NCI: Cancer that forms in tissues of the gallbladder. The gallbladder is a pear-shaped organ below the liver that collects and stores bile (a fluid made by the liver to digest fat). Gallbladder cancer begins in the innermost layer of tissue and spreads through the outer layers as it grows.,NCI: A malignant tumor arising from the epithelium of the gallbladder. It is usually associated with the presence of gallstones. Clinical symptoms are not specific and usually present late in the course. Morphologically, most gallbladder carcinomas are adenocarcinomas; squamous cell carcinomas, adenosquamous carcinomas, signet ring carcinomas, and undifferentiated carcinomas can also occur.