Oral Tumors in Dogs - An Overview

By Malcolm Weir, DVM, MSc, MPH; Jan Bellows, DVM, Dipl. AVDC, ABVP; Christopher Pinard, DVM

What is an oral tumor?

An oral tumor is an abnormal growth and unregulated replication of cells that occur within the mouth. A dog’s mouth, similar to our own, is made up of several different cell types; for example, there are epithelial (or skin) cells, bone cells, fibrous cells, and others all of which can become cancerous. Some tumors may grow slowly and do not typically spread, meaning they are benign, whereas others are aggressive and spread elsewhere (malignant tumors).

What causes these types of tumors?

The reason why a particular dog develops this, or any tumor is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. Male dogs appear to be twice as likely to develop oral cancer than female dogs. Several breeds seem to be more predisposed to oral cancers including Akita, Cocker spaniel, Collie, Boxer, Dachshund, Doberman pinscher, Golden Retrievers, Poodle, Scottish terrier, and Shetland sheepdog.

What are the clinical signs of oral tumors?

Oral tumors come in many forms and your dog’s clinical signs will depend on location of the tumor, tumor type, tumor size, and presence of spread. Melanomas appear pigmented or non-pigmented, and may be nodular or cauliflower-like in appearance. These tumors may appear as swellings on the gums around the teeth, or on the hard or soft palates. They frequently ulcerate (break open) and bleed. They may also become infected. These tumors may look small but may extend deeper into the tissues than expected, invading the underlying bone.

"Oral pain is usually apparent, especially in dogs with tumors that have extended into the underlying bone."

Oral pain is usually apparent, especially in dogs with tumors that have extended into the underlying bone. Signs may include bad breath (halitosis), drooling, panting, movement or loss of teeth, lack of appetite, difficulty eating, reluctance to be touched on the head, facial swelling, and swelling of the lymph nodes.

How is oral cancer diagnosed?

An accurate diagnosis of oral tumors requires microscopic examination of tumor tissue. Fine needle aspiration (FNA) may be pursued in cases of oral tumors. FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope. In some cases, results from FNA may not be entirely clear and biopsy may be necessary.

A biopsy is a surgical excision of a piece of the tumor. Pieces of the tumor are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology is not only helpful to make a diagnosis but can indicate how the tumor is likely to behave (probability of local recurrence or spread to other areas).

How does this cancer typically progress?

Benign oral tumors usually progress slowly, while malignant tumors rapidly enlarge and invade adjacent tissues. Depending on the type of oral tumor present, it may be more locally aggressive, meaning it will invade the closely associated tissues and structures (tooth roots, bone, and other soft tissues). Others may be very aggressive and metastasize (spread elsewhere) to local lymph nodes, the lungs, and abdominal organs. Depending on tumor type, metastasis be as high as 80%.

In cases of a malignant oral tumor, full staging (searching for potential spread to other locations in the body) is highly recommended, as malignant tumors can spread. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If local lymph nodes are enlarged or feel abnormal, further sampling by FNA may be pursued to determine if spread is present.

"Depending on tumor type, metastasis be as high as 80%."

The results of the histopathology report will indicate how the tumor is likely to behave. The veterinary pathologist will include information about the probability of local recurrence or metastasis.

What are the treatments for oral tumors?

Surgical removal is the most common recommendation to treat oral tumors.Magnetic resonance imaging (MRI) or CT scans of the head/neck are usually performed prior to surgery to determine the extent of disease, as well as for surgical planning. If local lymph nodes are affected, they may be removed at the same time as tumor removal. Your veterinarian may recommend removal of these lymph nodes as a preventive measure to ensure there is no spread.

"Surgical removal is the most common recommendation to treat oral tumors."

If the tumor has invaded bone, its removal may be difficult, and it may be necessary to remove a portion of your dog’s jaw (upper or lower). Although this type of surgery sounds daunting for you and your dog, many of these tumors are painful and surgical removal provides relief. The appearance of a dog after removal of the left lower jaw is pictured to the right.

After surgery, the tissues are examined by a pathologist to predict, as best as possible, the probability of local recurrence or metastasis (spread to other areas). If the entire tumor is submitted, the pathologist may be able to assess if the tumor was completely removed or if additional therapies are necessary.

If a malignant tumor has not been completely removed, a second surgery may be required or follow-up treatments with radiation therapy. In some cases, surgery may not be possible or warranted. Radiation therapy can also be considered as a primary treatment option if surgery is not possible. Certain tumors such as ameloblastomas have an excellent response to radiation therapy (though surgery is still preferred if possible).

Targeted electromagnetic thermoablation (localized hyperthermia has been studied for the treatment of some oral tumors in dogs, although this method of treatment is not widely available.

A DNA melanoma vaccine, called Oncept®, is available as an immunotherapy for oral melanoma in dogs. The vaccine is usually used after surgical debulking of the primary tumor and is designed to target possible microscopic metastasis. This treatment has shown benefits over surgery alone in some studies.

In cases where metastasis has been observed, your veterinarian may discuss chemotherapy as a treatment option for your dog.

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