Small renal tumours (< 3 cm) have been regarded in the past as adenomas rather than carcinomas. Unfortunately, the size of a renal mass is not a valid criterion for differentiating a benign from a malignant mass. There are reports of tumours that have produced metastases when less than 3 cm, although this is uncommon. Needle biopsy of a small lesion is not helpful in differentiating benign from malignant tumours, as most solid masses are composed of a heterogeneous population of cells and sampling error is common.
Oncocytomas are tubular adenomas with a specific histological appearance characterized by the oncocyte. They have previously been considered benign, but it is now recognized that they can metastasize. Oncocytomas can occur at any age and are often asymptomatic at presentation. They can vary in size from 1 to 20 cm in diameter, but tend to be large. Although they are usually solitary and unilateral, they can be multiple (5%) and bilateral (3%). Ultrasound demonstrates a solid mass with internal echoes, which occasionally has a stellate hypoechoic centre. However, the echogenicity of the mass can be variable. Contrast-enhanced CT demonstrates a well-defined solid mass (Fig 1) which, when large, can contain a low attenuation central scar. Large lesions can extend into and engulf the perinephric fat, and can be mistaken for angiomyolipomas. There are no features on MRI that will differentiate an oncocytoma from renal carcinoma. Arteriography is also of limited value in discrimination between an oncocytoma and renal cell carcinoma.
Figure 40.12 Oncocytoma. CT demonstrates multiple well-defined enhancing masses in both kidneys which were confirmed by percutaneous biopsy to be oncocytoma. Follow-up examination at 12 months did not demonstrate any growth.