UNUSUAL FAT-CONTAINING TUMORS OF THE KIDNEY - A DIAGNOSTIC DILEMMA

Unusual fat-containing renal tumors in a series of 27 cases comprised five categories: atypical and complicated angiomyolipomas (AMLs) (n = 15), including AMLs with extrarenal growth (n = 5), AMLs with undetect able fat (n = 4), and hemorrhagic AMLs (n = 6); fat-containing renal c ell carcinomas (RCCs) (n = 9), lipoma (n = 1); liposarcoma (n = 1); an d fat-containing renal oncocytoma (n = 1), Fat was present within RCCs by the following mechanisms: lipid-producing necrosis within a large RCC (n = 2), intratumoral bone metaplasia with fatty marrow elements a nd calcification within a small RCC (n = 2), and entrapment of periren al (n = 4) or sinus (n = 1) fat by large irregular RCCs. Fat-containin g RCC must be considered in cases of fat-containing renal tumors, even though the presence of intratumoral fat is characteristic of AML, A d edicated computed tomography scanning protocol and strict diagnostic c riteria are mandatory for accurate diagnosis, Malignancy should be sus pected on the basis of the following criteria: presence of intratumora l calcifications; large, irregular tumor invading the perirenal or sin us fat; large necrotic tumor with small foci of fat; and association w ith nonfatty lymph nodes or venous invasion.