Gastrinoma

Gastrinoma is a rare, functional neuroendocrine tumor (NET) of the pancreatic islet cells or, less commonly, the duodenum or other gastrointestinal sites, that autonomously secretes excessive gastrin. This leads to hypergastrinemia, which stimulates profound gastric acid secretion, resulting in severe, recurrent, and often refractory gastroduodenal ulceration, esophagitis, and diarrhea. In veterinary medicine, gastrinoma is most frequently diagnosed in middle-aged to older dogs, with a slight breed predisposition in certain purebreds, and is extremely rare in cats. The tumor is often malignant, with a high metastatic potential to regional lymph nodes and the liver. The clinical syndrome is analogous to Zollinger-Ellison syndrome in humans. The disease is characterized by a classic triad of hypergastrinemia, gastric acid hypersecretion, and severe peptic ulceration. Early recognition and aggressive management are critical, as the condition is often fatal due to complications such as perforation, peritonitis, or metastatic disease.

The exact etiology of gastrinoma is unknown, but it is believed to arise from neoplastic transformation of pancreatic G cells or neuroendocrine cells in the duodenum. No specific viral, bacterial, or environmental cause has been identified. In humans, gastrinomas are associated with multiple endocrine neoplasia type 1 (MEN1) syndrome, but a similar hereditary syndrome is not well-documented in dogs or cats. Sporadic mutations in genes involved in cell cycle regulation, such as MEN1, have been hypothesized but not confirmed in veterinary species. The tumor secretes gastrin in an unregulated manner, leading to hypergastrinemia. Chronic hypergastrinemia causes hyperplasia of gastric enterochromaffin-like (ECL) cells and parietal cells, further exacerbating acid secretion. The neoplastic cells may also secrete other peptides, such as pancreatic polypeptide, but gastrin is the primary driver of clinical signs.

Gastrinoma is an extremely rare tumor in dogs and cats. In dogs, it typically affects middle-aged to older animals, with a median age of around 8-10 years. No strong sex predilection is reported, but some studies suggest a slight male predominance. Certain breeds may be overrepresented, including Golden Retrievers, Labrador Retrievers, German Shepherds, and mixed-breed dogs, but this is based on limited case series. In cats, gastrinoma is exceedingly rare, with only a few case reports. There is no geographic or seasonal variation. The tumor is often malignant, with metastasis to regional lymph nodes and the liver at the time of diagnosis in up to 50-80% of cases. Because of its rarity, the true incidence is unknown, but it should be considered in any dog with severe, recurrent gastrointestinal ulceration or diarrhea that is unresponsive to standard therapy.

Gastrinoma pathophysiology centers on the unregulated secretion of gastrin from neoplastic neuroendocrine cells. Gastrin acts on cholecystokinin-2 (CCK2) receptors on gastric parietal cells to stimulate acid secretion, and on enterochromaffin-like (ECL) cells to release histamine, which further augments acid production. The resultant hyperchlorhydria overwhelms the mucosal defense mechanisms, leading to peptic ulceration in the stomach, duodenum, and sometimes the jejunum. The high acid load also inactivates pancreatic enzymes and damages the intestinal mucosa, causing maldigestion and malabsorption, which manifests as chronic diarrhea and weight loss. Additionally, hypergastrinemia induces trophic effects on the gastrointestinal mucosa, leading to hypertrophy of gastric folds and ECL cell hyperplasia. The tumor itself may cause local effects such as pancreatic mass, biliary obstruction, or invasion into adjacent structures. Metastatic spread, particularly to the liver and regional lymph nodes, contributes to systemic illness and poor prognosis.

Predisposing factors for gastrinoma are poorly understood due to its rarity. Genetic predisposition is suspected, as certain breeds appear to be overrepresented, but no specific genetic markers have been identified. Age is a risk factor, with most cases occurring in middle-aged to older dogs. There is no evidence for dietary, environmental, or infectious causes. Concurrent endocrine neoplasia, such as multiple endocrine neoplasia (MEN) syndromes, has been reported in humans but is not well-documented in veterinary patients. Chronic hypergastrinemia from other causes, such as renal failure or gastric outlet obstruction, may lead to ECL cell hyperplasia but does not directly cause gastrinoma. Immunosuppression or chronic inflammation has not been linked to gastrinoma development.

Clinical signs of gastrinoma are primarily related to severe gastrointestinal ulceration and hypersecretion. The most common signs include chronic vomiting (often with blood), diarrhea (sometimes with melena), anorexia, weight loss, and lethargy. Abdominal pain may be evident on palpation. In some cases, signs of acute abdomen may occur due to gastric or duodenal perforation, leading to peritonitis and shock. Physical examination may reveal pale mucous membranes due to anemia from chronic blood loss, dehydration, and poor body condition. In advanced cases, a cranial abdominal mass may be palpable if the pancreatic tumor is large. Cats with gastrinoma may present with similar signs, but vomiting and weight loss are more prominent. The severity of clinical signs can wax and wane, but the disease is progressive. Some patients may present with signs mimicking inflammatory bowel disease or exocrine pancreatic insufficiency, leading to delayed diagnosis.

Differential diagnoses for gastrinoma include: 1) Chronic gastritis (e.g., Helicobacter-associated, lymphocytic-plasmacytic) – typically less severe ulceration, responsive to dietary and medical management; 2) Inflammatory bowel disease (IBD) – chronic diarrhea and vomiting, but no hypergastrinemia or severe ulceration; 3) Gastrointestinal lymphoma – may cause similar signs, but diagnosis via biopsy; 4) Exocrine pancreatic insufficiency (EPI) – chronic diarrhea and weight loss, but no vomiting or ulceration; 5) Renal failure – can cause vomiting and gastrointestinal ulceration due to uremic gastritis, but hypergastrinemia is not a feature; 6) Mast cell tumor (visceral) – can cause gastric ulceration due to histamine release, but diagnosis via cytology/histopathology; 7) Foreign body ingestion – acute onset, imaging findings; 8) Gastric adenocarcinoma – older dogs, weight loss, vomiting, but no hypergastrinemia; 9) Duodenal ulceration due to nonsteroidal anti-inflammatory drugs (NSAIDs) – history of drug administration; 10) Hepatic disease – may cause gastrointestinal signs, but no hypergastrinemia. Definitive differentiation relies on measurement of serum gastrin levels, imaging, and histopathology.

The diagnostic algorithm for gastrinoma begins with a thorough history and physical examination, focusing on chronic vomiting, diarrhea, and weight loss. Baseline laboratory tests (CBC, biochemistry, urinalysis) are performed to assess for anemia, electrolyte imbalances, and organ dysfunction. If gastrointestinal ulceration is suspected, abdominal imaging (radiography, ultrasonography) is recommended to identify masses, thickened bowel, or free fluid. Endoscopy is valuable to visualize ulcers and obtain biopsies of the stomach and duodenum. The key screening test is serum gastrin concentration, which should be measured after a 12-hour fast. If gastrin is markedly elevated (>100 pg/mL, often >200 pg/mL), gastrinoma is suspected. A gastrin stimulation test with secretin or calcium can be performed to differentiate gastrinoma from other causes of hypergastrinemia, but this is rarely done in veterinary practice. Advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may help localize the primary tumor and detect metastases. Definitive diagnosis requires histopathological confirmation of a neuroendocrine tumor, typically via surgical biopsy or fine-needle aspiration of the pancreatic mass or metastatic lesions.

Laboratory findings in gastrinoma often reflect chronic gastrointestinal blood loss and acid-base disturbances. Complete blood count (CBC) may show anemia (regenerative or non-regenerative) due to chronic bleeding, with microcytosis if iron deficiency develops. Serum biochemistry may reveal hypoproteinemia (hypoalbuminemia) due to protein-losing enteropathy, and electrolyte abnormalities such as hypokalemia, hypochloremia, and metabolic alkalosis due to vomiting. Liver enzymes may be elevated if metastatic disease is present. Urinalysis is typically unremarkable, but may show low urine specific gravity if concurrent renal disease. Blood gas analysis may demonstrate metabolic alkalosis. The most specific laboratory finding is elevated serum gastrin concentration, typically >100 pg/mL, and often >200 pg/mL. In dogs, normal fasting gastrin is usually <50 pg/mL. Other biomarkers such as pancreatic polypeptide may be elevated, but are not routinely measured. Fecal occult blood testing may be positive due to gastrointestinal bleeding.

Imaging plays a crucial role in the diagnosis and staging of gastrinoma. Abdominal radiography may reveal a soft tissue mass in the cranial abdomen, but is often unremarkable. Ultrasonography is more sensitive and may show a pancreatic mass (often hypoechoic), thickened stomach wall, or evidence of metastasis to the liver or lymph nodes. Doppler ultrasound can assess vascular invasion. Computed tomography (CT) is the preferred imaging modality for detecting small pancreatic masses and metastases, with contrast-enhanced CT showing hyperenhancing lesions. Magnetic resonance imaging (MRI) may be useful for characterizing hepatic lesions. Endoscopy is essential for evaluating gastric and duodenal ulcers, and can also be used to obtain biopsies. Endoscopic ultrasound (EUS) is not widely available in veterinary medicine but can provide detailed imaging of the pancreas. Thoracic radiographs are recommended to rule out pulmonary metastases, although these are rare.

Cytological evaluation of fine-needle aspirates (FNA) from pancreatic masses or metastatic lesions may reveal clusters of round to polygonal cells with moderate amounts of cytoplasm, granular chromatin, and occasional nucleoli, consistent with neuroendocrine neoplasia. However, cytology may be nondiagnostic due to hemorrhage or necrosis. Histopathology is the gold standard for diagnosis. On biopsy, gastrinomas exhibit a characteristic neuroendocrine growth pattern, with cells arranged in nests, trabeculae, or sheets, often with a rich capillary network. The cells have round nuclei with stippled chromatin (salt-and-pepper appearance) and moderate eosinophilic cytoplasm. Immunohistochemistry is confirmatory, with positive staining for chromogranin A, synaptophysin, and gastrin. Ki-67 index may be used to assess proliferative activity. Metastatic lesions in the liver or lymph nodes show similar histologic features. Special stains, such as Grimelius silver stain, can highlight neuroendocrine granules.

Treatment of gastrinoma involves a multimodal approach: medical management to control gastric acid hypersecretion and ulceration, and surgical resection if feasible. Emergency stabilization may be required for patients with perforation or severe hemorrhage, including fluid therapy, blood transfusions, and surgical intervention. Medical therapy primarily consists of proton pump inhibitors (PPIs) such as omeprazole (0.7-1.5 mg/kg PO q12h) or pantoprazole (0.7-1.0 mg/kg IV q24h) to suppress acid secretion. Histamine-2 receptor antagonists (e.g., famotidine 0.5-1 mg/kg PO/IV q12h) may be used as adjuncts but are less effective. Sucralfate (0.5-1 g per dog PO q8h) can be administered as a cytoprotectant. Octreotide, a somatostatin analogue, may reduce gastrin secretion and is used in humans, but its efficacy in dogs is variable; dosage is 10-20 µg/kg SC q8-12h. Surgical resection of the primary tumor and metastases is the treatment of choice if complete excision is possible. However, many tumors are metastatic at diagnosis, making surgery palliative. In such cases, medical management is the mainstay. Chemotherapy (e.g., streptozocin) has been reported but is not well-established. Supportive care includes a bland, easily digestible diet, and management of diarrhea with antidiarrheals if needed.

The prognosis for gastrinoma is generally poor to guarded. The tumor is often malignant, with metastasis present in up to 80% of cases at diagnosis. Median survival times in dogs are reported to be around 1-2 years with medical management alone, but can be longer if complete surgical resection is achieved. Negative prognostic factors include the presence of metastasis, high tumor grade, and poor response to medical therapy. Patients with severe ulceration or perforation have a high mortality rate. With aggressive medical management, quality of life can be improved, but cure is rare. In cats, the prognosis is similarly poor. Regular monitoring of serum gastrin levels and imaging is essential to assess disease progression.

Follow-up for gastrinoma patients involves regular re-evaluations every 1-3 months initially, then every 3-6 months if stable. At each visit, a thorough physical examination, CBC, serum biochemistry, and serum gastrin measurement should be performed. Abdominal ultrasonography or CT should be repeated every 3-6 months to monitor for tumor growth or metastasis. If the patient is on omeprazole, dose adjustments may be needed based on clinical response and gastrin levels. For patients undergoing surgery, post-operative monitoring is crucial to detect recurrence. Long-term management includes continued PPI therapy, dietary modifications, and monitoring for complications such as anemia or protein-losing enteropathy. Owners should be educated on signs of ulcer perforation, such as acute abdominal pain, collapse, or vomiting blood, and advised to seek immediate veterinary care.

Pearls: 1) Always consider gastrinoma in any dog with severe, recurrent gastrointestinal ulceration or diarrhea that is unresponsive to standard therapy. 2) Serum gastrin measurement is a simple, cost-effective screening test; a fasting gastrin >100 pg/mL is highly suggestive. 3) Proton pump inhibitors are the mainstay of medical therapy; use high doses and administer on an empty stomach for optimal effect. 4) Surgical resection offers the best chance for long-term survival if the tumor is localized. 5) Octreotide can be a useful adjunct in some cases. Pitfalls: 1) Do not rely on imaging alone to rule out gastrinoma, as small tumors may be missed. 2) Avoid using histamine-2 receptor antagonists as sole therapy, as they are often insufficient. 3) Do not delay surgery if a resectable mass is identified, as metastasis can occur early. 4) Be cautious with corticosteroid use, as they can exacerbate ulceration. 5) Do not forget to check for concurrent endocrine tumors, especially in breeds predisposed to multiple endocrine neoplasia.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are recommended for gastrinoma: 1) Omeprazole: Dogs: 0.7-1.5 mg/kg PO q12h; Cats: 0.7-1.0 mg/kg PO q12h. Administer on an empty stomach 30 minutes before meals. For severe cases, may use IV pantoprazole at 0.7-1.0 mg/kg IV q24h. 2) Famotidine: Dogs: 0.5-1 mg/kg PO/IV q12h; Cats: 0.5-1 mg/kg PO/IV q12h. Less potent than PPIs, use as adjunct. 3) Sucralfate: Dogs: 0.5-1 g PO q8h; Cats: 0.25-0.5 g PO q8h. Administer 1 hour before or 2 hours after other medications. 4) Octreotide: Dogs: 10-20 µg/kg SC q8-12h; Cats: 10-20 µg/kg SC q8-12h. May cause vomiting or diarrhea; monitor glucose. 5) For pain management, use opioids such as buprenorphine (0.01-0.02 mg/kg IV/IM q8-12h) or fentanyl CRI (2-5 µg/kg/hr) in hospitalized patients. Avoid NSAIDs due to ulcer risk. 6) Antiemetics: Maropitant (1 mg/kg IV/PO q24h) or ondansetron (0.1-0.2 mg/kg IV q8-12h) for vomiting. 7) Antibiotics are not routinely indicated unless perforation or sepsis is present. Adjust dosages in renal or hepatic impairment. Monitor for drug interactions, especially with other hepatically metabolized drugs.

Evidence on gastrinoma in veterinary medicine is limited to case reports and small case series. A retrospective study by Lurye et al. (2002) described 10 dogs with gastrinoma, reporting that most had metastasis at diagnosis and median survival was 1 year with medical management. Another study by English et al. (1988) reported successful surgical resection in a dog with long-term survival. In cats, only isolated case reports exist. Consensus guidelines from ACVIM on gastrointestinal tumors recommend serum gastrin measurement in cases of unexplained gastrointestinal ulceration. There are no randomized controlled trials due to the rarity of the disease. Treatment protocols are extrapolated from human medicine, where PPIs and somatostatin analogues are standard. A recent case report described the use of octreotide in a dog with gastrinoma, showing temporary improvement. Overall, the evidence base is weak, and management is largely individualized.