Synchronous Succinate Dehydrogenase Subunit B (SDHB)-Deficient Gallbladder Paraganglioma and Periduodenal Well-Differentiated Neuroendocrine Tumor: A Case Report.

Paragangliomas are extra-adrenal neuroendocrine neoplasms (NENs), and those arising in the gallbladder are exceedingly rare. Pathogenic variants in the succinate dehydrogenase (SDH) complex are well-established drivers of paragangliomas and pheochromocytomas, and have also been implicated in epithelial neuroendocrine tumors (NETs). We report a 50-year-old woman with a likely pathogenic succinate dehydrogenase subunit B (SDHB) germline variant (p.D118Y, c.352G>T), who underwent surveillance imaging due to a family history of malignant carotid paraganglioma. Magnetic resonance imaging revealed a 2.5-cm retroperitoneal mass adjacent to the duodenum and pancreatic head, consistent on biopsy with a well-differentiated, low-grade NET showing loss of SDHB expression. Endoscopic evaluation incidentally identified a 1.2-cm enhancing nodule on the hepatic surface of the gallbladder wall. 64Cu-DOTATATE positron emission tomography/computed tomography (PET/CT) showed somatostatin receptor-positive uptake in both lesions. The patient underwent cholecystectomy with liver wedge resection and excision of the periduodenal mass. Histologic examination revealed two distinct tumors: a gallbladder paraganglioma showing a Zellballen pattern, positive for synaptophysin and chromogranin but negative for cytokeratin, and a periduodenal well-differentiated NET positive for cytokeratin AE1/AE3, synaptophysin, and chromogranin. Both lesions demonstrated complete loss of SDHB immunoreactivity, confirming cross-lineage SDH deficiency. The concurrent absence of SDHB expression in both tumors provides compelling phenotypic evidence supporting the pathogenicity of the germline SDHB variant, and reinforces the biologic coherence of the genotype-phenotype correlation. The postoperative course was uneventful, and follow-up imaging at eight months showed no recurrence. This unique case of synchronous SDHB-deficient gallbladder paraganglioma and periduodenal NET illustrates cross-lineage tumorigenesis within a single germline background and emphasizes the value of SDH immunohistochemistry and genotype-guided surveillance.