Perirenal Dedifferentiated Liposarcoma Excised as a Multiloculated Kidney Mass by Imaging

Liposarcomas involving the kidney are rare, with most appearing as secondary involvement from large pararenal liposarcomas. A 77-year-old male patient presented with acute flank pain. CT scans demonstrated a large, thick-walled cystic lesion with rim calcifications "arising from the right inferior kidney." MRI revealed a 16.3 cm complex cystic mass with a pushing border against the uninvolved right lower renal pole. A primary cystic renal cell neoplasm was favored based on radiological findings. The patient underwent a right radical nephrectomy and adrenalectomy. Gross examination revealed a 15 cm well-circumscribed cystic mass abutting the renal capsule with a friable cut surface and extensive hemorrhage and necrosis. Microscopically, the tumor originated in the perirenal adipose tissue within renal fascia without direct infiltration into the renal parenchyma. The viable tumor accounted for approximately 10% of the mass. The tumor was surrounded by a compressed fibrous pseudocapsule. The viable tumor demonstrated a transition from a well-differentiated liposarcoma (15%) to a high-grade dedifferentiated liposarcoma (DDLPS; 85%). The dedifferentiated component was characterized by sheets of large pleomorphic tumor cells arranged in a haphazard pattern, with focal lipogenic differentiation. In contrast, the well-differentiated component consisted of adipocytic proliferation with atypical stromal cells, along with variation in adipocyte size. Immunohistochemical staining revealed diffuse nuclear positivity for MDM2 and CDK4 in both components. Fluorescence in situ hybridization confirmed amplification of the <i>MDM2</i> gene. The patient was in good health and free of recurrence at the 6-month follow-up. We report a perirenal DDLPS and discuss the associated diagnostic challenges.