Tufts Cummings School of Veterinary Medicine, Tufts Medical Center Molecular Oncology Research Institute, North Grafton, MA, USA
Stomach cancer (gastric carcinoma) is considered a rare cancer in dogs, an impression that is reinforced by published literature on this disease. Most papers feature fewer than 20 cases, thus providing a very limited sketch of this disease. Several years ago, we reported that Chow Chows have a significantly increased risk for gastric cancer and began to study the disease in this breed. Subsequently, we have expanded our research to include other breeds at high risk, including the Belgian Tervuren, Belgian Sheepdog, Keeshond, Irish Setter, Bouvier, Norwegian Elkhound, Akita, and Scottish Terrier. Other breeds may also be at risk. We suspect that difficulty in accurately diagnosing dogs with this cancer may result in underreporting of its prevalence in dogs of all breeds, although in certain breeds the disease is quite common.
Several years ago we established the Canine Gastric Cancer Repository and Database to provide a tool to learn about canine gastric cancer and to develop strategies to prevent, diagnose and treat this aggressive and nearly uniformly fatal disease.
What Is Gastric Cancer?
Cancer develops from cells that grow uncontrollably and invade normal tissues. Cancer in the stomach can derive from a number of different cell types; therefore, many types of cancer can occur in the stomach. However, most cancers in the stomach derive from the epithelium or lining cells and are called gastric carcinoma (or adenocarcinoma). Therefore, most of the time, gastric cancer is considered synonymous with gastric carcinoma.
Varieties of classification systems that are based on microscopic appearance and position of the cancer in the stomach have been used to classify stomach cancer in people. One of the traditional systems (the Lauren System) consists of two groups: Intestinal type and diffuse type. Intestinal type forms lumps or masses on the surface, while diffuse type invades directly into the wall of the stomach, causing thickness without the development of a surface mass. While dogs appear to be capable of developing both types of gastric carcinoma, it appears that diffuse type is most common. A systematic review of the histology from more than 100 canine gastric cancer cases is currently underway.
What Causes Gastric Cancer in Dogs?
The short answer to this question is that we don't know. However, in humans, both environmental causes and genetics play a role. Environmental contributors include diet (salt and nitrites) and a bacterial organism called Helicobacter pylori. H. pylori does not appear to naturally infect dogs.
The occurrence of gastric cancer in particular breeds strongly suggests that genetics are important in the canine disease. We have found families with multiple individuals affected over multiple generations, which also supports this notion. Other evidence for this includes the high prevalence of diffuse carcinoma in dogs, which is associated with familial gastric cancer in people. The mode of inheritance is not clear. We are collaborating with Elaine Ostrander's lab at the NHGRI to identify gene(s) that cause canine gastric carcinoma.
What Are the Signs of Gastric Cancer in Dogs?
The signs of gastric cancer are usually insidious, particularly in the early stages. Consistently, we see loss of appetite and weight loss. In many cases, we also see vomiting, although it may be very intermittent and easy to ignore, because all else seems normal. Occasionally there is diarrhea or dark, tarry stool, which indicates intestinal bleeding. Because the signs of stomach cancer are very vague and nonspecific, it is unusual for veterinarians to see a dog until the disease is quite advanced.
How Is a Diagnosis of Gastric Cancer Made?
A presumptive diagnosis of gastric cancer can often be made based on abdominal ultrasonography. The difficulty is that gas in the stomach interferes with this imaging. Furthermore, a distinct mass is often lacking in these cases. Thickness of the gastric wall may be the best indication of cancer. This underscores the importance of routinely evaluating wall thickness, particularly in dogs of high-risk breeds with gastrointestinal signs.
Definitive diagnosis of gastric cancer is based on biopsy. The least invasive way to obtain a biopsy is with endoscopic biopsy. However, in many cases the diagnosis is missed on endoscopy. The inaccuracies in endoscopic biopsy stem from the nature of most stomach cancers in dogs in which the surface lining may look relatively normal even though there is substantial infiltration of the stomach wall by cancer cells. Therefore, many times veterinarians cannot determine where biopsies should be collected. Furthermore, endoscopic biopsies may miss the cancer cells that are more deeply embedded. Finally, the areas of affected stomach often become very firm and almost rubbery in consistency, and biopsying these areas may yield inadequate tissue.
Surgical biopsy has the best chance of providing an accurate diagnosis, but this is, of course, more invasive. However, in addition to biopsy, there may also be the opportunity to surgically remove the cancer.
How Is Gastric Cancer Treated?
Surgery is the treatment of choice for gastric carcinoma. However, the removal of a cancer from the stomach is challenging. When the cancer is very advanced, involving a large proportion of the stomach, removal is not usually feasible. The location of the cancer is also a deciding factor. In our experience, removal of the cancer is not always possible, and it is uncommon for the surgeon to be able to remove it completely. Even partial removal can provide some relief to the dog, though this is not always true. We are aware of a single dog that lived for an additional 5 years following removal of a gastric tumor and several others that survived for a year or more. However, the vast majority of dogs will die of stomach cancer in time.
Several chemotherapy drugs have been used in dogs with stomach cancer, although the effectiveness of these is questionable.
The future of the management of stomach cancer will rely on development of genetic tests to identify at-risk individuals, selective breeding in some cases, development of screening tests, and the development of new agents that target the molecular constitution of stomach cancer. These types of advances are only possible with better understanding of the genetics and molecular biology of canine stomach cancer.