CATEGORIES OF SPINAL TUMOR
Spinal tumors are categorized as either primary or metastatic.  Primary tumors may be outside the covering of the spinal cord (extradural), inside the covering of the spinal cord but not within the cord itself (extramedullarlyl) or within the spinal cord (intramedullary).  Extradural tumors are the most common type of spinal tumor, and intramedullary tumors are the least common.  Metastatic tumors usually are found in the bones housing the spinal cord, the vertebrae.  It is uncommon that metastasis occurs to the spinal cord itself, but it can happen with certain tumors.  Tumors of the lymphatic system, like lymphoma, can occur in the spine as a primary tumor or as a metastatic tumor.  This is also true of tumors found within the vertebrae. 

If a spinal tumor is suspected, a host of tests are performed.  Bloodwork and radiographs of the chest are recommended to ensure that anesthesia will be safe, and to make sure that a spinal tumor has not metastasized to the chest.   Lymph nodes may be evaluated by taking a needle aspirate.  Radiographs of the spine may show cancer of the bones or displacement of certain areas of the spinal column that suggest the presence of a mass.  A myelogram may be recommended if a diagnosis cannot be made on radiographs.  With a myelogram, a contrast agent is injected into the area between the spinal cord and its covering to delineate any pressure being placed on it, or enlargement of it.  A CT scan or an MRI may be needed to further delineate a mass in the bone, around the spinal cord, or in the spinal cord.

SPECIFIC TUMORS OF THE SPINE
Peripheral nerve sheath tumors are derived from cells that surround the axons of peripheral nerves, specifically originating from the Schwann cell.  They are slow growing, locally invasive and rarely metastasize, and are rare in cats.  The area of the spinal cord where nerves go off to the forelimbs (brachial plexus) is involved in roughly 80% of cases.  Clinical signs include progressive lameness with a decrease in muscle mass in most cases.  Occasionally, some changes in the eye may be noticed on the same side as the tumor.  In one study, a myelogram was abnormal in all cases of this type of tumor.  If the tumor is within the brachial plexus, then amputation or mass removal may be performed.  If the tumor is within the spinal canal, surgery may be done to remove bone around the spinal tumor to decompress the spine.  If the tumor is located in both places, then amputation along with decompression would be necessary.  This tumor has a 72% recurrence rate and a median survival time of 2 months to 2 years.  Longer survival is possible with tumors located within the limb, where complete excision is possible.

Nephroblastoma (also known as neuroepithelioma, medulloepithelioma, ependymoma) is a type of tumor that originates from fetal kidney material entrapped within the dural during fetal development.   It may be located within the covering of the spinal cord, or within the spinal cord itself.  This is a rare type of tumor.  It is usually a solitary tumor, but metastasis may occur within the spinal cord.  Young dogs, 5-36 months of age, and especially German Shepherd Dogs are at risk for this type of tumor.  Clinical signs include progressive inability to use the rear limbs well.  A myelogram, CT scan or MRI may be recommended for diagnosis.  Treatment is surgical removal of the mass if possible, and survival may be prolonged by several years.  Chemotherapy and radiation therapy may be used after surgery.

Spinal meningiomas are tumors of the covering of the layers that cover the spinal cord.  They are located within the outer covering, the dura.  The median age for dogs with this tumor is 9 years, and for cats 12 years.  The clinical signs are usually slow and progressive, with dogs often having the tumor located in the neck and cats often having the tumor located in the spine as it runs through the chest.  A myelogram, CT scan and/or an MRI may be used to diagnose this tumor.   Treatment is usually surgery +/- radiation therapy.  Survival rates are long with complete excision—about 3.8 years for dogs and cats.  The prognosis decreases if the meningioma occurs where the nerves come off the spinal cord to the legs, if the tumor is located underneath the spinal cord, or if it is invasive into the cord. 

Spinal lymphosarcoma may occur as a primary tumor, or it may be the result of metastatic disease.  Primary lymphosarcoma is usually a solitary epidural mass, while metastatic lymphosarcoma usually infiltrates various areas of the spinal cord.  In cats, it is usually a young cat disease, with 70% of cats less than 3 years of age.  Mean age in dogs is 5-7 years.  Clinical signs in dogs and cats differ because of location; dogs usually have the brain and neck affected, and clinical signs include seizures, difficulty walking, behavior changes and circling.  Cats typically have lymphosarcoma in the back, with a progression over about 1 week or less.  The majority of these cats are positive for feline leukemia virus.  A myelogram typically compression over multiple areas, and cancer cells may be detected in the spinal fluid.  A variety of treatments may be recommended, including surgery, chemotherapy, steroids and radiation treatment.  Surgical treatment is not used with the mindset of removing all of the cancer that may be present; instead, it  aids in diagnosis with recovery of biopsy specimen and may relieve spinal cord compression.  Chemotherapy has been used with a complete response for 14 weeks in cats and for 3-12 weeks in dogs.  The prognosis is poor. 

Tumors of the vertebrae cause weaking of the bone, leading to fractures of the bone that cause abnormal pressure on the spinal cord. Tumors may arise from bone, cartilage, fibrous tissue and blood vessels.  Sarcomas are the most common, with osteosarcoma the most common tumor of the vertebrae.  Metastasis is common to the lungs.  Chondrosarcomas develop from cartilage of the vertebrae.  They expand and infiltrate soft tissues, but are slower growing than osteosarcomas.  They metastasize later and less often.  Fibrosarcomas come from the connective tissue and cause bone destruction over months; occasionally very aggressive forms may cause rapid destruction.  Multiple myelomas come from plasma cells that frequently are seen in the vertebrae bone marrow.  Metastatic tumors found in bone may be from hemangiosarcoma, lymphosarcoma, melanoma, chemodectoma, carcinoma, adenocarcinoma.  They are seeded by the vascular supply and lymphatics.  Microscopic pieces of tumor may be released into circulation and small vessels are blocked in the vertebrae, and tumor growth occurs.  More commonly, the bloodstream carries cells to the bones of the vertebrae.  The prognosis is obviously poor for tumors in these locations.  Possible treatment options include surgical stabilization, palliative radiation therapy and chemotherapy depending on the tumor type.

Mast cell tumors, myxoma/myxosarcoma, oligodendroglioma, intramedullary hemagioblastoma, mixed germ cell tumor, chordoma, leptomeningial malignant histiocytosis, ganglioneuroma, liposarcoma, giant cell sarcoma, and a variety of benign tumors have been reported in the spinal cord of cats and dogs.