Hypovascular insulinoma with reduced microvessel density on histopathology: a case report.

Pancreatic neuroendocrine tumors (PanNETs) are generally hypervascular and readily detectable on imaging tests. However, hypovascular PanNETs are clinically problematic, requiring multiple imaging tests and tissue analyses to differentiate them from pancreatic ductal cancers. A 41 year-old man presented with Whipple's triad; 72 h fasting test followed by glucagon challenge test suggested insulinoma. However, contrast-enhanced computed tomography image showed a 17 mm tumor with poor enhancement and unclear borders in the tail of the pancreas. Abdominal magnetic resonance imaging and contrast-enhanced endoscopic ultrasonography (EUS) indicated cystic degeneration and necrosis at the same site; EUS-guided fine-needle aspiration cytology indicated a PanNET Grade 1 tumor. Although the imaging was inconclusive, diazoxide treatment ameliorated the hypoglycemia-related symptoms and insulinoma was deemed likely; following tail pancreatectomy and splenectomy, the symptoms disappeared. Pathological examination revealed a tumor positive for insulin and classed as PanNET Grade 1 according to the 2019 WHO classification. The microvessel density (MVD) of the tumor was found to be as low as 3.9%, which may partly account for the inconclusive images. The present case was difficult to diagnose preoperatively due to hypovascularity on imaging because of reduced MVD. It is clinically important to evaluate MVD in cases of hypovascular PanNETs by multiple preoperative imaging studies to differentiate them from pancreatic cancers and to validate the findings by postoperative pathological analysis.