Case 180126-2

Three-year-old, male castrated English bulldog.

This animal was presented two months earlier with ventral strabismus and swelling in the dorsal conjunctival region of the left eye causing elevation of the superior palpebra. No pain was noted upon opening of the mouth or on attempted retropulsion of the globe. The veterinarian recommended biopsy, but it was declined at that time. The dog was sent treated with prednisone and clavamox, but clinical signs worsened two months later. At that time the owners elected for enucleation of the globe and the globe.

A 2.5 x 3 x 2.5 cm, multinodular, firm, pale tan mass is expanding and replacing the retrobulbar connective tissue and is compressing the adjacent ocular globe.

Three sections of a large, partially encapsulated, densely cellular mass surround and infiltrate myocytes and nerves. The mass is composed of dense sheets of round to occasionally spindle cells within a fine fibrovascular stroma. Cells have distinct cell borders, a scant amount of eosinophilic cytoplasm, and an oval to irregularly shaped nucleus with finely stippled chromatin and one to three basophilic nucleoli. Anisocytosis and anisokaryosis are marked. Thirty-five mitoses are observed in ten 400x fields. A few, large, multinucleated cells are scattered throughout the tumor, some of which have brightly eosinophilic cytoplasm. Single-cell necrosis is common and areas of hemorrhage are scattered throughout.

Immunohistochemistry for desmin: Neoplastic cells exhibit variably strong cytoplasmic and membranous immunoreactivity.

Retrobulbar mass: Rhabdomyosarcoma.

Rhabdomyosarcomas are relatively rare neoplasms in domestic species. They are classified as embryonal, botryoid, alveolar, or pleomorphic. The significance of the subtype in domestic species has not been fully established; however, a recent review article compares a collection of 62 cases. In this article it mentions that most rhabdomyosarcomas occur in dogs less than two years of age (63%) and most are of the botryoid subtype (43%). The most common location for them to occur is in the urogenital tract or viscerally (49%). One case included was retrobulbar.

Cases which most commonly metastasized were those classified as “not otherwise specified”, indicating that they did not fall into one of the specific histologic subtype categories. In this case the neoplastic cells exhibit features of the alveolar subtype in some regions with the presence of large multinucleate cells, and vague organization into an “alveolar” pattern. The mitotic count was high, indicating a more aggressive nature of the neoplasm.

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