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.
