Stomach and intestinal tumours in dogs

Overview

A tumour develops when control of cell multiplication in the stomach or intestine is disrupted, producing abnormal tissue growth. Most tumours in these organs are malignant, meaning they can invade surrounding tissues and spread to distant sites.

Stomach tumours are estimated to account for less than 1% of all tumours in dogs and cats. In the same small-animal group, intestinal tumours account for less than 10% of all tumours.

In dogs, adenocarcinoma, a cancer of glandular tissue, is the most common tumour of the stomach and colon. Lymphoma, a cancer of white blood cells, is the most common small-intestinal tumour.

The intestine can also develop gastrointestinal stromal tumours, or GISTs, which arise from the interstitial cells of Cajal that regulate intestinal movement. Smooth muscle tumours include leiomyoma and leiomyosarcoma.

A benign polyp or leiomyoma behaves differently from rapidly progressive intestinal lymphoma.

Causes

The cause of a tumour usually cannot be identified

A single cause of a stomach or intestinal tumour is usually unknown. A combination of inherited and environmental risk factors may be involved.

The same direct link between Helicobacter and stomach cancer as in people has not been demonstrated in dogs. Intestinal lymphoma may form a local mass or thicken the bowel wall over a large area.

Intestinal lymphoma can also affect other organs, such as liver tissue. A growing mass can narrow the digestive passage and disrupt normal function.

Ulceration of a tumour's surface can cause bleeding into the digestive tract. Some muscle tumours release insulin-like substances that can lower blood glucose.

Tumour disease can also increase protein loss through the intestine.

Risk factors

Stomach and intestinal tumours are usually identified in middle-aged or older dogs, often around 6–9 years old. An older typical age group of around 9–13 years has also been described for intestinal tumours.

Benign gastric leiomyomas are found particularly in dogs aged 10–15 years. Predisposition to gastric adenocarcinoma is increased in the Tervueren and Groenendael varieties of Belgian Shepherd Dog.

Predisposition to gastric carcinoma has also been reported in the Norwegian Lundehund. Stomach tumours occur slightly more often in males than females.

A similarly clear sex predisposition has not been identified for intestinal tumours. Intestinal smooth muscle tumours occur particularly in large dogs, especially German Shepherd Dogs.

Typical symptoms

Signs depend on whether the tumour is in the stomach, small intestine, colon or rectum.

• Your dog vomits intermittently over weeks or months.

• Appetite declines and your dog gradually loses weight.

• Vomit contains blood or dark material resembling coffee grounds.

• Stools become black because of gastrointestinal bleeding.

• With a colonic tumour, stools may contain red blood and defecation requires straining.

• Stools may become narrower than usual or ribbon-like.

• Your dog is subdued or its abdomen looks swollen.

Diarrhoea can occur with both small- and large-intestinal tumours, but is not present in every dog. Benign polyps may cause no symptoms at all.

Low blood sugar associated with some muscle tumours can also cause trembling, confusion or seizures. Tumour-related increases in blood calcium can increase drinking and urination.

Complications and consequences

Obstruction, bleeding and spread elsewhere

An ulcerating tumour can perforate the stomach or intestinal wall and cause life-threatening peritonitis. Long-term bleeding can lead to anaemia, reducing red blood cells and the blood's oxygen-carrying capacity.

Up to 77% of intestinal adenocarcinomas have spread to distant sites by diagnosis. For gastric adenocarcinoma, a corresponding proportion of up to 32% has been reported.

Metastases may be found in regional lymph nodes, the liver or lungs. Persistent straining associated with a colonic tumour can also lead to prolapse of the rectal lining.

The outlook for adenocarcinoma depends on its location

In a surgical follow-up of gastric adenocarcinoma, 34 of 40 dogs, or 85%, were discharged from hospital. Median survival among those discharged was 6 months after surgery.

Reported median survival for small-intestinal adenocarcinoma is 4–18 months. Among dogs with small-intestinal adenocarcinoma, 40–60% have been alive at 1 year.

For adenocarcinoma of the colon and rectum, median survival after surgery has been 2–4 years.

Different forms of lymphoma and GIST behave differently

With high-grade gastrointestinal lymphoma, median survival is often only 2–3 months despite treatment. Colonic lymphoma is an exception, with reported median survival after treatment of 5.5–6 years.

Dogs with low-grade intestinal lymphoma have had median survival times of 14–21 months. Around 10% of canine low-grade intestinal lymphomas transform into high-grade lymphoma.

Completely removed GISTs and leiomyosarcomas have had median survival of around 42 months. However, metastases from GISTs and leiomyosarcomas have been reported in up to 30% of dogs.

How survival figures apply depends on tumour type, spread and the success of removal.

Diagnosis at the vet

Examination and imaging locate the change

The vet assesses weight and general condition and feels the abdomen for a mass or pain. A change in the colon or rectum may also be found during rectal examination.

A blood count and blood biochemistry help assess bleeding, nutritional status and organ function. However, routine blood tests do not provide a definite tumour-specific diagnosis.

Abdominal ultrasound may show bowel-wall thickening and loss of the normal layered structure. Ultrasound also helps assess regional lymph nodes and plan the sampling site.

A normal ultrasound does not rule out a tumour, particularly in the stomach. Chest radiographs or computed tomography look for possible lung metastases before treatment is decided.

Computed tomography may help assess a colonic or rectal lesion within the pelvic cavity that ultrasound cannot reach.

A tissue sample determines the tumour type and treatment approach

Endoscopy examines the inner surface of the stomach or intestine with a camera, and suspicious areas can be biopsied. Endoscopic samples usually reach only the lining, so a tumour deeper in the wall may be missed.

Endoscopy also cannot examine much of the dog's jejunum. Surgery allows a full-thickness bowel biopsy and samples from lymph nodes or the liver.

On histopathology, a pathologist identifies the tumour type and assesses how aggressively it is likely to behave. Distinguishing high-grade from low-grade lymphoma affects treatment selection.

Identifying lymphoma's B-cell or T-cell subtype provides additional prognostic information alongside grade. Special stains may distinguish GIST from other bowel-wall tumours, for example.

Uncertain lymphoma can also be investigated using PARR testing, which looks for a population of similar lymphocytes. Such a clonal finding can occur with inflammation too, so results are interpreted alongside histopathology and other findings.

Treatment

A localised tumour is often treated surgically

Your vet selects treatment according to tumour type, spread and your dog's general condition. Supportive care starts immediately in a seriously affected dog while investigation of the cause proceeds.

Surgical removal of a localised adenocarcinoma, GIST or leiomyosarcoma is attempted if no metastases have been identified. When removing a small-intestinal tumour, the surgeon aims where possible for healthy margins of at least 3 cm on either side.

Removing a tumour at the stomach outlet may require construction of a new connection between the stomach and intestine. Complex surgery carries risks of peritonitis, bleeding and other serious complications.

An obstruction caused by a tumour that has spread may sometimes be relieved with palliative surgery. The aim of such treatment is to ease symptoms and improve quality of life.

Lymphoma and some GISTs need medication

The main treatment for intestinal lymphoma is usually chemotherapy acting throughout the body. A local lymphoma mass may sometimes be removed, but medication is still recommended for high-grade disease.

A glucocorticoid and an alkylating chemotherapy drug may be used for low-grade small-cell lymphoma. Complete remission in lymphoma means that detectable signs of disease disappear during treatment.

The benefit of chemotherapy after adenocarcinoma surgery has not been reliably demonstrated. A targeted tyrosine kinase inhibitor may be considered for an inoperable or aggressive GIST.

In a small GIST follow-up, a measurable treatment response was identified in 4 of 7 dogs. Median progression-free time in that group receiving a targeted medicine was 26 months.

Radiotherapy may be considered in selected cases, such as some colonic adenocarcinomas. After stomach surgery, a dog may need smaller but more frequent meals.

Successful removal of a muscle tumour may also resolve the effects of its hormone-like substances elsewhere in the body.

Prevention

There is no known reliable way to prevent stomach and intestinal tumours in all dogs. Several genes contribute to the inherited risk of gastric adenocarcinoma.

A study involving DNA from 470 dogs identified more than 15 genes associated with lifetime gastric adenocarcinoma risk. A test or assessment based on inherited risk is still being developed to help identify susceptible dogs.

The aim of the risk assessment under investigation is to reduce stomach cancer through planned breeding decisions and target early investigations towards dogs identified as susceptible.

Important for dog owners

Record your dog's weight, appetite, timing of vomiting and stool changes. Photograph any unusual colour in stools or vomit for the appointment. Explain whether your dog has lost weight despite an unchanged diet and whether weakness or abdominal pain has accompanied the signs.

Weight loss and malnutrition can reduce treatment options alongside other diseases.

Ask the veterinary practice for written information on tumour type, grade and spread. Agree on the aim of treatment and which changes to monitor at home. If surgery or medication is not chosen, agree on symptom-relieving care and situations requiring reassessment.

Frequently asked questions

Is an intestinal tumour always cancer?

The intestine develops both benign and malignant tumours, although most are malignant. Tumour type is confirmed from a tissue sample. The result helps assess behaviour and select suitable treatment. Even a benign tumour can disrupt bowel function as it grows.

Does a normal ultrasound rule out stomach cancer?

A normal ultrasound does not definitively rule out a stomach tumour. Endoscopy allows inspection of the stomach lining and biopsy collection. If a lesion lies deeper in the wall, diagnosis may require a surgical sample. A superficial mucosal biopsy does not always reach a deeper wall tumour.

Is intestinal lymphoma treated like adenocarcinoma?

The main treatment for lymphoma is usually chemotherapy. Localised adenocarcinoma is primarily treated surgically if it can be removed and no metastases have been identified. In lymphoma, grade influences medication selection. The effectiveness of chemotherapy for adenocarcinoma, however, is uncertain.

How long can a dog live with a tumour?

The outlook depends on tumour type, location and spread. Median survival of 4–18 months has been described for small-intestinal adenocarcinoma. For colonic and rectal adenocarcinoma, median survival after surgery has been 2–4 years. Completeness of removal also affects an individual dog's outlook.

Can diet replace tumour treatment?

Treatment of a localised adenocarcinoma or GIST usually relies on tumour removal. Lymphoma generally requires chemotherapy. Feeding arrangements support recovery, and meal size and frequency may need changing after stomach surgery. Weight loss and malnutrition can worsen the overall situation.