Schwannoma are an infiltrative peripheral nerve sheath tumor similar to neurofibroma, and characterized by benign neoplastic Schwann cells (normally produce the insulating myelin sheath covering peripheral nerves). In dogs, these tumors may present as either benign or malignant.
Although schwannomas are entirely formed of neoplastic Schwann cells, neurofibromas are a mixture of Schwann cells, perineurial cells and fibroblasts, with the Schwann cell the primary neoplastic cell.
A collective term 'benign peripheral nerve cell tumor' has been advocated instead of schwannoma or neurofibroma due to the close histological appearance of these two tumor types as well as both benign schwannomas and benign neurofibromas behave similarly in clinical setting.
Schwannomas are most frequently associated with cranial nerve VIII (acoustic or vestibulocochlear nerve) but extradural (spinal column and brachial plexus), intracranial, testicular and malignant epithelial schwannomas have been reported.
Affected dogs often present with ipsilateral forelimb lameness, neck pain, head tilt, episodic circling or difficulty opening the mouth.
Diagnosis is based on presenting clinical history, and needle biopsy or tissue biopsy during surgical excision.
These tumors have an infiltrative growth, with a characteristic microscopic appearance of spindle-shaped neoplastic cells arranged in sheets and concentric whorls. They may exhibit different morphological patterns, specifically Antoni type A where spindle cells are arranged in palisades and Antoni type B which are less cellular and organized, made of oval to round loosely arranged cells.
Many heterogeneous patterns have been described such as epithelioid, rhabdomyoblastic, cartilaginous, osseous, angiomatous, glandular and lipoblastic.
Definitive diagnosis usually requires immunohistochemical staining for S-100 antigen, neuron-specific enolase, and glial fibrillary acidic protein. S-100 antigen staining appears to be selective with benign schwannomas.
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