Case 2 - 171020

10-year-old, male castrated, Labrador retriever mix

The patient presented to the UC Davis emergency service for left forelimb lameness and falling over following a one month history of having an abnormal gait. MRI revealed cystic mass at the C6-C7 articular process. Clinicians felt like complete surgical excision was not an option and debulking would likely lead to regrowth. Owners elected euthanasia.

Submitted for necropsy is a 35 kg, 10-year-old, male castrated Labrador retriever, who was euthanized on June 30, 2017 at 4:30 PM. The necropsy is performed on July 1, 2017 at 10:45 AM, approximately 18.25 hours post-mortem. The body is in good post-mortem condition with ample internal and subcutaneous adipose stores. There is mild atrophy of the muscles of the thoracic limbs bilaterally. Within the vertebral canal, a soft, friable, poorly demarcated, gray to tan mass is present beginning at the C6 vertebral foramen and extending dorsally to caudal C7. There are multiple cavitations noted within the vertebral body of C6.

This slide has one section of vertebra, in which an infiltrative, poorly demarcated, densely cellular neoplasm composed of thin, anastomosing trabeculae, sheets, and clefts of spindle cells embedded in a thin fibrovascular stroma focally invades into the vertebral bone and replaces cortical bone and marrow. Neoplastic cells have indistinct cell borders and a scant to moderate amount of eosinophilic cytoplasm. Nuclei are round to ovoid with finely stippled chromatin and one to two variably distinct nucleoli. Anisocytosis and anisokaryosis are mild, and there are no mitotic figures per ten 400x fields. Few multinucleate cells are noted with up to 5 nuclei. Multiple variably-sized fragments of pre-existing bone are scattered throughout the neoplastic population.

IBA-1: negative. CD18: negative

Pan-cytokeratin: negative.

C6 vertebral body, vertebral canal: synovial cell sarcoma

Neoplastic processes were noted within the C6 vertebral body, canal and the left adrenal gland. The mass noted within the vertebral canal likely arises from the C6-C7 articular process and extends into the vertebral body of C6, forming a multilocular, cyst-like structures that replace bone. The morphology of neoplastic cells is consistent with a sarcoma.

Cells frequently pile along crude papillary projections, or pallisade along an interstitium,forming clefting spaces. Multinucleate syncytial cells are present but not a predominant feature. Immunohistochemistry was performed to further characterize neoplastic cells; cells were IBA-1, CD18, and pan-cytokeratin negative. The tumor is most consistent with a synovial cell sarcoma.Skeletal muscle atrophy of the thoracic limbs and changes within the spinal cord and nerves are likely associated with the mass in the vertebral canal (disuse or neurogenic atrophy) and instability within the vertebral column.Synovial cell sarcomas are uncommon in the dog but most often affect the stifle. Thought to arise from undifferentiated mesenchymal cells that differentiate into synovioblasts.In humans, synovial sarcomas arise from pluripotential mesenchymal stem cells and are found outside the joint capsule. Tumors can be monophasic (spindle cells only) or biphasic (spindle and epithelial). Identify of the tumor is confirmed by idnetification of a specific genetic translocation.Epithelial components will be cytokeratin positive. Normal synoviocytes are not cytokeratin positive. Type A synoviocytes are CD18+, while there are no specific markers for type B synoviocytes.A retrospective study (Craig et al, 2002) suggests that many of the historically diagnosed synovial cell sarcomas were actually histiocytic neoplasms or other sarcomas.

Craig, L. E., Julian, M. E., Ferracone, J. D. (2002). The diagnosis and prognosis of synovial tumors in dogs: 35 cases. Veterinary Pathology, 39(1), 66-73. Harasen, G. (2002). Joint tumors. The Canadian Veterinary Journal, 43(12), 975976.

Jubb, K. V. F., Peter C. Kennedy, and Nigel Palmer. Pathology of Domestic Animals. St. Louis, MO: Elsevier, 2016. Print. Meuten, Donald J. Tumors in Domestic Animals. Ames: Wiley Blackwell, 2017. Print.