Background 

VIPoma are rare functional pancreatic neuroendocrine tumours (pNETs) that typically present with severe secretory diarrhoea, hypokalaemia, and metabolic acidosis, and are often metastatic at diagnosis. Delayed diagnosis is uncommon because the characteristic clinical syndrome usually prompts early investigation. This case highlights an exceptionally indolent functional VIPoma with prolonged clinical and radiological stability. 

Case 

A 75-year-old man presented in 2021 with two admissions for severe watery diarrhoea (>10 motions/day), acute kidney injury, profound hypokalaemia, and metabolic acidosis (nadir pH 7.02). Fasting vasoactive intestinal peptide (VIP) levels were repeatedly elevated (>120 pmol/L; ULN 25), but the diagnosis was missed. He was treated for presumed pancreatic exocrine insufficiency based on a low faecal elastase, with gradual resolution of diarrhoea over two years and eventual cessation of pancreatic enzyme replacement. 

In 2025, an episode of pancreatitis led to incidental detection of a 3 cm pancreatic head lesion, retrospectively stable on imaging since 2021. Endoscopic ultrasound-guided biopsy demonstrated a well-differentiated grade-1 pNET (Ki-67 1%). DOTATATE PET showed avid uptake confined to the pancreatic lesion without FDG-avid metastatic disease. Repeat fasting VIP remained elevated (114–118 pmol/L), and immunohistochemistry staining on tissue biopsy confirmed tumoural VIP expression, establishing the diagnosis of VIPoma. Lanreotide 60 mg monthly was commenced, resulting in transient gastrointestinal adverse effects but complete resolution of diarrhoea after the third dose. Following multidisciplinary discussion, the patient elected radiological surveillance rather than surgery. 

Conclusion 

This case highlights an exceptionally indolent functional VIPoma with prolonged radiological stability, spontaneous remission of classical secretory symptoms despite persistent VIP elevation, and absence of metastatic disease four years after presentation. The discordance between persistent biochemical activity and clinical phenotype demonstrates the heterogeneity of VIPoma and the limited evidence guiding management of atypical presentations. Individualised treatment balancing disease biology, treatment toxicity, and patient preference remains essential.