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Background: The most common location of malignant tumors of the peripheral nerve sheath in the spinal cord is the intradural-extramedullary region, and is rare in the spinal nerve roots in the lumbar region. They mainly affect large female dogs over 6 years of age. Imaging tests assist in the presumptive diagnosis, but confirmation requires histopathological and immunohistochemical examination. The prognosis is guarded. Diagnostic imaging, anatomopathological and immunohistochemical findings of a malignant tumor of the intradural-extramedullary peripheral nerve sheath with medullary infiltration in the lumbar region in a young dog are reported. Case: A body of a 6-year-old Poodle dog was donated for necropsy and diagnostic clarification. In the history, there was a suspicion of lumbar intramedullary neoplasia, detected by computed tomography (CT), with a 4 years progressive chronic evolution. Additionally, the dog had hidden spina bifida (L7 to S3), as detected by radiography and CT. On post mortem radiographic examination (X-ray), there was an enlargement of the vertebral canal (T10 to S2), intense osteolysis (L1 to S2), spinous processes (L5 to L7), and ankylosis (L3 to L7). Necropsy revealed ankylosis (L3 to L7) and intradural-extramedullary mass (9.5 × 2.6 × 2.3 cm) (L2 to L6). No metastases were identified. On microscopy, there was neoplastic proliferation of cells with intense pleomorphism, arranged in bundles interlaced in palisades and sometimes solid mantles. The mitotic index was high, ranging from 10 to 12 mitoses per field. There was also necrosis, hemorrhage, edema, and focal axonal demyelination of the adjacent white matter in the spinal cord. Masson Trichrome staining highlighted an intense diffuse conjunctive stroma. There was a suspicion of a malignant tumor of the peripheral nerve sheath and an immunohistochemical panel was performed for confirmation. There was strong and diffuse positivity for vimentin and S-100 and partial positivity for neuron-specific enolase (NSE), negative for anti-factor VIII, glial fibrillary acidic protein (GFAP), a-actin for smooth muscle, cytokeratin, neurofilament, and desmin. Thus, the diagnosis of malignant neoplasm of the peripheral nerve sheath was confirmed. Discussion: Peripheral nerve sheath tumors are classified as benign or malignant. In dogs, they are frequent in elderly, females, and large breeds. In this case report, the animal was young, female, and small breed. The location of the spinal nerve roots is uncommon, and is more commonly found in the brachial plexus. In the animal reported, the tumor was observed as lumbar swelling. Clinical signs vary with the affected region, however, neurogenic claudication and muscle atrophy are more frequent, as observed in this report. Imaging examinations such as X-rays and CT assist in the presumptive diagnosis. In this case report, spina bifida was identified on radiography, and CT suggested the presence of intramedullary neoplasia and allowed to monitor tumor growth. Post mortem X-ray imaging revealed intense osteolysis and ankylosis, which were confirmed at necropsy, which also elucidated its intradural-extramedullary location with infiltration into the spinal cord. The confirmation of the neoplasm was made by histopathological and immunohistochemical examination; the latter should be made a panel, not restricted to the use of antibodies S-100 and vimentin only. The prognosis of malignant peripheral nerve sheath tumors (MPNST) of the spinal cord is poor, and although there are palliative methods, there is no curative treatment, as complications can interfere with the quality of life of the animal. MPNST should be included in the differential diagnosis of spinal disorders, even in young dogs and small breeds. CT helps in early diagnosis to make decisions aimed at the animal's well-being. [ABSTRACT FROM AUTHOR]