An aggressive, high-grade and poorly differentiated carcinoma with neuroendocrine differentiation that arises from the gallbladder. It is characterized by the presence of malignant small cells. The disease is gallbladder small cell neuroendocrine carcinoma (MONDO_0006219, a Monarch Disease Ontology entry). Also known as: Oat cell carcinoma of gallbladder, Oat cell carcinoma of the gallbladder, gall bladder small cell carcinoma, gallbladder Oat cell carcinoma, gallbladder small cell NEC, gallbladder small cell carcinoma, small cell carcinoma of gall bladder, small cell carcinoma of gallbladder.