Answer: Dedifferentiated liposarcoma

Histology: Encasing the kidney is a spindle cell tumor which in areas is composed of fibrous appearing cells that are not specifically arranged in long sweeping fascicles. These cells have moderate cellularity along with definite moderate nuclear pleomorphism. Focal necrosis is identified along with scattered mitotic figures. In the adjacent adipose tissue within bands of fibrosis associated with a lymphocytic infiltrate are large hyperchromatic nuclei with smudgy chromatin.

Discussion: This case was sent in as a possible angiomyolipoma. Angiomyolipomas may involve the retroperitoneum adjacent to the kidney yet do not have this morphology. The non adipose component of an angiomyolipoma more typically resembles smooth muscle or in some cases may be composed of epithelioid cells resembling renal cell carcinoma. A fibrous appearance is not one of the patterns of angiomyolipoma. Fibromatosis is characterized by long fascicles cut in different planes of section of relatively hypocellular fibrous tissue. In addition to the different growth pattern of fibromatosis, the current case shows a much greater degree of nuclear pleomorphism than seen within fibromatosis. Similarly, leiomyosarcoma would be composed of long interlacing fascicles of smooth muscle cells which is lacking in the current case. The key to the correct diagnosis in this case is recognizing that the adipose tissue adjacent to the more cellular fibrous appearing component is not normal adipose tissue but rather integral to the lesion. Within the adipose tissue, the bands of fibrous tissue contain large hyperchromatic smudgy nuclei which are typical of well differentiated (lipoma-like) liposarcoma. Following the recognition of well differentiated liposarcoma, the more cellular fibrous appearing areas with more overtly malignant features then becomes recognizable as dedifferentiated liposarcoma. Most cases of dedifferentiated liposarcoma are Grade 3 (high grade) as in the current case. There are rarer cases of low grade dedifferentiated liposarcoma resembling fibromatosis such that one should specify when diagnosing dedifferentiated liposarcoma the grade of the tumor. Lesions occurring within the retroperitoneum often experience local recurrence with distant metastases seen in approximately 15 to 20% of cases. Relative to other high grade sarcomas, dedifferentiated liposarcomas tend to have a relatively better prognosis.