The retroperitoneal space is derived from the mesoderm. Mesodermal derived tissue, muscle, bone, cartilage, fat, vessels and connective tissue can give rise to a great number of benign and malignant neoplasm’s.

The three most common malignant tumors found in the retroperitoneal space are lymphomas, liposarcomas and leiomyosarcomas, but only the latter two are mesenchymal neoplasms. Many neoplasms can present with spindle cells. IHC, cytogenetics, and FISH aid greatly in the differential.
Leiomyosarcomas are malignant smooth muscle tumors with a fascicular growth pattern. The spindle cells have nuclei with tapered or blunt ends, the degree of nuclear pleomorphism depends on grade. Multinucleation, mitosis and the degree of necrosis also depends on grade. They can be differentiated from benign spindle cell neoplasms by their more frequent mitosis (greater than 5 per 50 HPF), the presence of some necrosis and at least some nuclear irregularities. Typically, leiomyosarcomas will stain for desmin and smooth muscle actin and are negative for CD117, S100, CD34.
Other fascicular patterned tumors, like peripheral nerve sheath tumors, the fibroblastic/ myofibroblastic tumors, the vascular tumors, when malignant can have spindle cells with some atypia. Therefore, choosing a basic immunohistochemical panel that differentiates between tissue types is the next step after an examination of the cytology and histology. A common panel used to differentiate soft tissue neoplasms include: CD34 for vascular tumors and GIST. S100 for peripheral nerve sheath tumors (and melanoma), desmin and smooth muscle actin for smooth muscle tumors. In our case DOG (discovered on gist) 1 and CD 117 were included since GIST is in the differential for retroperitoneal spindle cell tumors. A keratin marker can be useful when synovial sarcoma is in the differential.