Renal oncocytoma growth rates before intervention

Study Type - Prognosis (case series) Level of Evidence 4 What's known on the subject? and What does the study add? Growth of small renal masses has been shown to be variable, with mean growth rates ranging from 1 to 5 mm/year in several series. Small numbers of oncocytomas in these series show similar but variable growth rates of 0.5-5.2 cm/year with no statistically significant difference compared with RCCs. This study gathered growth rate data from the largest pool (n= 33) of pathologically proven oncocytomas to date, using current imaging techniques. Primarily, it confirms the variable growth of renal oncocytomas over an extended period of time with mean (sd) growth of 2.9 (2.6) mm/year over a mean of 36 months. Observed oncocytoma growth was equivalent to that previously established for RCCs, and small renal masses generally, under surveillance. Secondarily, we found considerable similarity in tumour measurements between observers, suggesting that even low growth rates may be reproducibly documented. OBJECTIVE To describe the growth rates of oncocytomas before treatment with surgical resection or percutaneous ablation. PATIENTS AND METHODS This single-institution retrospective study included 33 consecutive, pathologically proven renal oncocytomas with serial contrast-enhanced computed tomography scans spanning at least 1 year before intervention, from 2000 to 2009. Tumours were measured by two radiologists, and growth rates and interobserver variability were calculated. The mean (range) pre-procedural imaging surveillance period was 36 (12-124) months (median 33 months). RESULTS The mean (sd) oncocytoma size was 17 (11) mm (range 4-47 mm, median 15 mm) in maximum transverse diameter on initial imaging and grew to a mean (sd) of 26 (5) mm (range 10-83 mm, median 23 mm) by the time of treatment. Overall, the mean (sd) and median growth rates were 2.9 (2.6) mm/year and 2.7 mm/year, respectively (range -1.2-10.9 mm/year). After weighting by the se of each tumour's growth rate, the mean (sd) change was 2.1 (1.2) mm/year. The mean (range) interobserver variability for each tumour measurement was 1 (0-7) mm with an intraclass correlation coefficient of 0.99. CONCLUSIONS Renal oncocytomas grow at a rate similar to reported growth rates of renal cell carcinoma. As the observation of growth does not distinguish between benign and malignant renal tumours, growth of small renal masses under active surveillance should be carefully considered before a switch is made to intervention.