Encephalic fibrosarcoma in a dog

Intracranial sarcomas are rarely reported in dogs. A 9-y-old, castrated male mixed-breed dog was evaluated and euthanized because of a 1-wk history of lethargy, dullness, disorientation, right-sided head tilt, and circling to the right. Grossly, a pale-tan, firm, 1.5-cm mass effaced ~60% of the right-ventral aspect of the pons. Histologically, the mass was composed of elongate neoplastic cells with abundant cytoplasm and round-to-oval nuclei arranged in bundles and supported by collagenous stroma. Anisocytosis and anisokaryosis were moderate, with 11 mitoses in 2.37 mm² (10 FN22/40× fields). The neoplasm was well-demarcated with occasional areas of infiltration in the surrounding neuroparenchyma. Neoplastic cells had widespread cytoplasmic immunolabeling for vimentin, patchy cytoplasmic immunolabeling for myoglobin, weak cytoplasmic immunolabeling for desmin, and rare cytoplasmic immunolabeling for myogenin. Transmission electron microscopy (TEM) revealed bundles of oval-to-elongate neoplastic cells with small-to-medium cytoplasmic aggregates of rough endoplasmic reticulum cisterns. The centrally-to-peripherally located nuclei were round and had one or more compact-to-reticulate nucleoli. Collagen bundles were in intimate contact with the plasma membrane of neoplastic cells and formed the abundant extracellular matrix. Histologic, immunohistochemical, and ultrastructural findings were consistent with an encephalic fibrosarcoma. The myogenic differentiation suggested by immunohistochemistry could not be confirmed by TEM.