Extraskeletal osteosarcoma in a dog

By Emma Scurrell

A 1-year-old male neutered Dalmatian presented with a firm 4x3x3cm subcuticular mass overlying the right chest wall. There was no clinical or radiological association of the mass with the underlying ribs and no other lesions were found. A fine needle aspirate was performed and following the results, the mass was surgically resected and submitted for histopathology.

Fig 1. Cytology of the subcuticular mass reveals large pleomorphic spindeloid to polygonal neoplastic cells (modified Wright’s stain, x 400). The cytological diagnosis was a malignant neoplasm, consistent with a sarcoma.

Fig 2. Some of the cells contain pink intracytoplasmic granules such as in this cell near the centre of the image. These pink granules are not specific to osteosarcoma and can also be seen in chondrosarcomas and less commonly in other sarcomas (modified Wright’s stain, x 400).

Fig 3. Histopathology confirms the presence of a highly cellular infiltrative neoplasm composed of pleomorphic spindeloid to polygonal cells. Separating the neoplastic cells are seams of a bright pink matrix which is typical of osteoid and typical of an osteosarcoma. Mitotic figures can be seen at the 7 o’clock and 9 o’clock position in the image.

Final Diagnosis

Extraskeletal osteosarcoma

Discussion

The presence of a solitary subcuticular neoplasm in the absence of primary bone involvement is typical of an extraskeletal osteosarcoma. The majority of reported extraskeletal osteosarcomas are in the mammary gland and other sites less frequently involved include the subcutaneous tissue, liver, spleen, thyroid gland, intestinal tract, eye and muscle. Oesophageal osteosarcomas in association with Spirocerca lupi infection in dogs is a well-recognised phenomenon. Unfortunately, similar to primary bone osteosarcomas, the risk of metastasis is high.