Salivary Gland Tumors

By Tammy Hunter, DVM; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What are salivary glands?

Salivary glands are organs that produce and release saliva, a substance that has important digestive functions. There are five major salivary glands in cats (parotid, mandibular, sublingual, molar and zygomatic), and four in dogs (parotid, mandibular, sublingual, and zygomatic).

What is a salivary gland tumor?

A salivary gland tumor is caused by abnormal replication of gland cells resulting in enlargement of the gland. Salivary gland tumors are rare in dogs and cats. Although they can be benign or malignant, they are usually malignant.

Salivary gland tumors occur most commonly in the parotid gland (located by the base of the ear) in dogs and the mandibular gland (located at the level of the lower jaw) in cats. The most common type of salivary gland tumor is a malignant (cancerous) tumor called adenocarcinoma. Less common types of cancerous tumors include squamous cell carcinoma, mucoepidermoid carcinoma, anaplastic carcinoma, and complex carcinoma. Benign adenomas (non-cancerous) account for only 5% of salivary tumors in dogs and cats.

Cancers in other areas of the body will sometimes spread to, and grow in, the salivary glands, appearing to be a primary salivary gland tumor. These types of cancer include fibrosarcomas, lipomas, mast cell tumors, and lymphomas.

What causes this cancer?

The reason why a particular pet may develop this, or any cancer, is not always straightforward. Very few tumors or cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic. There are no environmental or genetic factors known to cause these tumors. In dogs, poodle and spaniel breeds seem to be more affected. In cats, the Siamese breed appears to be at a higher risk. Male cats are twice as likely to be affected. Overall, salivary gland tumors tend to occur in older animals.

What are the signs of a salivary gland tumor?

The most common sign of a salivary gland tumor is a painless swelling of the upper neck, ear base, or upper lip. Your pet may have bad breath (halitosis) and may drool. You may notice difficulty eating (reluctance to chew or dropping food) or trouble swallowing. Your pet may eat less than usual or stop eating altogether, lose weight, and become lethargic. Your pet’s voice (bark or meow) may change. You may notice sneezing. Your pet’s ability to yawn may become limited, and your pet may refuse to allow you to open their mouth. The eye nearest to the swelling may protrude (bulge) outwards.

How are salivary gland tumors diagnosed?

Salivary gland tumors are tentatively diagnosed by a fine needle aspiration (FNA) and cytology. FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the swelling and placing them on a microscope slide. A veterinary pathologist then examines the cells under a microscope (cytology). FNA and cytology can determine if the swelling is neoplastic or non-neoplastic (i.e., related to other conditions such as inflammation or infection).

"A biopsy and histopathology are required for a conclusive diagnosis."

A biopsy and histopathology are required for a conclusive diagnosis. A biopsy is a surgical excision of one or more pieces of the tumor. The pieces are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology not only helps make a diagnosis but can also indicate how the tumor is likely to behave.

How do these types of tumors typically progress?

Without treatment, salivary gland tumors will continue to grow and invade the surrounding tissues, and in some cases will spread to other areas of the body (metastasize), most commonly to the nearby lymph nodes, but occasionally to other areas of the body as well. Staging is highly recommended (searching for potential spread to other locations in the body). Staging may include bloodwork, urinalysis, X-rays (radiographs) of the lungs, and an abdominal ultrasound. If a CT (computerized tomography) scan is recommended prior to treatment, a full body scan may be advised to screen for metastasis.

What are the treatments for salivary gland tumors?

The most common treatment is surgery to remove the tumor. As salivary gland tumors can be quite invasive, a CT scan or MRI (magnetic resonance imaging) may be recommended to plan the surgical approach. Radiation therapy may also be recommended, if needed. Even if the tumor cannot be fully excised, surgery may still be recommended to reduce the size of the tumor and relieve the symptoms. Radiation therapy is usually recommended for tumors that are incompletely removed and can be a stand-alone treatment for tumors that cannot be surgically removed. Chemotherapy may be recommended if there are signs of metastasis, but the role (and effectiveness) of chemotherapy is largely unknown.

Is there anything else I should know?

If a tumor is suspected or diagnosed, your veterinarian may refer you to a veterinary oncologist, as salivary gland tumors are rare. Surgery and radiation therapy may control the cancer and offer your pet a long survival time. The prognosis depends on the type of tumor, the extent to which it has invaded local tissues, and whether it has metastasized, as well as the initial health status of your pet.

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