An approximately four-year-old spayed female mixed breed dog was presented to the Oncology Service for a mass on the left ventral neck noted approximately 2 months prior. The client reported the dog had been yawning with decreased frequency but otherwise was asymptomatic for the mass. Physical examination was normal except for the mass, which was approximately 4 cm, firm, semi-fixed, and on the left ventral neck just lateral to the laryngeal cartilages. Blood work (complete blood count, serum biochemistry panel, and Snap 4Dx) was unremarkable, and three view thoracic radiographs showed no abnormalities. Cytology of the mass was consistent with a spindle cell tumor. A CT scan of the neck and thorax was recommended for complete staging and surgical planning which was performed the following week.
On the CT scan, a well circumscribed, heterogeneously contrast enhancing mass measuring 3.8 cm x 4.9 cm x 4.9 cm was identified (Figure 1). The mass was bordered dorsally by the ventral aspect of the left tympanic bulla, medially by the hyoid apparatus, and caudolaterally by the mandibular salivary gland. The margins of the mass appeared well defined except for its dorsal extent, which was ill defined and displaced the carotid artery. Stippled to patchy mineralization was present throughout the mass. The left medial retropharyngeal lymph node was mildly enlarged; all other cervical lymph nodes measured normal in size. No evidence of pulmonary metastasis was seen.
Local control options were discussed including surgical resection and radiation therapy. An incisional biopsy was offered prior to definitive treatment. The client elected to forego incisional biopsy, and surgical resection of the mass was scheduled one week later.
The mass was approached via an approximately 8 cm paramedian incision on the left ventral neck. The subcutaneous tissue and platysma were incised to expose the mass, which was carefully dissected from the surrounding tissue and removed using a combination of bipolar electrocautery and a vessel sealing device. The dog recovered smoothly from anesthesia and did well postoperatively. She was discharged from the hospital the following day. Histopathology showed extraskeletal osteosarcoma with 2-4 mitoses per high powered field and large areas of necrosis. The mass was invasive into surrounding tissue, although margins appeared free of tumor (measured between 2-5 mm).
The dog was re-evaluated approximately 2 weeks later, at which time she was doing well and had a healed incision. Complete blood count was within normal limits, and carboplatin chemotherapy was initiated.
Figure 1. Contrast-enhanced axial CT image of the neck showing a left-sided, heterogeneous mass lateral to the larynx and ventral to the tympanic bulla. Mineralization is present throughout the mass.
Extraskeletal osteosarcoma is a rare, highly malignant mesenchymal neoplasm characterized by the formation of tumor osteoid within viscera or soft tissues without bone or periosteal involvement.1 In dogs, the most common location for this tumor is the spleen.2-4 Extraskeletal osteosarcoma has also been associated with retained surgical sponges.1,5
Currently, a paucity of comprehensive literature exists on this tumor type in canine patients. In the studies to date, predominantly older dogs are affected with no clear breed or sex predilection.1,3,4 Metastasis has been reported in 24-57% of dogs at the time of initial diagnosis; the most common sites include the regional lymph nodes, lungs, liver, and kidney.1-4 Prognosis is considered poor, with median survival times ranging from 23 to 90 days.1-4 Importantly, dogs that receive wide surgical resection appear to have a significant survival advantage over those receiving palliative treatments.1 Similarly, adjuvant chemotherapy has been shown to improve survival.4
- Duffy D, Selmic LE, Kendall AR, Powers BE. Outcome following treatment of soft tissue and visceral extraskeletal osteosarcoma in 33 dogs: 2008-2013. Vet Comp Oncol 2017; 15: 46-54.
- Patnaik AK. Canine extraskeletal osteosarcoma and chondrosarcoma: a clinicopathologic study of 14 cases. Vet Pathol 1990; 27: 46-55.
- Langenbach A, Anderson MA, Dambach DM et al. Extraskeletal osteosarcoma in dogs: a retrospective study of 169 cases (1986-1996). J Am Anim Hosp Assoc 1998; 34: 113-120.
- Kuntz CA, Dernell WS, Powers BE, Withrow S. Extraskeletal osteosarcoma in dogs: 14 cases. J Am Anim Hosp Assoc 1998; 34:26-30.