at the first followup appointment after RMB. We extracted demographic and clinical information to assess conversion to active treatment (surgical excision or radiofrequency ablation [RFA]), metastasis, and survival. 

Results: We included 123 patients with a median age at diagnosis of 66.7(56.9–74.4) years and tumor size of 2.9 (1.8–3.4) cm. The median time on AS was 4.5 (2.8–8.0) years. Conversion to active treatment occurred in 22 patients (18%), with 18 undergoing nephrectomy and four undergoing RFA. Reasons for treatment included provider concern regarding growth rate (n=12, 55%), worrisome features of renal cell carcinoma (RCC) on followup (n=7, 32%), and patient preference (n=3, 14%). Among operated patients, the overall concordance rate from original RMB to nephrectomy pathology was 79% (14/18). In the four discordant patients, final surgical pathology revealed: 1) eosinophilic RCC (pT1a); 2) succinate-dehydrogenase-deficient RCC (pT1b); 3) mixed oncocytic, chromophore, and eosinophilic RCC (pT3a); and 4) unclassified RCC (pT3a). No patients had evidence of metastasis or disease-specific mortality in followup for AS or after treatment.

Conclusions: AS for biopsy-proven renal oncocytomas is oncologically safe in the intermediate-term. The majority of patients in our cohort remained on AS for years, with approximately 18% undergoing treatment. Of these, only four patients had discordant final surgical pathology. No metastases or disease-specific mortality were observed.