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Gastrointestinal Neoplasia |  |
| Neoplasia of the GI system is uncommon and represents <1% of all cancers in small animals. GI cancer most commonly develops in the rectum and colon of dogs and in the small intestine of cats. Older animals are predisposed, and adenocarcinoma and lymphosarcoma are seen more frequently in male dogs. Colorectal tumors are more prevalent in Boxers, German Shepherds, Poodles, Great Danes, and spaniels. |
| Etiology and Pathophysiology: |
| No specific cause(s) has been identified for most GI tumor types, although alimentary lymphoma in cats is believed to be caused by the feline leukemia virus (FeLV), even in FeLV-negative cats. It has been suggested that the severe form of inflammatory bowel disease in cats may transform into lymphosarcoma, although to date no substantive evidence exists. Intestinal neoplasms tend to be malignant in small animals (88% of nonlymphoid tumors in dogs and all nonlymphoid tumors in
cats). |
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| Clinical Findings: |
| Clinical signs vary depending on the location and extent of the tumor and associated paraneoplastic consequences, eg, hypercalcemia (
Hypercalcemia in Dogs and Cats: Overview). Vomiting sometimes with blood, diarrhea also with blood, weight loss, constipation, tenesmus, abdominal pain, ascites, and peritonitis associated with rupture of affected bowel have been reported. Affected animals may also have clinical signs of anemia. |
| Adenomas are extremely rare in dogs and cats. In dogs, adenomas most often involve the rectum but have also been found in the stomach and colon. These tumors tend to form polyps that may range in size from 0.5-3 cm. Adenomatous polyps may progress to carcinoma in dogs. Affected dogs may have clinical signs of GI obstruction or bleeding. |
| Adenocarcinoma is the most common GI tumor type in dogs, in which the stomach, colon, and rectum are most frequently involved. In cats, the small intestine is most often affected. Gastric adenocarcinomas of the stomach tend to affect the pylorus and may ulcerate or invade the muscularis, which gives the stomach a “leather bottle-like” appearance. Metastasis to mesenteric lymph nodes is common, and adhesions and peritoneal implantation occur. |
| Leiomyomas and leiomyosarcomas appear as firm, white, lobulated masses throughout the GI tract, most commonly in the cecum and jejunum. Gastric leiomyomas appear as round, smooth, mucosal-covered masses, most often located along the lesser curvature near the cardia. They may be ulcerated. Leiomyosarcomas are locally invasive and slow to metastasize. Dogs with leiomyosarcoma tend to be aged (~12 yr). Biopsy is required to differentiate the benign from the malignant forms of the
disease. |
| Lymphosarcomas may arise anywhere throughout the GI tract but are more often seen in the stomach of dogs and in the ileum of cats. Lymphoma was thought to be the most common tumor involving the GI tract in cats, but more recent reports indicate that adenocarcinoma may be the most common malignant neoplasm of the large intestine in cats. Lymphoma tends to affect older cats, most of which are FeLV-negative. These tumors tend to have a large, swollen, ellipsoid appearance and may
invade all layers of the gut. Ulceration and metastasis to local mesenteric lymph nodes are common. |
| Mast cell tumors are found in cats and are located in the muscularis of the small intestine. They are also reported in the large intestine of cats. These tumors appear pale, firm, and diffusely swollen. A primary mast cell tumor of the ileocecal area has been reported in a dog. |
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| Diagnosis: |
| Diagnosis is based on an appropriate history and physical examination and confirmed by histologic evaluation. A mass may be detected on abdominal palpation and confirmed by plain and contrast radiographs or by abdominal ultrasound. Rectal examination may reveal evidence of a palpable mass lesion or bleeding. Biopsy samples may be taken at the time of a laparotomy or endoscopically. If samples are taken during endoscopy, it is important to take several samples from different
areas and to take deep biopsies of suspect areas in an attempt to establish the diagnosis and to determine its extent. Deep-seated tumors, eg, leiomyomas and leiomyosarcomas, may not be detected by endoscopic biopsy because generally only the mucosa and superficial submucosa are sampled. In another study, 30% of canine rectal lesions were incorrectly diagnosed by endoscopic biopsy because samples were insufficient. |
| A CBC, biochemical profile, and urinalysis should be completed to determine the extent of concurrent illness. Other than anemia or hypoproteinemia, these tests are often normal. Pulmonary metastasis is rarely documented on thoracic radiographs. |
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| Treatment: |
| Surgical resection is the preferred treatment. Margins of 4-8 cm should be included in the resected area. Biopsies of suspect lesions in other areas should also be taken to determine the extent of metastatic disease. |
| Prognosis depends on tumor type and the ability to remove all of it. The prognosis for longterm survival of dogs is poor with leiomyosarcoma but excellent with leiomyomas. Recurrence after surgical resection is uncommon. Metastasis develops in up to 74% of dogs with gastric adenocarcinoma; efficacy of chemotherapy is unknown, and clinical signs have recurred from 3 days to 10 mo after surgery. The longterm prognosis is poor. |
| Local canine lymphoma responds better to chemotherapy than the diffuse form of disease. Local resection of feline intestinal lymphosarcoma has occasionally been curative. Cats with alimentary lymphoma treated with L-asparaginase, vincristine, cyclophosphamide, methotrexate, and prednisone had a mean survival time of 25.3 wk. In the same study, cats that received prednisone alone had a mean survival time of 7.3 wk. Cats with alimentary lymphoma are reported poorly responsive to
treatment with vincristine, cyclophosphamide, and prednisone (median survival 50 days), but a few affected cats may have long survival times. |
| High-dose radiotherapy may be considered for select cases of rectal adenocarcinoma. The affected section of rectum is prolapsed with stay sutures, and doses of radiation are directly applied to the tumor and immediate area. Cats with colonic adenocarcinoma should undergo subtotal colectomy; adding doxorubicin may increase survival times. |
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