Answer: Embryonal rhabdomyosarcoma

Histology: This bladder wall demonstrates some evidence of polypoid cystitis, likely due to the irritative effects of chemotherapy. The bladder wall is focally hypercellular, and in the lamina propria one can see that the extra cells are spindled with well-developed ,eosinophilic cytoplasm that has cross-striations. Mitotic figures are rare, likely due to prior therapy. These are the typical findings of Embryonal Rhabsomyosarcoma, and the microscopic findings would fit the gross description of a botryoid tumor.

Discussion: Papillary Urothelial Carcinoma would be characterized by thin, fibrovascular septa lined by malignant urothelial cells. Inflammatory Myofibroblastic Tumor is a spindle cell tumor that may involve the bladder, and typically has an extensive associated inflammatory infiltrate. The spindle cells are often immunoreactive for ALK. Alveolar Rhabdomyosarcoma is typically a round cell tumor, as opposed to Embryonal Rhabdomyosarcoma which is a spindle cell tumor.