Renal cell carcinomas with intratumoral fat and concomitant angiomyolipoma: potential pitfalls in staging and diagnosis.

Intratumoral fat and angiomyolipomas (AMLs) occurring within renal cell carcinomas (RCCs) have rarely been reported but may be mistaken for tumor invasion into perinephric or renal sinus fat or misdiagnosed as tumor exhibiting sarcomatoid differentiation. We report 16 such cases. In 14 RCC cases, there was intratumoral fat, 9 of which had fat located peripherally near the capsule (n = 6), renal sinus (n = 1), or both (n = 2). Inflammatory infiltrates and osseous metaplasia were identified in the intratumoral fat in 7 and 8 cases, respectively. Two cases had intratumoral AML foci located at the periphery of RCC. Intratumoral fat or AML at the periphery of RCC simulated the invasion into the fat, while the smooth muscle component of AML resembled spindle cell, or sarcomatoid, differentiation. Our study highlights the potential pitfalls in staging and diagnosis when intratumoral fat or AML is found within RCC.