Most Renal Oncocytomas Appear to Grow: Observations of Tumor Kinetics With Active Surveillance

PurposeWe examined the growth of tissue proven renal oncocytoma on serial imaging to improve our understanding of its natural history.Materials and MethodsWe reviewed the charts of 69 patients with oncocytoma diagnosed by biopsy or surgery between 2004 and 2010. A total of 29 cases were managed by active surveillance for at least 12 months and had 3 or more imaging events. Tumor size was documented and the average tumor growth rate was calculated using a random coefficient model. Interaction terms were used to investigate correlations between variables of interest, including age at diagnosis, gender, symptom status, laterality, initial tumor size, surveillance duration and number of imaging events.ResultsAt a mean surveillance duration of 40 months 80% of oncocytomas increased in size. Based on the random coefficient model the estimated average growth rate was 0.16 mm monthly (95% CI 0.097–0.228, p <0.0001). We identified no variables that significantly correlated with growth.ConclusionsDespite its low metastatic potential renal oncocytoma appears to progress locally with a growth rate similar to that of RCC. Thus, absent tumor growth on serial imaging is not a robust prognostic factor for benign histology. Biopsy remains the mainstay of diagnosis. At centers where it can be performed safely and accurately, active surveillance of tissue proven oncocytoma appears to be safe in the short term. Alternative management includes partial nephrectomy and minimally invasive approaches. To our knowledge this is the largest study of oncocytoma natural history.