Unusual malignant epithelial tumors of the gallbladder.

Most malignant epithelial tumors of the gallbladder are heterogenous and classified as adenocarcinomas. The authors report a previously unrecognized group of unusual gallbladder carcinomas including two cases of signet ring cell carcinoma in situ, three of cribriform invasive adenocarcinoma, and one of adenocarcinoma with pseudoangiosarcomatous features. In addition, the authors review their material and update the information on clear cell, small cell, and undifferentiated carcinoma of the gallbladder. A new morphologic variant of undifferentiated carcinoma mimicking lobular carcinoma of the breast is identified. The clinical, morphologic, and immunohistochemical features of these unusual neoplasms are emphasized.