Value of uroplakin III in distinguishing variants of primary bladder urothelial carcinoma from malignancy metastatic to the urinary bladder.

Urothelial carcinoma (UC) variants can be difficult to differentiate from carcinoma metastatic to the bladder. We examined immunostaining for uroplakin III in 43 cases of primary bladder UC variants including micropapillary UC (n=19), nested variant of UC (n=2), pleomorphic giant-cell carcinoma (n=8), plasmacytoid UC (n=4), lymphoepithelioma-like carcinoma (n=2), large cell undifferentiated carcinoma (n=2), UC with abundant myxoid stroma (n=3) and lipid cell variant (n=3) and in 11 tumors from other organs metastatic to the bladder. These tumors included invasive ductal carcinoma of the breast (n=2), colorectal adenocarcinoma (n=4), endometrioid adenocarcinoma (n=1) and serous papillary carcinoma of the uterus (n=1) melanoma (n=1), embryonal carcinoma of the testis (n=1), and renal clear cell carcinoma (n=1). Out of the 43 UC variants, 35 (81%) were positive for uroplakin III, including micropapillary, lipid cell variant and UC with abundant myxoid stroma. Pleomorphic giant cell carcinoma, plasmacytoid UC and nested variant of UC were less commonly positive. Of the 11 metastatic tumors, six were found to be positive for uropIakin III: metastatic colorectal adenocarcinoma, clear cell carcinoma of the kidney and embryonal carcinoma of testis. UP III Positivity for uroplakin III is not found only in primary bladder UC variants, but in some tumors that have metastatized to the bladder. Staining for uroplakin III alone should not be taken as evidence of UC.