Intestinal Cancer in Dogs

Intestinal Tumors in Dogs

The abnormal proliferation and dysregulated replication of cells anywhere throughout the intestinal tract, which includes the small and large intestines, results in intestinal cancer in dogs. The duodenum, jejunum, and ileum are the three different parts of the small intestine. The cecum, colon, and rectum are the three different parts of the large intestine. Tumors of the intestine’s inner lining or the muscle that surrounds it are the most common sources of intestinal tumors.

Benign or Malignant?

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.  

Intestinal Tumor Types in Dogs

In dogs, three types of intestinal tumors are seen: 

While lymphomas tend to occur anywhere along the intestinal tract, adenocarcinomas occur more often in the large intestine, and leiomyosarcomas more often in the small intestine.

Lymphoma is the most frequent intestinal malignancy in cats, primarily affecting the small intestine. Adenocarcinoma, which most commonly arises in the large intestine, is the second most frequent cancer, followed by mast cell tumor and leiomyosarcoma.

Leiomyomas, gastrointestinal stromal tumors (GISTs), plasmacytomas, adenomas, adenomatous polyps, carcinoids, and osteosarcomas are some of the other types of intestinal tumors found in cats and dogs. Polyps are more common in the duodenum (upper small intestine) in cats and the colon or rectum in dogs, and they can become malignant in dogs.

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.

Causes of Intestinal Cancer in Dogs

It’s difficult to say why a particular dog might get this, or any other tumor or disease. Only a small percentage of tumors and malignancies have a single identified etiology. The majority appear to be caused by a complicated combination of risk factors, some of which are environmental in nature and others which are genetic or hereditary. Age, sex, and breed all appear to be risk factors for intestinal cancers.

Intestinal cancer, like many other cancers, becomes more common as dogs get older. Intestinal tumors are more common in middle-aged to older dogs, usually between the ages of 6 and 9, and older cats, usually between the ages of 10 and 12. Some tumors, such as leiomyomas, are more common in older dogs (on average, 16 years old). Males are more likely than females to develop benign and malignant tumors.

Certain breeds are particularly predisposed to developing certain intestinal cancers indicating that that specific genetic predisposition exists.

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  • In dogs, leiomyomas and leiomyosarcomas (both smooth muscle tumors) tend to occur in the 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.
  • Tumors of the colon and rectum are more prevalent in Boxers, German Shepherds, Poodles, Great Danes, and Spaniels.

“Certain breeds are particularly predisposed to developing certain intestinal cancers indicating that that specific genetic predispositions exist.”

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.

You may notice occasional vomiting, decreased appetite, lethargy, and gradual weight loss if your dog has a small intestine tumor. The vomit may have a bloody tint or resemble coffee grounds. This can happen when cancers in the upper small intestine (such as the duodenum) ulcerate (open) and cause bleeding.

Stool may appear blackish if there is bleeding from tumors anywhere throughout the small intestine. Anemia (few circulating red blood cells) can cause pale gums as a result of chronic bleeding. You may also hear rumbling or gurgling sounds coming from your dog’s intestines, as well as regular gas passing.

“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.”

You may notice bleeding from the anus, blood in the stool, and difficulty passing stools if your dog has a tumor of the large intestine. Rectal prolapse can occur as a result of continuous straining (protrusion of the rectal lining through the anus). The stools might become slender and ribbon-like in appearance at times.

When a tumor is found at the intersection of the small and large intestines, your dog may exhibit a variety of symptoms (i.e., signs of both small and large intestinal tumors). Diarrhea is uncommon, however, it can occur with both small and large intestinal cancers. Fluid build-up in the abdomen can cause the belly to become swollen on occasion.

In dogs with intestinal leiomyomas or leiomyosarcomas, blood glucose levels might drop below normal (hypoglycemia). This is a form of paraneoplastic syndrome, which occurs when cancer cells release compounds that interfere with the operation of other organs.

Restlessness, weakness, trembling, confusion, and convulsions are all symptoms of low blood glucose (blood sugar). Tumor-related nephrogenic diabetes insipidus is another paraneoplastic condition associated with leiomyomas and leiomyosarcomas. Excessive drinking and urination are symptoms of this condition. Hypercalcemia is a paraneoplastic condition caused by lymphoma and intestinal adenocarcinoma (high blood calcium). Excessive drinking and urination are additional symptoms of hypercalcemia.


In an older dog with poor physical condition and signs of gastrointestinal disturbance, your veterinarian may suspect intestinal cancer. A physical examination yields a variety of results. When your veterinarian palpates (feels) your dog’s abdomen, they may discover a tumor or some swollen, uncomfortable portions of the small intestine. The lymph nodes and certain organs (such as the liver) may swell with fluid, and the abdomen may be bloated. A mass could also be discovered during a rectal examination.

“The findings with a physical examination are variable.”

Blood tests, urinalysis, radiology, endoscopy, and a biopsy will all be performed by your veterinarian. Blood tests and urinalysis can be used to detect alterations linked to paraneoplastic disorders. Radiographs (X-rays) may reveal an abdominal mass, while specialized imaging (a barium swallow) may reveal intestine ulcers, restricted intestinal motility, or obstruction. Ultrasound can also be useful for inspecting the layers of the intestinal wall and obtaining an ultrasound-guided fine needle or core needle biopsy.

This procedure involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a slide. A veterinary pathologist then examines the slide under a microscope.

Endoscopy, a procedure that uses an endoscope (a thin tube with a light and tiny camera at the end and through which forceps can be passed to take tissue samples) to diagnose the presence of a tumor in most areas of the intestine, can be helpful in diagnosing the presence of a tumor in most areas of the intestine. It can also be used to gather biopsy samples. These samples, like needle biopsies, are not always useful for diagnosis, and a surgical biopsy may be required to achieve a definite (correct) diagnosis.

Surgical biopsies can be obtained using laparoscopy, a technique involving the use of a laparoscope (a narrow, tube-like tool with a light and lens), or laparotomy, a procedure involving the use of a laparoscope (a small, tube-like instrument with a light and lens) (a surgery to open the abdomen). A veterinary pathologist examines the biopsies under a microscope to determine the type of cancer. This is referred to as histology. Histopathology can not only help with diagnosis, but it can also predict how the tumor will behave in the future.

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

Progression of Intestinal Cancer

The type of tumor and how it affects the body determine how intestinal cancer grows. Tumors grow at different rates. Some grow slowly, while others grow quickly. Both benign and malignant tumors will continue to grow if they are not treated, interfering with intestinal function and raising the risk of ulceration or intestinal blockage. Ulcerative tumors can cause perforation of the gut, allowing intestinal contents to pour into the belly, resulting in a life-threatening infection.

Since most intestine tumors are cancerous, they frequently spread to other parts of the body, including the lymph nodes, liver, and lungs, as well as other organs including the abdomen’s inner lining. Staging (looking for potential spread to other parts of the body) is extremely advised when there’s a probability of metastasis.

Blood tests, urinalysis, X-rays of the lungs, and potentially an abdomen ultrasound are all possible tests. More advanced imaging techniques, such as computed tomography (CT) scans or magnetic resonance imaging (MRI), are occasionally utilized (MRI). If any lymph nodes are larger or feel abnormal, further sampling may be done to see if there is any spread.

Traditional Treatment

The treatments for intestinal tumors 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 excision of metastasized tumors is primarily for palliative purposes, to alleviate symptoms and improve quality of life. In these cases, the long-term outlook is usually poor, with surgery offering only a few months of relief until the metastatic growths become problematic or the tumor grows back. Chemotherapy or radiation therapy may be used in conjunction with surgery in some circumstances. In some circumstances, such as colon adenocarcinomas, radiation therapy may be suggested.

“With most intestinal tumors, surgery is the treatment of choice.”

In dogs, intestinal lymphoma can be localized, growing as a mass in one region, or diffuse, spreading out as a general thickening of the intestinal wall. If surgery is possible, it will help to ease cancer’s symptoms, but whether or not the tumor is removed, lymphoma is usually treated with chemotherapy. When it comes to intestinal lymphoma in dogs and cats, chemotherapy is sometimes the best option. Radiation therapy may be recommended in specific instances.

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

Final Thoughts

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 of the cancer can be removed.

The degree of debilitation (e.g., weight loss and malnourishment) or other health concerns your pet has can sometimes restrict the prognosis. Your veterinarian or veterinary oncologist can advise you on the best course of action for your pet.

Read more:

Prognostic factors for dogs with surgically resected gastrointestinal stromal tumors

Gastrointestinal Stromal Tumours (GISTs)

Stomach cancer in dogs

Stomach tumors in dogs and cats

Published by Amber L. Drake

Dr. Amber L. Drake is a celebrated author and a distinguished cancer specialist, renowned for her comprehensive research in canine cancer prevention and nutrition. She is widely recognized for her commitment to helping dogs lead long and joyful lives, as well as for her contributions to veterinary medicine education. As the CEO of Canine Companions Co., the Founder of the Drake Dog Cancer Foundation and Academy, and the Co-Founder of Preferable Pups, she has become a respected and influential figure in the canine community, earning the admiration and respect of dog enthusiasts around the globe.

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