What is a tumor of the stomach?
A tumor of the stomach is an abnormal proliferation and dysregulated replication of the cells that make up the stomach. Stomach tumors usually grow from the cells of the inner lining of the stomach or muscle that surrounds the lining.
Stomach 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 stomach are malignant.
Gastric (stomach) tumors are uncommon in dogs and cats. Leiomyosarcoma is the most common cancer in dogs, and lymphoma the most common cancer in cats. Other gastric tumors in dogs include lymphoma, the smooth muscle tumors (leiomyomas and leiomyosarcomas), adenocarcinomas, mast cell tumors, fibrosarcomas, plasmacytomas, and gastrointestinal stromal tumors (GISTs), among others. Rarely, dogs can have benign adenomatous polyps.
"Gastric (stomach) tumors are uncommon in dogs and cats. Gastric (stomach) tumors are uncommon in dogs and cats."
In cats, the next most common tumors include leiomyosarcomas, adenocarcinomas, mast cell tumors, plasmacytomas, carcinoids, GISTS, and others. Cats can also, but less commonly, develop benign tumors, including adenomatous polyps, adenomas, and leiomyomas.
What causes this cancer?
The reason why a particular pet may develop this, or any other tumor or cancer, 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. In the case of stomach tumors, age, sex, and breed appear to be risk factors.
Stomach tumors tend to develop in older dogs and cats. Males are generally at higher risk than females, both for benign and malignant tumors. Certain breeds are at an increased risk, including Beagles for leiomyoma, and Chow Chows, Rough Collies, Staffordshire Terriers, Belgian Shepherds, Norwegian Lundehunds, and Dutch Tervueren Shepherds for adenocarcinoma. This may indicate a genetic predisposition. Siamese cats also appear to be predisposed to adenocarcinoma.
In dogs, gastric adenocarcinoma has been linked to the long term feeding of nitrosamines (a type of chemical found in many foods) in the diet.
What are the signs of stomach tumors?
The signs of stomach tumors develop gradually over weeks to months. They include chronic intermittent vomiting, lack of appetite, lethargy, and weight loss. Vomit may be blood-tinged or have a “coffee grounds” appearance. This is related to tumor ulceration (opening) causing bleeding. The bleeding may cause the stool may become blackish. Chronic bleeding can lead to anemia (low circulating red blood cells) causing paleness of the gums. Sometimes excessive salivation may be seen, a symptom of nausea.
Pets with gastric leiomyomas or leiomyosarcomas (muscle tumors) can develop a lower than normal blood glucose (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.
Cats and dogs with adenomatous polyps often have no symptoms of cancer.
How is this cancer diagnosed?
Your veterinarian may suspect stomach cancer in older dogs or cats with a history of chronic vomiting, lack of appetite, and weight loss. The findings with a physical examination are variable. If your pet’s stomach is markedly enlarged, your veterinarian may be able to palpate (feel) a gastric mass or gastric thickening (sometimes painful). Abdominal palpation may also reveal enlarged lymph nodes.
"Bloodwork and urinalysis are helpful to find the changes associated with the paraneoplastic syndromes."
Your veterinarian will perform tests such as blood tests, urinalysis, imaging, endoscopy, or surgery and a biopsy. Bloodwork and urinalysis are helpful to find the changes associated with the paraneoplastic syndromes. Radiographs (X-rays) may show a thickened gastric wall or displacement of the stomach, and specialized imaging (a barium swallow) may show ulceration in the stomach, reduced stomach movement, or gastric obstruction.
Ultrasound is also helpful, especially to examine the layers of the stomach wall and to obtain 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 (i.e., a thin tube with a light and tiny camera at the end, and through which forceps can be passed to take tissue samples) can be useful to diagnose the presence of a tumour and collect biopsy samples. As with needle biopsies, these samples are not always valuable for diagnosis and instead a surgical biopsy may be needed to obtain a definitive (accurate) 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). To identify the type of cancer, the biopsies are 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.
How does this cancer typically progress?
How cancer of the stomach 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 will continue to grow, increasingly interfering with stomach functioning and risking ulceration or gastric obstruction. In some cases ulcerative tumors can lead to perforation of the stomach, with the spillage of stomach contents into the abdomen leading to a life-threatening infection (septic peritonitis).
As most stomach tumors are malignant, most 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. This 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. Staging ‘sets the stage’ for the most appropriate plan of care.
What are the treatments for this type of tumor?
The treatments for stomach tumors depend on the type of tumor and extent to which it has grown and spread. With most stomach tumors, surgery is the treatment of choice.
"The treatments for stomach tumors depend on the type of tumor and extent to which it has grown and spread."
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.
Gastric lymphoma in dogs and cats can be localized (growing as a mass in one location) or diffuse (spread out as a general thickening of the stomach wall). Surgery is advised if it is possible, as it will help alleviate the signs of the cancer, but typically lymphoma is treated with chemotherapy whether or not the tumor is removed. Sometimes chemotherapy is the preferred treatment approach with gastric 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 the paraneoplastic syndrome will resolve. After surgery, smaller, more frequent meals may be necessary.
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.