A 5-wk-old, 10.6-kg, intact female Leonberger dog was presented for evaluation of a mass on the left ventrolateral thorax that had been present since birth. A biopsy of the mass revealed an invasive, unencapsulated spindle-cell population arranged in bundles and concentric whorls (pseudo-onion bulb formations) with multifocal melanocytic differentiation. Neoplastic cells in pseudo-onion bulbs immunolabeled strongly for glial acidic fibrillary protein and PGP9.5 and moderately for S100 and Sox10. The supporting matrix had strong immunolabeling for laminin. Cells had multifocal immunolabeling for NeuN, melan A, and PNL2. Collectively, these histopathologic characteristics support a diagnosis of congenital nerve sheath tumor, which is rarely described in dogs.
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