Gastrointestinal neoplasms in dogs and cats (Proceedings)
Primary gastrointestinal (GI) neoplasia is an important differential diagnosis for a dog or cat with vomiting or diarrhea, especially chronic vomiting or diarrhea, anorexia and weight loss, particularly animals that are middle-aged and older. Some GI neoplasms have been associated with paraneoplastic syndromes such as hypoglycemia and polyuria/polydipsia, which could provoke additional clinical complaints. Depending on the tumor type and location, patients with GI neoplasia can have extensive disease at the time of definitive diagnosis, often worsening the prognosis. Thus, early diagnosis becomes important to improving clinical outcomes.
Etiology and pathophysiology
The most common primary gastric tumor in dogs is adenocarcinoma in various forms; lymphosarcoma and smooth muscle tumors (leiomyomas, leiomyosarcomas) are considered the next most common tumors. Dogs with gastric tumors are most commonly middle-aged to older dogs. The antrum and lesser curvature are the most common sites in which adenocarcinomas are found in dogs. Common metastatic sites of gastric adenocarcinomas include regional lymph nodes, liver, spleen, omentum/peritoneal cavity, and lung. The most common primary gastric tumor in cats is lymphosarcoma, which can have a diverse array of appearances from infiltrative to polypoid lesions. Gastric adenocarcinomas can be seen in cats, but are considered rare. In both dogs and cats, gastric lymphosarcoma is commonly a part of more diffuse gastrointestinal lymphosarcoma, but can be seen as a solitary tumor in the stomach. Extramedullary plasmacytomas have been described in both dogs and cats as primary gastric or bowel tumors.The most common intestinal tumor in cats is lymphosarcoma; in dogs, intestinal lymphosarcoma intestinal adenocarcinoma and intestinal smooth muscle tumors are seen with around the same frequency although lymphosarcoma is reported a bit more often than the other tumor types. Other tumor types that have been described in the bowel include leiomyomas, leiomyosarcomas; gastrointestinal stromal tumors (GIST), plasma cell tumors, mast cell tumors, carcinoids (tumors of neuroendocrine origin) and extraskeletal osteosarcomas. Benign polyps can be found in the duodenum of cats or the rectum of dogs. GISTs have been recently described as tumors that arise from the interstitial cells of Cajal, cells that regulate intestinal motility and peristalsis. GISTs have histological features that are very similar to intestinal smooth muscle tumors, but are distinguished by positive staining for c-kit (CD117). In one report that characterized GIST, there was a predilection for this tumor in dogs to be seen in the cecum and large intestine whereas the smooth muscle tumors were more likely to be seen in the stomach and small intestine; only the dogs with GIST had hypoglycemia. The paraneoplastic syndromes attributed in the older literature to leiomyomas/leiomyosarcomas have the potential to be a result of GIST. Intestinal adenocarcinomas in dogs are seen more often in the large intestine than the small intestine.
Animals with GI tumors are most commonly middle-aged to older. Common metastatic sites of non-lymphoid intestinal tumors include regional lymph nodes, liver, spleen, omentum/peritoneal cavity and lung.
GI tumors can disrupt normal gastrointestinal motility likely as a consequence of disruption of the intestinal smooth muscle wall and/or accompanying inflammatory changes, or cause obstructive disease. Ulceration of the GI mucosa is a feature of GI tumors in some patients, but will not be present in all. In patients with GI bleeding secondary to mucosal ulceration, chronic blood loss can lead to iron deficiency anemia, which can be either regenerative or non-regenerative. Because indices and erythrocyte morphology don't always betray the existence of regenerative responses, obtaining a reticulocyte count in patients with undefined anemia becomes an important diagnostic step as reticulocytosis indicates that anemia is from either hemorrhage, or hemolytic disease.