GI neoplasia is uncommon, with gastric tumors representing <1% and intestinal tumors <10% of all cancer in dogs and cats. Neoplasms tend to be malignant. The average age of dogs with GI neoplasia is 6–9 yr, and of cats is 10–12 yr.
Etiology and Pathophysiology
Specific etiologic agents for GI neoplasia have not been identified. The increased risk of Belgian Shepherds for gastric carcinoma, and of Siamese cats for intestinal adenocarcinoma and lymphoma, may reflect genetic predispositions. Feline leukemia has been suggested to be an underlying factor in the development of feline GI lymphoma, even in cats with a negative retroviral status. Helicobacter infections are associated with gastric neoplasia in people, but similar direct links have not been established in dogs or cats.
Adenocarcinomas are the most common canine GI neoplasia, and are most often found in the duodenum, colon, and rectum; gastric adenocarcinomas frequently affect the lower third of the stomach (ie, the lesser curvature and pyloric region). In cats, adenocarcinoma is commonly identified in the jejunum and ileum. Adenocarcinomas are aggressive and frequently metastasize to regional lymph nodes, liver, and lung. At the time of diagnosis in dogs, up to 44% of intestinal and up to 95% of gastric adenocarcinomas have metastasized.
Adenomas and carcinomas in situ are uncommon in the GI tract of dogs and rare in cats. These polyp-like masses are usually solitary and located in the colon or rectum in dogs. They are considered local diseases, although there is speculation that they may transform into aggressive adenocarcinomas over time.
Lymphoma is the most common feline GI neoplasia and is also common in dogs. GI lymphoma most often affects the small intestine as well as extra-intestinal organs such as the liver. Two subtypes of feline GI lymphoma have been reported: a low-grade, small-cell lymphocytic lymphoma and a poorly differentiated, aggressive subtype of lymphoma. Canine GI lymphoma is usually poorly differentiated and aggressive.
Leiomyomas and leiomyosarcomas are uncommon in dogs and extremely rare in cats. Tumors are slow growing; ∼80% of leiomyosarcomas have not metastasized at the time of diagnosis. These stromal tumors are associated with a paraneoplastic hypoglycemia that should resolve once the tumor is resected. Other uncommonly reported GI neoplasms in dogs and cats include fibrosarcomas, mast cell tumors, and plasmacytomas.
Clinical signs of GI lymphoma depend on the location and extent of the tumor and its possible metastases or paraneoplastic syndromes (eg, hypercalcemia, hypoglycemia). The most common clinical signs associated with GI neoplasia include vomiting (with or without blood), anorexia, weight loss, diarrhea, and lethargy. Signs of constipation or tenesmus may accompany colonic and rectal tumors. An abdominal mass or organomegaly may be palpable. Abdominal pain and ascites may reflect peritonitis secondary to a ruptured portion of neoplastic bowel.
Routine laboratory testing and plain radiographs do not show specific changes associated with GI neoplasia. Hypoglycemia is often associated with leiomyoma/leiomyosarcomas. Hypercholesterolemia and elevated alkaline phosphatase activity has been observed in some nonlymphomatous neoplasia. Microcytic anemia with or without hypoproteinemia is a common finding with ulcerated masses and chronic blood loss. Electrolyte and acid-base disturbances may reflect ongoing vomiting and can include hypochloremia, hypokalemia, and metabolic alkalosis or acidosis. Paraneoplastic hypercalcemia has been associated with lymphoma and intestinal adenocarcinoma.
Contrast abdominal radiographs may reveal masses in the GI tract or areas of ulceration. Abdominal ultrasonography may show focal or diffuse thickening of the GI tract and loss of normal layering. Regional lymph nodes may be enlarged, and splenomegaly and/or hepatomegaly may be seen in some cases of GI lymphoma. Ultrasonography can facilitate fine needle aspiration or needle biopsy sample collection for cytologic or histologic analysis.
Upper and lower GI endoscopy can facilitate identification and partial thickness biopsy. The small size and superficial nature of biopsy samples collected during endoscopy are not definitive because some tumors are submucosal. A false diagnosis of gastritis or enteritis may reflect inflammation of the mucosa overlying a neoplastic process. Full-thickness surgical biopsies collected via laparoscopy or laparotomy are more suitable for establishing a diagnosis and allow for biopsy of regional lymph nodes and liver to evaluate for metastasis.
Treatment and Prognosis
Surgical excision is recommended for nonmetastatic, nonlymphomatous neoplasia; margins ≥4 cm are recommended, if possible.
The median survival time for GI adenocarcinoma in dogs is 10–15 mo if the tumor is focal and completely resected but only 3 mo if metastasis is present at the time of diagnosis. Effective chemotherapy for treatment of GI adenocarcinoma has not been reported.
Gastrointestinal lymphoma is typically treated with chemotherapy. Well-differentiated, low-grade small-cell lymphoma is treated with prednisone (5 mg, PO, sid-bid) and chlorambucil (either 2 mg, PO, every other day or 15 mg/m2, sid for 4 days, every 3 wk). Prognosis associated with small-cell GI lymphoma is good, with a median survival time of 765 days reported in a recent study. Poorly differentiated GI lymphoma in dogs and cats is poorly responsive to chemotherapy. If treatment is undertaken, a multidrug chemotherapy protocol (eg, Wisconsin-Madison) is recommended, but reported median survival times are usually <2 mo. Focal lymphoma may be surgically excised. Follow-up chemotherapy may be recommended depending on the type and nature of the neoplastic process.
Malignant GI neoplasia usually has a poor prognosis (ie, <6 mo survival), even with surgical and medical therapy. Benign lesions, such as leiomyomas and colorectal adenomas, have a good prognosis with surgical excision.
Last full review/revision March 2012 by Shauna L. Blois