Diagnosis: D. Leiomyosarcoma

Histology: Sections reveal intersecting fascicles of spindle cells with blunt-ended (cigar shaped) nuclei, scattered enlarged and hyperchromatic nuclei, indistinct cell borders, pink cytoplasm, and brisk mitotic activity. The neoplasm closely approximates a large vein and appears to either arise from it or invade into it.

Discussion: This tumor is a retroperitoneal leiomyosarcoma arising from the renal vein. Retroperitoneal leiomyosarcomas in females are usually gynecologic in origin (e.g., uterine), but leiomyosarcomas can arise from the smooth muscle wall of blood vessels. Leiomyosarcomas label for markers of smooth muscle differentiation, including smooth muscle actin and desmin. The differential diagnosis of a spindle cell neoplasm of the retroperitoneum should always include dedifferentiated liposarcoma, as liposarcomas are the most common retroperitoneal sarcoma. It may not be possible to definitively classify a spindle cell neoplasm on core biopsy if the diagnostic features are not present due to partial sampling. Features that would favor a dedifferentiated liposarcoma include the presence of a well-differentiated component (which may be very focal) and MDM2 amplification (although this can be seen in other sarcomas). Features that would favor a leiomyosarcoma include absence of a well-differentiated liposarcoma, well developed histology of fascicular architecture and uniform blunt-ended nuclei, and association with a specific site of origin such as a large vessel or the uterus.