Beyond renal cell carcinoma: rare and unusual renal masses.

The vast majority of primary renal masses represent clear cell or papillary renal cell carcinomas, angiomyolipomas, or transitional cell carcinomas. However, a number of more rare masses can also be encountered, many of which can be very difficult to differentiate from these more common entities based on their imaging features. These uncommon entities include metanephric adenoma, epithelioid angiomyolipoma, medullary renal cell carcinomas, multilocular cystic nephroma, hemangiopericytoma, hemangioma, leiomyoma, leiomyosarcoma, solitary fibrous tumor, renal plasmacytomas, and mixed epithelial and stromal tumors. In some cases, certain clinical and imaging features can allow one of these unusual entities to be placed in the differential diagnosis, including patient age, degree of tumor enhancement, presence of underlying sickle cell trait or sickle cell disease, the presence of a cystic component to the tumor, and tumor morphology. Even if a radiologist is unable to make a specific diagnosis, knowledge of these entities is important, as it allows radiologists to guide post-surgical follow-up, as well to understand the most common sites of metastatic disease.