Answer: Infiltrating poorly differentiated carcinoma with squamous differentiation

Histology: The tumor consists of sheets of undifferentiated cells with large prominent nucleoli and lightly eosinophilic cytoplasm. Scattered amongst these undifferentiated cells are cells with similar nuclei yet very brightly dense eosinophilic cytoplasm.

Discussion: All of the above choices are various histological patterns of urothelial carcinoma. Infiltrating urothelial carcinoma with rhabdoid differentiation and plasmacytoid differentiation are extremely rare and associated with a relatively poor prognosis. Rhabdoid tumors have eccentric cytoplasmic hyaline globules and plasmacytoid tumors have eccentric amphophilic cytoplasm resembling plasma cells; both of these features are lacking in the current case. Large cell undifferentiated carcinoma is a controversial term. I use this term for cases where, if one did not know the tumor was in the bladder, one would not be able to recognize it as being of urothelial differentiation. Others make the assumption that any tumor in the bladder without overt differentiation is of urothelial origin and hence would not use the term “undifferentiated carcinoma.” Instead, they would call these tumors “high grade urothelial carcinomas.” It is unclear whether bladder tumors showing no specific differentiation have a worse prognosis than high grade urothelial carcinoma with at least some urothelial differentiation. In the current case, cells with dense eosinophilic cytoplasm demonstrate squamous differentiation. This particular pattern of squamous differentiation is extremely uncommon. More typically, one sees nests of squamous cells showing variable keratin production. In the current case, rather, one sees isolated cells showing squamous differentiation in a background of an undifferentiated carcinoma. The finding of aberrant (squamous or glandular) differentiation in a urothelial carcinoma is of no prognostic significance.