Peripheral Nerve Sheath Tumors In Dogs

Just like humans, dogs have both a central nervous system and a peripheral nervous system. The latter includes the nerves throughout the body that are not within the brain or spinal cord. These peripheral nerves are enveloped and insulated by sheaths, similar to a sleeve covering your arm. Ironically, despite this protective role, peripheral nerve sheaths can develop tumors, including malignant (cancerous) tumors.

Peripheral nerve sheath tumors are one of the more common soft tissue sarcomas in dogs. They are also referred to as schwannomas because a type of cell that helps form the sheath are called Schwann cells. Overall, however, schwannomas are uncommonly diagnosed. Fortunately, even for dogs that are stricken, treatment can sometimes be curative or put the cancer in extended remission. This especially holds true if the tumor is not terribly aggressive and the cancer has not metastasized. Notably, metastasis is rare.

It’s important to know that benign tumors can cause issues, too. If a benign tumor becomes sizeable enough, for instance, these masses can press on the nerve and cause pain or discomfort. Early diagnosis thus is very important for any type of mass, especially canine peripheral nerve sheath tumors. To that end, it’s helpful to know what signs to look for in order to detect nerve sheath tumors in dogs in a timely manner.

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Symptoms of Nerve Sheath Tumors in Dogs

Peripheral nerve sheath tumors in dogs are typically a single mass. They may be visible on the skin or you may feel a lump below the skin. The mass may be just a few millimeters in size but nerve sheath tumors also can grow much larger. On the rare occasions when a dog develops more than one tumor, it is likely that each is a primary tumor rather than a case of metastasis. While spread is rare, checking the lymph nodes for swelling is a good practice. Any suspicious lumps merit a trip to see your veterinarian.

Because a canine peripheral nerve sheath tumor can develop anywhere on the peripheral, or outside, nerves, it is important to have any new mass evaluated by your veterinarian.  At times, you may not notice a mass until it is quite large, perhaps causing discomfort to your dog.   A larger tumor could affect a stricken dog’s energy level, making him or her lethargic. He or she also could lose weight, even while maintaining a normal appetite. A larger tumor also could cause nerve impingement, impairing function or limiting a dog’s range of motion in a limb, for instance. A dog might appear to be “dragging” a limb or may even turns its paw over, walking on his or her knuckles with the pad facing upward, which is often referred to as “knuckling.” Since other types of tumors and conditions also can cause these symptoms, you should see your veterinarian to get a proper diagnosis.  Though uncommon, when this type of cancer metastasizes, it will spread to the lungs or the local lymph nodes.

Diagnosing Schwannoma in Dogs

Lumps and bumps on and under a dog’s skin can have a wide range of causes. To specifically diagnose a canine peripheral nerve sheath tumor, a veterinarian would work to rule out other possibilities, including but not limited to other types of sarcomas, a sebaceous adenocarcinoma, a mast cell tumor or a basal cell tumor. The mass also could be benign. Tests and procedures that will help a veterinarian reach a diagnosis include:

• A complete blood count (CBC) and a blood chemistry profile will speak to the dog’s overall health.
• Testing urine also can reveal abnormalities that help a veterinarian in treating your dog.
• Aspiration or biopsy of the mass. An aspiration involves using a fine needle to take a sample of cells for cytologic evaluation, while a biopsy refers to the removal of a portion of the mass or even the entire mass at times. In both cases, the sample is sent to a lab, where a veterinary pathologist will determine whether it’s cancer and examine it for characteristics such as the mitotic rate (the rate at which cells are dividing).
• Lymph node sampling. If there is concern that the cancer has spread to the regional lymph node(s), your veterinarian may suggest an aspiration cytology or a biopsy of this organ.
• 3-view X-rays. While most peripheral nerve sheath tumors in dogs do not spread to the lungs, some will. Chest X-rays can determine whether there has been spread to the lungs.
• CT Scan. On occasion, your veterinarian may suggest a CT scan to assess the extent of the tumor as well as evaluate the surgical feasibility.

Prognosis and Treatment For Peripheral Nerve Sheath Tumors In Dogs

Factors such as the location of a tumor, its size, its grade, and how far the cancer has advanced will all be considered before a veterinarian recommends a treatment course. There are a few different treatment options that have proved successful, but a dog’s prognosis can vary with each individual case.

Surgery

The goal of surgery is to remove the mass and achieve wide, clean margins. That means the edges of the removed portion have no microscopic cancer cells left behind, which would indicate that the malignancy has been completely removed. If such margins are achieved, surgery can be curative.  If all of the microscopic cancer cells cannot be fully excised, these tumors tend to recur in a matter of months.

Radiation Therapy

Radiation might be recommended in a variety of circumstances:

• If clean margins are not obtained through surgery
• To shrink the mass before surgery if the tumor is too big for simple surgical removal
• In place of surgery if surgery is not recommended or is declined for any reason

There are two primary types of radiation: conventionally fractionated radiation therapy (CFRT) and stereotactic radiation (SRS). While both types direct high-energy rays at cancerous cells to destroy them, SRS is more advanced, using higher dosages and precision targeting. It is often very successful. Another big advantage of SRS is that just one to three treatments are typically required vs. at least 15 for CFRT. Because dogs must be anesthetized for radiation, this also means fewer such events, which can be a key consideration depending on the dog’s age and overall condition. SRS has achieved good success at controlling tumors and putting cancer in remission for 600 to 700 days. PetCure Oncology specializes in SRS as a treatment option.

Chemotherapy

Chemotherapy is another treatment option when a dog is not a good candidate for surgery or if surgery and radiation are declined for any reason. Chemo also could be recommended if a canine peripheral nerve sheath tumor is aggressive and/or has metastasized if the cancer has become systemic rather than just localized.

Metronomic chemotherapy — which entails administering low doses of oral chemo typically every other day on an ongoing basis — tends to help in about 50% of cases for up to six months or a year. A veterinarian might also recommend chemo as an adjuvant therapy — meaning as a supplement to the primary treatment (surgery, radiation therapy), with the aim of reducing the risk of cancer returning.

Forgoing Treatment

Left untreated, peripheral nerve sheath tumors in dogs will continue to grow. They tend to be slow-growing tumors, however, and in some cases, it may take months or much longer before problems present themselves.

PetCure Oncology Treats Peripheral Nerve Sheath Tumors

At PetCure Oncology, we understand how much your dog means to you. We’ve made it our mission to provide the best care possible for dogs stricken with cancer, including peripheral nerve sheath tumors. Our wide range of treatment options includes SRS radiation therapy. We specialize in SRS — and also in offering compassion that matches our expertise. We’re here to help provide your pet with the best quality of life possible while extending your time together. For more information about our cancer treatment options, find a location near you and contact us today.

The contents of this article were provided in part by Dr. Renee Alsarraf, DVM, DACVIM (Oncology), a board-certified veterinary medical oncologist and member of PetCure Radiation Oncology Specialists (PROS).