Answer: All of the above

Histological Description:
Within the lamina propria and muscularis propria is an infiltrating tumor composed to loosely cohesive individual cells. Most of the cells superficially resemble plasma cells with eccentric cytoplasm although the nuclei lack the chromatin of plasma cells and rather have central prominent nucleoli. Other tumor cells have cytoplasmic vacuoles or pink cytoplasmic inclusions resembling signet ring cell and rhabdoid cells, respectively. Many of the cells also look similar to infiltrating lobular carcinoma of the breast.

Discussion:
Contemporary thinking suggests that signet ring cell, plasmacytoid, lobular-like carcinoma, and rhabdoid variants of urothelial carcinoma as belonging to the same spectrum of tumors and are likely to represent the same disease. In fact, virtually all cases of signet ring cell carcinoma contain a variable (and sometimes predominant) number of cells with plasmacytoid, rhabdoid –like, or lobular-like features while occasional signet ring cells may be present in both plasmacytoid and rhabdoid-like variants of urothelial carcinoma. Other shared features of these loosely cohesive tumors include a diffuse pattern of infiltration, advanced stage at presentation, poor clinical outcome and shared molecular abnormalities.
Confirming origin in the bladder by finding carcinoma in situ may be very difficult, particularly on biopsy specimens. These tumors tend to infiltrate the bladder wall diffusely giving it an indurated and thickened quality similar to the “linitis plastica” seen in gastric signet ring cell carcinomas. The tumor also commonly infiltrates extensively throughout adjacent soft tissue, making primary resection for cure virtually impossible. Because of their advanced stage at presentation and poor prognosis, it is important to make the diagnosis on the TUR specimen, if at all possible. In the differential diagnosis, one must rule out direct extension, usually from a rectal carcinoma, or metastasis from stomach or lobular carcinoma of the breast. Both breast carcinoma and this urothelial carcinoma variant are positive for GATA3 and can express ER so that only GCDFP and possibly uroplakin would be helpful in distinguishing them. CDX2 can also be seen in urothelial carcinoma so is not definitive for a GI primary. One must also be aware that CD138 labels both plasma cells and plasmacytoid variant of urothelial carcinoma. In the absence of a conventional in-situ or invasive component, one must often put a disclaimer in the pathology report, that this tumor is consistent with a bladder primary but a breast (in a female) or GI primary should be excluded on clinical grounds.