Mouth Cancer (Adenocarcinoma) in Dogs

The salivary glands produce saliva which lubricates the mouth and begins the process of digestion. They are located in four places in dogs: at the back of the jaw bone, close to the ear, underneath the tongue, and around both cheek bones. When cancerous mutations affect the glandular tissue, oral tumors can develop on these glands. Salivary gland neoplasia are not very common, with an overall incidence of .17 %, but they account for 30 % of all salivary gland disorders. This type of cancer is more common in older dogs. About 84% of all salivary gland tumors are adenocarcinoma, a malignant tumor that forms in the glandular epithelial layers. This type of cancer can be found on many different glands throughout the body, but in the mouth it is limited to the salivary glands. Tumors on the mandibular gland, at the very back of the lower jaw bone, are most common. Symptoms often include swelling at the top of the neck or the base of the ear. Depending on the location, dogs can have difficulty eating or swallowing as well as bad breath. Infection or displacement of the eyeball is also seen quite frequently. Local metastasis to the lymph nodes is common with salivary adenocarcinoma, but metastasis to more distant sites is relatively rare in dogs, occurring in about only 8 % of cases. If salivary tumors are removable without damage to vital organs, dogs often make a complete recovery. However if the tumor has invaded the surrounding tissue, a clean excision can be difficult and part of the cancer may be left behind to regrow.

Adenocarcinoma of the mouth is located on the salivary glands. Tumors at the back of the mandible are more common in dogs, but any of the four salivary gland areas can be affected. In the mouth, adenocarcinoma is usually limited to local metastasis and can often be treated effectively with surgical removal.

These are some of the symptoms you might notice in a dog with salivary adenocarcinoma. Early treatment is most effective, so take your dog to see a veterinarian immediately.

• Bad breath (halitosis)
• External protrusion of the eyeball (exophthalmos)
• Painless swelling at the base of the ear or the upper neck
• Swollen tongue
• Swollen lip
• Excessive salivation
• Difficulty eating
• Difficulty swallowing (dysphagia)
• Difficulty or pain opening the mouth
• Sudden onset blindness
• Ear or eye infection

Types

Adenocarcinoma can occur in any of the following salivary glands.

• Parotid – close to the ear
• Mandibular – on the lower jaw
• Sublingual – underneath the tongue
• Zygomatic –  in the cheek-bone

There is no definitive cause for this or any other type of cancer. Veterinarians believe that a combination of factors, including hormones, genetic predisposition, and environmental factors, like exposure to radiation and other carcinogens, are related. There is no sex predisposition for adenocarcinoma, but some veterinarians have found that salivary gland tumors are more common in Spaniel breeds. The disease could occur in any animal. Most cases are diagnosed over ten years of age.

The veterinarian will evaluate your dog’s symptoms. A salivary gland tumor may be suspected based on the location and symptoms, but other types of oral cancer, as well as abscess from a foreign body, may cause similar swelling, hypersalivation, and difficulty eating. If your dog is older, this will make a cancer diagnosis more likely. X-rays or CT scans and cytological examination of a biopsy sample will provide a definitive diagnosis. X-rays will help to show the exact placement of the tumor and its effect on the surrounding bones, while more detailed CT scans will show the extent of metastasis into the nearby lymph nodes. A biopsy sample will be obtained by inserting a hollow needle into the tumor and extracting a small sample. Microscopic examination will help the veterinarian determine the type and aggressiveness of the cancer, and make it easier to plan effective treatment. X-rays of the lungs or other vital organs may be ordered to determine if more widespread metastasis is present.

Surgical removal is the most effective treatment for salivary adenocarcinoma. The exact surgery will depend on the placement of the tumor and the degree to which vital organs are affected. Some tumors may be removable with minimally invasive surgery, while others will involve delicate procedures or may have infiltrated the surrounding tissue too much to be entirely removed. Surgeries around the neck area can result in complications that affect the eye reflex and limit the dog’s ability to blink. However, this side-effect can usually be corrected with further eyelid surgery and eye drops. 

Adenocarcinoma is often treated with radiation after surgery to minimize the likelihood of metastasis. Your dog may need to be hospitalized during this treatment also so that the veterinarian can adequately monitor the side-effects. Severe cases with a high degree of metastasis may not be treatable, or your dog may not be healthy enough for surgery. In this case, the veterinarian will manage your dog’s symptoms as long as possible.

Cancer is always a serious diagnosis in dogs, but salivary adenocarcinoma caught in the early stages can have a relatively good prognosis. In one study, twenty-four dogs survived for 550 days after surgical excision of a salivary gland tumor. If significant metastasis is already present upon diagnosis, this will greatly reduce treatment options and survival time. Your dog’s likelihood of making a full recovery will be evaluated by a veterinarian upon diagnosis.