Canine Gastrointestinal Adenocarcinoma
The incidence of intestinal tumors in dogs is low in comparison to people. These tumors tend to occur in older dogs (>9 years) and signs vary from anorexia, vomiting, weight loss, lethargy to diarrhea, depending on the location of the tumor. Males and females tend to develop these tumors at equal rates, although there is a higher incidence of gastric (stomach) tumor formation in males than females. The most common gastrointestinal tumors in dogs are adenocarcinoma, leiomyosarcoma, and lymphosarcoma with, adenocarcinoma being the most common. Adenocarcinomas are often classified as infiltrative, expansile (sometimes with ulcer formation), polypoid, nodular/cobblestone, and annular based on the biopsy and gross appearance. A separate condition of benign polypoid adenoma formation sometimes affects the colon.
The initial staging consists of a CBC, serum chemistry, chest radiographs, and an abdominal ultrasound. This helps determine if the cancer has spread. Endoscopy is occasionally performed to determine the extent of the disease. Once the staging has been performed, we can determine the best treatment option, which includes some combination of surgery, chemotherapy, and non-steroidal anti-inflammatory drugs (NSAIDs), which are in the same class as aspirin and have been shown to have some anti-cancer effects. The use of NSAIDs alone can be curative for the benign polypoid condition. Like aspirin, other NSAIDs can damage the lining of the stomach over time in sensitive individuals or affect the kidneys so these organs are monitored with bloodwork while the dog is receiving this medication.
The above listed workup for gastrointestinal adenocarcinoma helps determine the extent of the tumor and if the cancer has spread. Unfortunately, with adenocarcinomas, some patients present with advanced disease, meaning it has already spread to other areas of the body such as the lymph nodes, mesentery (abdominal connective tissue), liver and/or lungs.
The main method of treatment for these tumors is surgical resection. It has been shown that patients having complete tumor removal will do substantially better than those without. With adenocarcinomas, surgery is generally followed with chemotherapy using a drug called Adriamycin (Doxorubicin) +/- Cytoxan, or in some cases, Carboplatin.
Based upon the results of surgery and the staging, we can offer some idea of prognosis. Although there is not a great deal of information in the veterinary literature with this cancer, extrapolating from human medicine, there are some factors that we use. They include histologic type of cancer, the spread of cancer to the lymph nodes or other areas of the body, development of abdominal fluid with circulating cancer cells, incomplete tumor removal with surgery, or extension of the tumor to the outer layer/surface of the intestine. Loco-regional metastasis to the lymph nodes is common (up to 70-80% chance) followed by the liver and lungs later in the course of the disease. Metastasis portends a poor prognosis, and the risk of metastasis increases with the tumor grade.
The overall prognosis depends on the tumor type, grade, and location. For instance, dogs with gastric tumors have an average prognosis of 6 months following surgery. Dogs with small intestinal tumors have an average prognosis of 10 months following surgery. The survival time of dogs with colon tumors has been associated with the appearance of the tumor, with survival times of 36 months, 12 months, and 1.6 months noted for pedunculated masses, nodular/cobblestone masses, and annular ring masses, respectively. Other factors such as diffuse disease/inability to completely excise the tumor, mitotic rate/tumor grade (aggressiveness of the tumor), presence of metastasis, and invasion into the vascular or lymphatic system suggest a shortened overall survival. If the tumor spreads to the lymph nodes, 50% of patients will not survive beyond 6 months, although the survival time should improve with chemotherapy.
In general, surgery remains the cornerstone of effective therapy for gastrointestinal tumors in dogs. The benefits of chemotherapy are unknown, but in many cases the expected survival times may increase 2-3 times beyond that of which are expected with surgery alone.