Intestinal Tumors

By Malcolm Weir, DVM, MSc, MPH; Debbie Stoewen DVM, MSW, RSW, PhD

What are intestinal tumors?

Intestinal tumors develop as a result of the abnormal proliferation and dysregulated replication of cells anywhere along the intestinal tract, which includes the small and large intestines. The small intestine consists of three distinct regions: the duodenum, jejunum, and ileum. The large intestine also consists of three distinct regions: the cecum, colon, and rectum. Intestinal tumors usually grow from the cells of the inner lining of the intestine or from the muscle that surrounds the lining.

Intestinal tumors may be either benign (non-cancerous) or malignant (cancerous). Malignant tumors are invasive and prone to metastasize (spread to other areas of the body). Most tumors of the intestinal tract are malignant.

In dogs, three types of intestinal tumors are seen: lymphoma, adenocarcinoma, and leiomyosarcoma. While lymphomas tend to occur anywhere along the intestinal tract, adenocarcinomas occur more often in the large intestine, and leiomyosarcomas occur more often in the small intestine.

In cats, lymphoma is by far the most common intestinal tumor, occurring most often in the small intestine. The next most common is adenocarcinoma, which occurs most often in the large intestine, followed by mast cell tumor and leiomyosarcoma.

"Overall, digestive tract tumors are uncommon in dogs and cats, comprising approximately 2% of cancers in pets."

Other types of intestinal tumors in both cats and dogs include leiomyomas, gastrointestinal stromal tumors (GISTs), plasmacytomas, adenomas, adenomatous polyps, carcinoids, and osteosarcomas. Polyps are more likely to occur in the duodenum (upper small intestine) in cats and the colon or rectum in dogs, and in dogs, they can transform to become cancerous.

While the majority of intestinal tumors in dogs occur in the large intestine, in cats, the majority occur in the small intestine. Overall, digestive tract tumors are uncommon in dogs and cats, comprising approximately 2% of cancers in pets.

What causes this cancer?

The reason a pet develops intestinal tumors, or any other tumor or cancer, is not 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 or hereditary. In the case of intestinal tumors, age, sex, and breed appear to be risk factors.

As with many cancers, the incidence of intestinal cancer increases with age. Intestinal tumors tend to occur in middle-aged to older dogs, most often between 9 and 13 years, and older cats, usually older than 7 years. Some tumors, such as leiomyomas, tend to occur in old dogs (10 years old on average). There is no gender predisposition for dogs with intestinal tumors.

Certain breeds are particularly predisposed to developing certain intestinal cancers, indicating that specific genetic predispositions exist. In dogs, leiomyomas and leiomyosarcomas (both smooth muscle tumors) tend to occur in large breeds, most notably German shepherds. Adenocarcinomas occur mostly in German shepherds, collies, boxers, doberman pinschers, shar-peis, poodles, and west highland white terriers. Rectal polyps occur more commonly in German shepherds and collies. Mast cell tumors are more common in Maltese, as well as other miniature breeds. And tumors of the colon and rectum are more prevalent in boxers, German shepherds, poodles, Great Danes, and spaniels.

In cats, the Siamese breed is more likely to develop intestinal cancer compared with other cat breeds.

What are the signs of intestinal tumors?

The signs of intestinal tumors vary depending on the location of the tumor, the extent of the tumor, whether it has metastasized, and the associated consequences.

If your pet has a tumor of the small intestine, you may notice intermittent vomiting, reduced appetite, lethargy, and gradual weight loss. Vomit may be blood-tinged or have a “coffee grounds” appearance. This can occur with tumors of the upper small intestine (e.g., duodenum) that ulcerate (open), causing bleeding. Bleeding from tumors anywhere along the small intestine may cause the stool may become blackish. Chronic bleeding can lead to anemia (low circulating red blood cells), causing paleness of the gums. You may also notice intestinal rumbling or gurgling noises or that your pet is frequently passing gas.

"Bleeding from tumors anywhere along the small intestine may cause the stool may become blackish."

If your pet has a tumor of the large intestine, you may notice bleeding from the anus, blood in the stool, and it may be difficult for your pet to pass stools. Sometimes, chronic straining can lead to rectal prolapse (protrusion of the rectal lining through the anus). Sometimes the stools become narrow and ribbon-like in appearance.

When the tumor is located at the juncture of the small and large intestine, your pet may have a mixture of signs (i.e., signs of both small and large intestinal tumors). Diarrhea is not common but can occur with tumors of both the small and large intestine. Occasionally, the belly becomes distended due to fluid build-up in the abdomen.

Pets with intestinal leiomyomas or leiomyosarcomas can develop blood glucose levels that are lower than normal (hypoglycemia). This is a type of paraneoplastic syndrome, a condition when substances released by cancer cells affect the functioning of other organs. Signs of low blood glucose (blood sugar) include restlessness, weakness, trembling, disorientation, and seizures.

Another paraneoplastic syndrome with leiomyomas and leiomyosarcomas is tumor-associated nephrogenic diabetes insipidus. This condition causes excessive drinking and urination. Lymphoma and intestinal adenocarcinoma can also cause a paraneoplastic syndrome – hypercalcemia (high blood calcium). Hypercalcemia also causes excessive drinking and urination.

How is this cancer diagnosed?

Your veterinarian may suspect intestinal cancer in an older dog or cat with poor body condition and signs of gastrointestinal upset. Findings from a physical examination are variable. Your veterinarian may find a mass or some distended, painful areas of the small intestine when they palpate (feel) your pet’s abdomen. The lymph nodes and certain organs (such as the liver) may be enlarged, and the abdomen may be swollen with fluid. A mass may also be found during a rectal exam.

Your veterinarian will perform tests such as blood tests, urinalysis, imaging, endoscopy, and biopsy. Bloodwork and urinalysis help find the changes associated with paraneoplastic syndromes. X-rays (radiographs) may show a mass in the abdomen, and specialized imaging (a barium swallow) may show ulceration in the intestine, reduced intestinal movement, or intestinal obstruction.

Ultrasound is also helpful, especially to examine layers of the intestinal wall and to obtain an ultrasound-guided fine needle or core needle biopsy. In this procedure, a small needle with a syringe is used to sunction a sample of cells directly from the tumor and place them on a slide. A veterinary pathologist then examines the slide under a microscope.

Endoscopy can be useful to diagnose the presence of a tumor in most areas of the intestine. In this procedure, forceps are passed through an endoscope (a thin tube with a light and tiny camera at the end) to take tissue samples and biopsy samples. As with needle biopsies, these samples are not always valuable for diagnosis and a surgical biopsy may be needed to instead obtain a definitive diagnosis.

Surgical biopsies may be collected via laparoscopy, a procedure using a laparoscope (a thin, tube-like instrument with a light and lens), or laparotomy (a surgery to open the abdomen). A veterinary pathologist will examine the biopsies under the microscope to identify the type of cancer, a process called histopathology. Histopathology helps to make a diagnosis and can also indicate how the tumor is likely to behave.

In the case of rectal tumors, cells can be collected during a rectal exam.

How does this cancer typically progress?

How cancer of the intestines progresses depends on the type of tumor and how it affects the body. Some tumors grow very slowly, while others grow quite fast. Without treatment, both benign and malignant tumors continue to grow, increasingly interfering with intestinal function and risking ulceration or intestinal obstruction. In some cases, ulcerative tumors can lead to perforation of the intestine, with the spillage of intestinal contents into the abdomen leading to a life-threatening infection.

"Without treatment, both benign and malignant tumors continue to grow, increasingly interfering with intestinal function and risking ulceration or intestinal obstruction."

Because most intestinal tumors are malignant, they often metastasize to other areas of the body, including the nearby lymph nodes, liver, and lungs, as well as other organs and the inner lining of the abdomen. With the chance of metastasis, staging (searching for potential spread to other locations in the body) is highly recommended. Staging may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. Occasionally, more advanced imaging is used, such as computed tomography (CT) scans or magnetic resonance imaging (MRI). If any lymph nodes are enlarged or feel abnormal, further sampling may be pursued to determine if spread is present.

What are the treatments for this type of tumor?

Intestinal tumor treatments depend on the type of tumor and the extent to which it has grown and spread. With most intestinal tumors, surgery is the treatment of choice.

The surgical removal of tumors that have metastasized is primarily palliative, to ease symptoms and improve quality of life. The long-term outlook in these situations tends to be limited, with surgery providing a few months of relief before the metastatic growths become problematic or the tumor regrows. In some cases, surgery can be followed with chemotherapy or radiation therapy. Radiation therapy may be recommended in some cases, such as for adenocarcinomas of the colon.

Intestinal lymphoma in dogs and cats can be localized, growing as a mass in one location, or diffuse, spreading out as a general thickening of the intestinal wall. Surgery is advised, if it is possible, as it will help alleviate the signs of the cancer, but lymphoma is typically treated with chemotherapy, whether or not the tumor is removed. Sometimes chemotherapy is the preferred treatment approach for intestinal lymphoma in dogs and cats. In some cases, radiation therapy may also be recommended.

In the case of muscle tumors, when the tumor is removed, the signs of paraneoplastic syndrome will resolve.

Is there anything else I should know?

The outlook can vary from excellent to poor, depending on the type of tumor, whether it has spread to other areas of the body, the number of tumors present, and whether all the cancer can be removed.

Sometimes the prognosis is limited by the degree of debilitation (e.g., weight loss, and malnourishment) or other health conditions your pet has. Your veterinarian or veterinary oncologist will be able to provide guidance on the best plan of care for your pet.

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