Immunohistochemical distinction between primary adenocarcinoma of the bladder and secondary colorectal adenocarcinoma.

Primary adenocarcinoma of the urinary bladder sometimes causes a diagnostic dilemma because it can be indistinguishable morphologically from adenocarcinoma of colorectal origin secondarily involving the bladder by metastasis or direct extension. It is much less well studied than conventional urothelial carcinoma and colorectal adenocarcinoma because of its rarity. The current study was specifically designed to investigate whether an important mechanism involved in the pathogenesis of colorectal adenocarcinoma, beta-catenin dysregulation, was also important for the development of primary bladder adenocarcinoma and whether these two morphologically similar tumors could be distinguished immunohistochemically. Formalin-fixed, paraffin-embedded tissues from 17 primary adenocarcinomas of the urinary bladder, 16 colorectal adenocarcinomas involving the bladder, and 10 conventional urothelial (transitional) carcinomas were included in this study. Thirteen of the primary bladder adenocarcinomas were moderately to well differentiated (enteric type) and morphologically indistinguishable from colorectal cancers. The remaining four primary tumors were poorly differentiated (two cases) or of clear cell type (two cases). Immunohistochemical studies using a panel of monoclonal antibodies demonstrated positive nuclear staining for beta-catenin expression in 13 of the 16 (81%) colorectal adenocarcinomas secondarily involving the bladder but in none of the primary adenocarcinomas or the urothelial carcinomas. Instead, positive membranous (and some cytoplasmic) staining was present in all primary bladder tumors with the exception of two poorly differentiated adenocarcinomas where no beta-catenin staining was detected. All secondary colorectal adenocarcinomas stained negatively for CK7 and thrombomodulin (TM), whereas positivity for CK20 was observed in 15 (94%) cases. All urothelial carcinomas stained positively for CK7 and TM, and four of them also for CK20. Primary adenocarcinomas of the bladder showed mixed staining patterns for CK7, CK20, and TM with a positive rate of 65%, 53%, and 59%, respectively. These data indicate that dysregulation of beta-catenin, an important aberration seen in colorectal carcinogenesis, does not appear to play a role in the pathogenesis of the bladder adenocarcinoma. In addition, our data demonstrate that a panel of immunostains, including CK7, CK20, TM, and beta-catenin, is of diagnostic value in differentiating primary bladder adenocarcinoma from secondary adenocarcinoma of colorectal origin.