Answer: C. Dedifferentiated liposarcoma

Sections show a low-grade spindle cell tumor with admixed collagen. Scattered mitotic activity including atypical forms and focal necrosis is noted. The differential diagnosis is broad, and includes sarcomatoid carcinoma (urothelial or renal cell carcinoma) and various sarcomas. Lack of primary carcinoma elsewhere and the absence of an epithelial component argue against sarcomatoid carcinoma. Initially this tumor was interpreted as a renal mass and favored to represent renal cell carcinoma by imaging but dedifferentiated liposarcoma can abut the kidney and mislead radiologists. In this case, no stains were required because other sections showed well-developed areas of well-differentiated liposarcoma. Atypical mitotic figures would be unusual for neoplasms with simple karyotypes such as fibromatosis and low-grade fibromyxoid sarcoma, with negativity for MUC4 essentially excluding the latter. Keratin and PAX8 expression would support sarcomatoid renal cell carcinoma, but identification of a well-differentiated liposarcoma component is diagnostic of dedifferentiated liposarcoma in this case. The morphologic spectrum of dedifferentiated liposarcoma is immense and should be considered for any low or high-grade mesenchymal neoplasm, particularly in visceral sites. Identification of well-differentiated liposarcoma or MDM2 amplification or overexpression are confirmatory.