Case Report: Neuroblastoma-Like Schwannoma in a Domestic Short-Haired Cat

Abstract

An axillary mass was detected in a 6-year-old, neutered, male, domestic short-haired cat during a wellness exam. Gross examination following surgical removal revealed a discrete, deep subcutaneous, discoid mass that was between 0.5- and 0.7-cm-in-diameter and diffusely firm and white. Histologically, the mass was well-demarcated, partially encapsulated, and expanded the panniculus carnosus. It was composed of tightly packed, giant rosettes of radially arranged fusiform cells stacked in one to 10 layers with peripherally palisading nuclei and with centrally oriented, fibrillary, cytoplasmic processes, and collagenous fibers. Laminin immunoreactivity and ultrastructural examination highlighted a continuous basal lamina outside the plasma membrane of each neoplastic cell. Neoplastic cells were immunoreactive for GFAP, S100, periaxin, and Sox-10 and were immunonegative for synaptophysin, smooth muscle actin, and pancytokeratin. Collective findings were consistent with a diagnosis of neuroblastoma-like schwannoma. This is the first veterinary report of this rare variant of benign schwannoma.

Keywords: cat; neurilemoma; neuroblastoma; peripheral nerve sheath tumor; schwannoma.

Figure 1

Neuroblastoma-like schwannoma, cat, subcutaneous mass. The mass is composed of multiple lobules that are separated by variably thick bands of dense connective tissue (arrow). Note the hypercellular peripheral nerve fascicles (asterisk) in the vicinity of the mass, which is partially encapsulated by the epineurium. H&E stain (A). The lobules are composed of multiple, tightly packed, 5–300-μm-in-diameter, fibrillar, giant rosettes that exhibited a characteristic cellular polarity with a peripheralized nucleus and centrally oriented cytoplasm. Note that there is nuclear palisading. H&E stain (B). Each individual giant rosette is characterized by radially-arranged fusiform cells with round to oval, lymphocyte-like nuclei and eosinophilic fibrillary cytoplasm. H&E (C). Giant rosettes contain a centralized collagen core (in blue). Masson's trichrome stain (D). Each fibrillar and eosinophilic cytoplasmic neoplastic process, similar to each Schwann cell in healthy peripheral nerves (asterisk), diffusely and strongly immunoreacts to laminin. Immunohistochemistry for laminin (E). Each neoplastic cell is coated by a well-formed, continuous, 50 nm thick, electron-dense basal lamina investing long and interdigitating cytoplasmic processes. The cytoplasm contains a flattened, often invaginated nucleus, microfibrils and sparse organelles with a few mitochondria, ribosomes, and lysosome-like granules. TEM micrograph (F).

Figure 2

Neuroblastoma-like schwannoma, cat, subcutaneous mass. Immunohistochemistry. Neoplastic Schwann cells cytoplasmic processes diffusely and strongly immunoreact to periaxin (A). One hundred percent of nuclei of neoplastic cells are strongly positive for Sox-10 (inset) (B). Neoplastic Schwann cell cytoplasmic processes immunoreact to S-100 (C). Neoplastic fibrillary cytoplasmic processes are strongly labeled with glial fibrillary acid protein (D). Immunohistochemistry for synaptophysin confirms lack of immunoreactivity of neoplastic cells (E). Approximately 10% of the cells display nuclear ki-67 (MIB-1) immunostaining (F).