Small renal oncocytomas: differentiation with multiphase CT.

OBJECTIVES: To evaluate characteristic imaging findings of tumor attenuation in multiphase computed tomography (CT) between renal oncocytomas and clear-cell renal cell carcinoma (ccRCC) of small tumor size (≤5 cm).

METHODS: We retrospectively identified 20 patients with complete four-phase CT with either histologically confirmed small renal oncocytoma (N=10) or ccRCC (N=10) who underwent subsequent total or partial nephrectomy. Exclusion criteria for RCC were non-clear-cell components in histology and a tumor diameter>5 cm. The relative attenuation of solid renal lesions and normal renal cortex was determined in the unenhanced, corticomedullary, nephrographic and excretory phase. Statistical comparison was carried out by Wilcoxon Rank Sum Test.

RESULTS: Mean tumor size of renal oncocytomas was 2.8±0.4 cm (1.2-5) and of ccRCC 2.5±0.2 cm (1.7-4.4; p=0.57). All lesions were homogenous without extended areas of necroses. In the nephrographic phase, the difference of attenuation between renal cortex and tumor lesion was highest in both entities (oncocytoma, 48.1±5.2 HU; ccRCC, 67.5±12.1) but not between entities (p=0.30). In the corticomedullary phase, renal oncocytomas showed greater isodensity to the normal renal cortex (13.9±4.3 HU) compared to clear-cell RCC (51.5±5.0 HU; p=0.003). No further significant differences were found for the unenhanced and excretory phase.

CONCLUSIONS: In this study, the maximum tumor-to-kidney contrast coincided with the nephrographic phase which was thus the most reliable for the detection of a renal lesion<5 cm. For lesion characterization, the corticomedullary phase was most useful for differentiating both entities. This finding is particularly important for the preoperative planning of a partial nephrectomy.