[Perinephric liposarcoma mimicking cystic renal tumor].

Liposarcoma is one of the most common primary retroperitoneal neoplasms, and the perinephric region is a frequent location for them. Liposarcomas show a variety of radiographic features in terms of histological types and tumor sizes, so the specific diagnosis of liposarcoma is often difficult. We present a unique case of perinephric dedifferentiated liposarcoma mimicking cystic renal tumor. A 71-year-old man presented himself at our hospital with a palpable mass in his upper right abdomen. Abdominal computerized tomography (CT) revealed a well-defined cystic mass at the lower pole of the right kidney that contained heterogeneous solid components and small foci of fat. There were no signs of lymphadenopathy or tumor thrombus in the renal vein. Metastatic evaluation by chest x-ray and bone scan was negative. The probable diagnosis was cystic renal cell carcinoma or atypical angiomyolipoma. Because we could not exclude the possibility of cystic malignancy, a right radical nephrectomy was performed. Grossly, the tumor was predominantly encapsulated by a unilocular fibrous capsule and was filled with bloody fluid and debris. The anterior portion of the tumor was composed of various-sized soft and rubbery masses covered with necrotic tissue. The histological diagnosis was dedifferentiated liposarcoma arising in the perinephric retroperitoneum with extensive necrosis, and the cyst wall was composed of a necrotic tumor with a well differentiated liposarcoma and a fibrous capsule. Although the tumor widely covered the right kidney, there was no microscopic invasion of the kidney. No signs of tumor recurrence were noted six months after the operation.