Poorly Differentiated and Biologically Aggressive Oral Rhabdomyosarcoma in 34 Young Dogs.

Rhabdomyosarcoma is an aggressive neoplasm of primitive mesenchymal cells that have the potential to differentiate into skeletal muscle. Case selection required positive expression of both vimentin and desmin, confirmed by immunohistochemistry, and negative expression of cytokeratin. In this study of 34 young dogs, the average age at time of diagnosis was 16.5 months. The majority of tumors were located in the maxilla (n = 26, 76%), whereas a smaller subset (n = 4, 11%) occurred solely within the mandible. One (2.9%) involved the caudal mandible and maxilla, 1 of 34 (2.9%) involved the maxilla and palate, and 2 of 34 (5.8%) exclusively involved the soft palate. Of the maxillary tumors, 16 (59.2%) involved the nasal cavity. Thirty-one (91%) presented to their primary care veterinarian for acute facial swelling, with 17 (50%) cases also reporting concerns for ocular disease. Radiographically, 29 (87.8%) showed evidence of cortical lysis and loss of lamina dura surrounding the roots of teeth associated with the tumors. Histologically, neoplastic cells were small, round to spindle, and rarely had morphological features of myofiber differentiation. Radiographic findings consistent with metastasis to the lungs and/or lymph nodes were reported in 5 cases via thoracic imaging. Regional lymphadenopathy was identified in 19 (55%) cases, with metastasis cytologically confirmed in 2 patients. All 30 (100%) patients with follow-up data were euthanized due to progression of local or metastatic disease. The mean survival time was 59 days. Canine oral rhabdomyosarcomas are aggressive neoplasms that occur in young dogs and carry a poor prognosis.