An 11-year-old Yorkshire Terrier dog presented with progressive ataxia and paraparesis of 1 month duration. The abnormality was localized to the T13-L2 segment by neurologic examination and CT. MRI showed a space-occupying lesion compressing the spinal cord and adjacent osteolysis of the L1 vertebra. A diagnosis of poorly differentiated carcinoma with high malignancy was reached by histopathology of the biopsy samples. Metronomic chemotherapy was applied, but the patient was euthanized at 133 days after first presentation. Samples of the lumbar mass, pulmonary nodules, and lymph nodes were obtained during necropsy and evaluated with histopathology and immunohistochemistry (IHC). Undifferentiated carcinoma suspicious of transitional cell carcinoma was confirmed, but the exact origin of the tumor was undetermined. This is the first case report of an extradural primary carcinoma of unknown primary in the canine spinal cord and paravertebral region.

Key words: Carcinoma of unknown primary, Dog, Metastasis, Spinal cord neoplasia