Neuroendocrine Tumors of the Gallbladder: A Multicenter Case Series and Systematic Literature Review Indicating Predominantly Non-Aggressive Tumor Behavior and a Common Association with Cholesterol Polyps and Cholesterolosis

Current data on neuroendocrine tumors (NETs) of the gallbladder and cystic duct (GB-NETs) are highly limited, and the available evidence, largely derived from cancer registry data, suggests that these tumors exhibit a substantially more aggressive clinical behavior than NETs arising at other anatomical sites. We analyzed 26 GB-NETs. Female-to-male ratio: 1.9:1; median age: 50 years. They were typically incidental small (median: 0.8 cm, range: 0.08-2.3 cm) tumors, with 81% classified as grade 1 (G1), either pT1 or pT2, and 38% showing a paraganglioma-like pattern at least focally. Body/fundus tumors were mostly polypoid while neck/cystic duct examples showed mural growth. Symplastic pleomorphism was found in 11%, pseudoglandular structures in 50% and psammomatous intraluminal calcifications in 15%. Pancreatic polypeptide, somatostatin, and gastrin were detected in 5/7 (71%), 5/8 (62.5%), and 4/7 (57%) tested cases, respectively. ISLET1 and ARX were diffusely expressed in all 5 tested cases. Cholesterolosis was common, and six appeared to arise within cholesterol polyps. None had any evidence of metastasis and all 12 patients with available follow-up were alive without disease after a median follow-up of 30 months. Critical review of the literature (79 bona-fide GB-NETs) showed that tumors > 2 cm and grade 3 (G3) were enriched among metastatic GB-NETs. In conclusion, GB-NETs are generally indolent neoplasms, in contrast to gallbladder neuroendocrine carcinomas. Tumor size and grade may help predict metastatic potential, while the relatively frequent paraganglioma-like morphology, often associated with diffuse somatostatin expression, can represent a diagnostic pitfall. Their frequent association with cholesterol polyps also suggests a possible etiopathogenetic link.