Answer: C

Histology: This is a highly cellular spindle cell neoplasm. These spindle cells form short fascicles with pink cytoplasm, and they have angulated nuclei. Focally there is mitotic activity as well as cellular necrosis. However, while these features suggest a malignancy, several features suggests that this represents cellular schwannoma. First, the vessels within the lesion have thick walls and are hyalinized. Second, there is admixed chronic inflammation, and focal foam cells. Third, while the lesion is highly cellular, there is minimal cytologic atypia, and mitoses while present are not brisk. Fourth, the tumor cell nuclei are angulated which suggests schwann cells. Fifth, the lesion appears encapsulated, unlike a typical sarcoma. The lesion is diffusely immunoreactive for S100 protein, which supports the diagnosis of cellular schwannoma.

Discussion: A malignant peripheral nerve sheath tumor would not demonstrate the characteristic vasculature of cellular schwannoma, and would not typically be diffusely immunoreactive for S100 protein. Leiomyosarcoma would have more rectangular appearing nuclei, and more abundant pink cytoplasm. Leiomyosarcoma would label for muscular markers such as desmin and actin, but not for S100 diffusely. Gastrointestinal stromal tumor may occur outside of the tubular GI tract, but the paraspinal area would be an unusual location. These lesions would not be diffusely immunoreactive for S100, but rather would typically label for CD 117.
Cellular schwannoma is a well-recognized mimic of sarcoma, and should be considered before rendering a diagnosis of sarcoma in the paraspinal region.