Answer: Small cell undifferentiated carcinoma

Histology: The TUR is extensively replaced by sheets and large irregular nests of tumor. At low magnification, the tumor has a very basophilic appearance with numerous foci of geographic necrosis. The stroma separating the nests appears fibroblastic. At higher magnification, the tumor consists of a cellular infiltrate with nuclei showing high nuclear-to-cellular plastic ratio, small indistinct nucleoli, numerous apoptotic bodies, and scattered mitotic figures.

Discussion: Poorly differentiated adenocarcinoma of the prostate would have more abundant cytoplasm where the nuclei would typically show large central eosinophilic nucleoli. In addition, even poorly differentiated adenocarcinomas of the prostate typically show an attempt at glandular differentiation, even though it may be subtle, consisting of small rosette-like structures within sheets of cells as an attempt at poorly differentiated cribriform glandular formation. High grade infiltrating urothelial carcinoma still retains the characteristic features of urothelial cells with more abundant cytoplasm, a slight spindling of cells, and a nested appearance. I use the term “large cell undifferentiated urothelial carcinoma” for those urothelial tumors within the bladder which, if they were not occurring within the bladder, would not be recognizable as being of urothelial differentiation. Others may regard what I term “large cell undifferentiated urothelial carcinoma” as merely a variant of high grade infiltrating urothelial carcinoma. The crucial distinction in this case is to differentiate small cell undifferentiated carcinoma from non-small cell carcinoma. Typically, scant specimens from biopsies of small cell carcinoma show more molding and less distinct chromatin as compared to resections of small cell carcinoma. In the current case, with larger amounts of tissue available for examination, one may not recognize this lesion as small cell carcinoma since the nuclei do not show as much molding, have somewhat more open chromatin, and a very scant amount of cytoplasm. Nonetheless, this lesion lacks the more abundant cytoplasm and prominent nucleoli typical of non-small cell carcinoma of the bladder. In addition, large areas of geographic necrosis and numerous apoptotic bodies are typical of small cell carcinoma. One cannot distinguish between small cell carcinoma of the bladder or prostate unless one sees a non-small cell component typical of either organ. In my experience, small cell carcinomas of the prostate are almost always negative for PSA and PSAP in the small cell component. If one has difficulty in distinguishing small cell versus non-small cell carcinoma, immunohistochemical stains for neuroendocrine differentiation such as CD56 may be performed. However, the diagnosis of small cell undifferentiated carcinoma is still one based on the H&E stained sections rather than one based on immunohistochemistry. It is critical to make the distinction between small cell and non-small cell carcinoma as they are treated differently, with small cell carcinoma treated primarily by chemotherapy.